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Merge pull request #294 from ia0/doc

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Update the documentation to use linking by name
This commit is contained in:
Julien Cretin
2021-03-15 14:13:46 +01:00
committed by GitHub
parent e5313057f9 e50d89e28b
commit b1773d1cf3
8 changed files with 368 additions and 377 deletions
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38
libraries/persistent_store/src/buffer.rs
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@@ -12,6 +12,11 @@
// See the License for the specific language governing permissions and
// limitations under the License.
//! Flash storage for testing.
//!
//! [`BufferStorage`] implements the flash [`Storage`] interface but doesn't interface with an
//! actual flash storage. Instead it uses a buffer in memory to represent the storage state.
use crate::{Storage, StorageError, StorageIndex, StorageResult};
use alloc::borrow::Borrow;
use alloc::boxed::Box;
@@ -63,8 +68,8 @@ pub struct BufferOptions {
///
/// When set, the following conditions would panic:
/// - A bit is written from 0 to 1.
/// - A word is written more than `max_word_writes`.
/// - A page is erased more than `max_page_erases`.
/// - A word is written more than [`Self::max_word_writes`].
/// - A page is erased more than [`Self::max_page_erases`].
pub strict_mode: bool,
}
@@ -110,15 +115,13 @@ impl BufferStorage {
///
/// Before each subsequent mutable operation (write or erase), the delay is decremented if
/// positive. If the delay is elapsed, the operation is saved and an error is returned.
/// Subsequent operations will panic until the interrupted operation is [corrupted] or the
/// interruption is [reset].
/// Subsequent operations will panic until either of:
/// - The interrupted operation is [corrupted](BufferStorage::corrupt_operation).
/// - The interruption is [reset](BufferStorage::reset_interruption).
///
/// # Panics
///
/// Panics if an interruption is already armed.
///
/// [corrupted]: struct.BufferStorage.html#method.corrupt_operation
/// [reset]: struct.BufferStorage.html#method.reset_interruption
pub fn arm_interruption(&mut self, delay: usize) {
self.interruption.arm(delay);
}
@@ -130,10 +133,8 @@ impl BufferStorage {
/// # Panics
///
/// Panics if any of the following conditions hold:
/// - An interruption was not [armed].
/// - An interruption was not [armed](BufferStorage::arm_interruption).
/// - An interruption was armed and it has triggered.
///
/// [armed]: struct.BufferStorage.html#method.arm_interruption
pub fn disarm_interruption(&mut self) -> usize {
self.interruption.get().err().unwrap()
}
@@ -142,16 +143,14 @@ impl BufferStorage {
///
/// # Panics
///
/// Panics if an interruption was not [armed].
///
/// [armed]: struct.BufferStorage.html#method.arm_interruption
/// Panics if an interruption was not [armed](BufferStorage::arm_interruption).
pub fn reset_interruption(&mut self) {
let _ = self.interruption.get();
}
/// Corrupts an interrupted operation.
///
/// Applies the [corruption function] to the storage. Counters are updated accordingly:
/// Applies the corruption function to the storage. Counters are updated accordingly:
/// - If a word is fully written, its counter is incremented regardless of whether other words
/// of the same operation have been fully written.
/// - If a page is fully erased, its counter is incremented (and its word counters are reset).
@@ -159,13 +158,10 @@ impl BufferStorage {
/// # Panics
///
/// Panics if any of the following conditions hold:
/// - An interruption was not [armed].
/// - An interruption was not [armed](BufferStorage::arm_interruption).
/// - An interruption was armed but did not trigger.
/// - The corruption function corrupts more bits than allowed.
/// - The interrupted operation itself would have panicked.
///
/// [armed]: struct.BufferStorage.html#method.arm_interruption
/// [corruption function]: type.BufferCorruptFunction.html
pub fn corrupt_operation(&mut self, corrupt: BufferCorruptFunction) {
let operation = self.interruption.get().unwrap();
let range = self.operation_range(&operation).unwrap();
@@ -217,7 +213,8 @@ impl BufferStorage {
///
/// # Panics
///
/// Panics if the maximum number of erase cycles per page is reached.
/// Panics if the [maximum number of erase cycles per page](BufferOptions::max_page_erases) is
/// reached.
fn incr_page_erases(&mut self, page: usize) {
// Check that pages are not erased too many times.
if self.options.strict_mode {
@@ -243,7 +240,8 @@ impl BufferStorage {
///
/// # Panics
///
/// Panics if the maximum number of writes per word is reached.
/// Panics if the [maximum number of writes per word](BufferOptions::max_word_writes) is
/// reached.
fn incr_word_writes(&mut self, index: usize, value: &[u8], complete: &[u8]) {
let word_size = self.word_size();
for i in 0..value.len() / word_size {

4
libraries/persistent_store/src/driver.rs
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@@ -12,6 +12,10 @@
// See the License for the specific language governing permissions and
// limitations under the License.
//! Store wrapper for testing.
//!
//! [`StoreDriver`] wraps a [`Store`] and compares its behavior with its associated [`StoreModel`].
use crate::format::{Format, Position};
#[cfg(test)]
use crate::StoreUpdate;

176
libraries/persistent_store/src/format.rs
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@@ -12,6 +12,8 @@
// See the License for the specific language governing permissions and
// limitations under the License.
//! Storage representation of a store.
#[macro_use]
mod bitfield;
@@ -26,13 +28,14 @@ use core::convert::TryFrom;
/// Internal representation of a word in flash.
///
/// Currently, the store only supports storages where a word is 32 bits.
/// Currently, the store only supports storages where a word is 32 bits, i.e. the [word
/// size](Storage::word_size) is 4 bytes.
type WORD = u32;
/// Abstract representation of a word in flash.
///
/// This type is kept abstract to avoid possible confusion with `Nat` if they happen to have the
/// same representation. This is because they have different semantics, `Nat` represents natural
/// This type is kept abstract to avoid possible confusion with [`Nat`] if they happen to have the
/// same representation. This is because they have different semantics, [`Nat`] represents natural
/// numbers while `Word` represents sequences of bits (and thus has no arithmetic).
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct Word(WORD);
@@ -47,7 +50,7 @@ impl Word {
///
/// # Panics
///
/// Panics if `slice.len() != WORD_SIZE`.
/// Panics if `slice.len()` is not [`WORD_SIZE`] bytes.
pub fn from_slice(slice: &[u8]) -> Word {
Word(WORD::from_le_bytes(<WordSlice>::try_from(slice).unwrap()))
}
@@ -60,47 +63,49 @@ impl Word {
/// Size of a word in bytes.
///
/// Currently, the store only supports storages where a word is 4 bytes.
/// Currently, the store only supports storages where the [word size](Storage::word_size) is 4
/// bytes.
const WORD_SIZE: Nat = core::mem::size_of::<WORD>() as Nat;
/// Minimum number of words per page.
///
/// Currently, the store only supports storages where pages have at least 8 words.
const MIN_NUM_WORDS_PER_PAGE: Nat = 8;
/// Currently, the store only supports storages where pages have at least 8 [words](WORD_SIZE), i.e.
/// the [page size](Storage::page_size) is at least 32 bytes.
const MIN_PAGE_SIZE: Nat = 8;
/// Maximum size of a page in bytes.
///
/// Currently, the store only supports storages where pages are between 8 and 1024 [words].
///
/// [words]: constant.WORD_SIZE.html
/// Currently, the store only supports storages where pages have at most 1024 [words](WORD_SIZE),
/// i.e. the [page size](Storage::page_size) is at most 4096 bytes.
const MAX_PAGE_SIZE: Nat = 4096;
/// Maximum number of erase cycles.
///
/// Currently, the store only supports storages where the maximum number of erase cycles fits on 16
/// bits.
/// Currently, the store only supports storages where the [maximum number of erase
/// cycles](Storage::max_page_erases) fits in 16 bits, i.e. it is at most 65535.
const MAX_ERASE_CYCLE: Nat = 65535;
/// Minimum number of pages.
///
/// Currently, the store only supports storages with at least 3 pages.
/// Currently, the store only supports storages where the [number of pages](Storage::num_pages) is
/// at least 3.
const MIN_NUM_PAGES: Nat = 3;
/// Maximum page index.
///
/// Thus the maximum number of pages is one more than this number. Currently, the store only
/// supports storages where the number of pages is between 3 and 64.
/// Currently, the store only supports storages where the [number of pages](Storage::num_pages) is
/// at most 64, i.e. the maximum page index is 63.
const MAX_PAGE_INDEX: Nat = 63;
/// Maximum key index.
///
/// Thus the number of keys is one more than this number. Currently, the store only supports 4096
/// keys.
/// Currently, the store only supports 4096 keys, i.e. the maximum key index is 4095.
const MAX_KEY_INDEX: Nat = 4095;
/// Maximum length in bytes of a user payload.
///
/// Currently, the store only supports values smaller than 1024 bytes.
/// Currently, the store only supports values at most 1023 bytes long. This may be further reduced
/// depending on the [page size](Storage::page_size), see [`Format::max_value_len`].
const MAX_VALUE_LEN: Nat = 1023;
/// Maximum number of updates per transaction.
@@ -109,9 +114,15 @@ const MAX_VALUE_LEN: Nat = 1023;
const MAX_UPDATES: Nat = 31;
/// Maximum number of words per virtual page.
const MAX_VIRT_PAGE_SIZE: Nat = div_ceil(MAX_PAGE_SIZE, WORD_SIZE) - CONTENT_WORD;
///
/// A virtual page has [`CONTENT_WORD`] less [words](WORD_SIZE) than the storage [page
/// size](Storage::page_size). Those words are used to store the page header. Since a page has at
/// least [8](MIN_PAGE_SIZE) words, a virtual page has at least 6 words.
const MAX_VIRT_PAGE_SIZE: Nat = MAX_PAGE_SIZE / WORD_SIZE - CONTENT_WORD;
/// Word with all bits set to one.
///
/// After a page is erased, all words are equal to this value.
const ERASED_WORD: Word = Word(!(0 as WORD));
/// Helpers for a given storage configuration.
@@ -121,33 +132,31 @@ pub struct Format {
///
/// # Invariant
///
/// - Words divide a page evenly.
/// - There are at least 8 words in a page.
/// - There are at most `MAX_PAGE_SIZE` bytes in a page.
/// - [Words](WORD_SIZE) divide a page evenly.
/// - There are at least [`MIN_PAGE_SIZE`] words in a page.
/// - There are at most [`MAX_PAGE_SIZE`] bytes in a page.
page_size: Nat,
/// The number of pages in the storage.
///
/// # Invariant
///
/// - There are at least 3 pages.
/// - There are at most `MAX_PAGE_INDEX + 1` pages.
/// - There are at least [`MIN_NUM_PAGES`] pages.
/// - There are at most [`MAX_PAGE_INDEX`] + 1 pages.
num_pages: Nat,
/// The maximum number of times a page can be erased.
///
/// # Invariant
///
/// - A page can be erased at most `MAX_ERASE_CYCLE` times.
/// - A page can be erased at most [`MAX_ERASE_CYCLE`] times.
max_page_erases: Nat,
}
impl Format {
/// Extracts the format from a storage.
///
/// Returns `None` if the storage is not [supported].
///
/// [supported]: struct.Format.html#method.is_storage_supported
/// Returns `None` if the storage is not [supported](Format::is_storage_supported).
pub fn new<S: Storage>(storage: &S) -> Option<Format> {
if Format::is_storage_supported(storage) {
Some(Format {
@@ -163,21 +172,12 @@ impl Format {
/// Returns whether a storage is supported.
///
/// A storage is supported if the following conditions hold:
/// - The size of a word is [`WORD_SIZE`] bytes.
/// - The size of a word evenly divides the size of a page.
/// - A page contains at least [`MIN_NUM_WORDS_PER_PAGE`] words.
/// - A page contains at most [`MAX_PAGE_SIZE`] bytes.
/// - There are at least [`MIN_NUM_PAGES`] pages.
/// - There are at most [`MAX_PAGE_INDEX`]` + 1` pages.
/// - A word can be written at least twice between erase cycles.
/// - The maximum number of erase cycles is at most [`MAX_ERASE_CYCLE`].
///
/// [`WORD_SIZE`]: constant.WORD_SIZE.html
/// [`MIN_NUM_WORDS_PER_PAGE`]: constant.MIN_NUM_WORDS_PER_PAGE.html
/// [`MAX_PAGE_SIZE`]: constant.MAX_PAGE_SIZE.html
/// [`MIN_NUM_PAGES`]: constant.MIN_NUM_PAGES.html
/// [`MAX_PAGE_INDEX`]: constant.MAX_PAGE_INDEX.html
/// [`MAX_ERASE_CYCLE`]: constant.MAX_ERASE_CYCLE.html
/// - The [`Storage::word_size`] is [`WORD_SIZE`] bytes.
/// - The [`Storage::word_size`] evenly divides the [`Storage::page_size`].
/// - The [`Storage::page_size`] is between [`MIN_PAGE_SIZE`] words and [`MAX_PAGE_SIZE`] bytes.
/// - The [`Storage::num_pages`] is between [`MIN_NUM_PAGES`] and [`MAX_PAGE_INDEX`] + 1.
/// - The [`Storage::max_word_writes`] is at least 2.
/// - The [`Storage::max_page_erases`] is at most [`MAX_ERASE_CYCLE`].
fn is_storage_supported<S: Storage>(storage: &S) -> bool {
let word_size = usize_to_nat(storage.word_size());
let page_size = usize_to_nat(storage.page_size());
@@ -186,7 +186,7 @@ impl Format {
let max_page_erases = usize_to_nat(storage.max_page_erases());
word_size == WORD_SIZE
&& page_size % word_size == 0
&& (MIN_NUM_WORDS_PER_PAGE * word_size <= page_size && page_size <= MAX_PAGE_SIZE)
&& (MIN_PAGE_SIZE * word_size <= page_size && page_size <= MAX_PAGE_SIZE)
&& (MIN_NUM_PAGES <= num_pages && num_pages <= MAX_PAGE_INDEX + 1)
&& max_word_writes >= 2
&& max_page_erases <= MAX_ERASE_CYCLE
@@ -199,28 +199,28 @@ impl Format {
/// The size of a page in bytes.
///
/// We have `MIN_NUM_WORDS_PER_PAGE * self.word_size() <= self.page_size() <= MAX_PAGE_SIZE`.
/// This is at least [`MIN_PAGE_SIZE`] [words](WORD_SIZE) and at most [`MAX_PAGE_SIZE`] bytes.
pub fn page_size(&self) -> Nat {
self.page_size
}
/// The number of pages in the storage, denoted by `N`.
/// The number of pages in the storage, denoted by N.
///
/// We have `MIN_NUM_PAGES <= N <= MAX_PAGE_INDEX + 1`.
/// We have [`MIN_NUM_PAGES`] ≤ N ≤ [`MAX_PAGE_INDEX`] + 1.
pub fn num_pages(&self) -> Nat {
self.num_pages
}
/// The maximum page index.
///
/// We have `2 <= self.max_page() <= MAX_PAGE_INDEX`.
/// This is at least [`MIN_NUM_PAGES`] - 1 and at most [`MAX_PAGE_INDEX`].
pub fn max_page(&self) -> Nat {
self.num_pages - 1
}
/// The maximum number of times a page can be erased, denoted by `E`.
/// The maximum number of times a page can be erased, denoted by E.
///
/// We have `E <= MAX_ERASE_CYCLE`.
/// We have E ≤ [`MAX_ERASE_CYCLE`].
pub fn max_page_erases(&self) -> Nat {
self.max_page_erases
}
@@ -235,19 +235,18 @@ impl Format {
MAX_UPDATES
}
/// The size of a virtual page in words, denoted by `Q`.
/// The size of a virtual page in words, denoted by Q.
///
/// A virtual page is stored in a physical page after the page header.
///
/// We have `MIN_NUM_WORDS_PER_PAGE - 2 <= Q <= MAX_VIRT_PAGE_SIZE`.
/// We have [`MIN_PAGE_SIZE`] - 2 ≤ Q ≤ [`MAX_VIRT_PAGE_SIZE`].
pub fn virt_page_size(&self) -> Nat {
self.page_size() / self.word_size() - CONTENT_WORD
}
/// The maximum length in bytes of a user payload.
///
/// We have `(MIN_NUM_WORDS_PER_PAGE - 3) * self.word_size() <= self.max_value_len() <=
/// MAX_VALUE_LEN`.
/// This is at least [`MIN_PAGE_SIZE`] - 3 [words](WORD_SIZE) and at most [`MAX_VALUE_LEN`].
pub fn max_value_len(&self) -> Nat {
min(
(self.virt_page_size() - 1) * self.word_size(),
@@ -255,57 +254,50 @@ impl Format {
)
}
/// The maximum prefix length in words, denoted by `M`.
/// The maximum prefix length in words, denoted by M.
///
/// A prefix is the first words of a virtual page that belong to the last entry of the previous
/// virtual page. This happens because entries may overlap up to 2 virtual pages.
///
/// We have `MIN_NUM_WORDS_PER_PAGE - 3 <= M < Q`.
/// We have [`MIN_PAGE_SIZE`] - 3 ≤ M < Q.
pub fn max_prefix_len(&self) -> Nat {
self.bytes_to_words(self.max_value_len())
}
/// The total virtual capacity in words, denoted by `V`.
/// The total virtual capacity in words, denoted by V.
///
/// We have `V = (N - 1) * (Q - 1) - M`.
/// We have V = (N - 1) × (Q - 1) - M.
///
/// We can show `V >= (N - 2) * (Q - 1)` with the following steps:
/// - `M <= Q - 1` from `M < Q` from [`M`] definition
/// - `-M >= -(Q - 1)` from above
/// - `V >= (N - 1) * (Q - 1) - (Q - 1)` from `V` definition
///
/// [`M`]: struct.Format.html#method.max_prefix_len
/// We can show V ≥ (N - 2) × (Q - 1) with the following steps:
/// - M ≤ Q - 1 from M < Q from [M](Format::max_prefix_len)'s definition
/// - -M ≥ -(Q - 1) from above
/// - V ≥ (N - 1) × (Q - 1) - (Q - 1) from V's definition
pub fn virt_size(&self) -> Nat {
(self.num_pages() - 1) * (self.virt_page_size() - 1) - self.max_prefix_len()
}
/// The total user capacity in words, denoted by `C`.
/// The total user capacity in words, denoted by C.
///
/// We have `C = V - N = (N - 1) * (Q - 2) - M - 1`.
/// We have C = V - N = (N - 1) × (Q - 2) - M - 1.
///
/// We can show `C >= (N - 2) * (Q - 2) - 2` with the following steps:
/// - `V >= (N - 2) * (Q - 1)` from [`V`] definition
/// - `C >= (N - 2) * (Q - 1) - N` from `C` definition
/// - `(N - 2) * (Q - 1) - N = (N - 2) * (Q - 2) - 2` by calculus
///
/// [`V`]: struct.Format.html#method.virt_size
/// We can show C ≥ (N - 2) × (Q - 2) - 2 with the following steps:
/// - V ≥ (N - 2) × (Q - 1) from [V](Format::virt_size)'s definition
/// - C ≥ (N - 2) × (Q - 1) - N from C's definition
/// - (N - 2) × (Q - 1) - N = (N - 2) × (Q - 2) - 2 by calculus
pub fn total_capacity(&self) -> Nat {
// From the virtual capacity, we reserve N - 1 words for `Erase` entries and 1 word for a
// `Clear` entry.
self.virt_size() - self.num_pages()
}
/// The total virtual lifetime in words, denoted by `L`.
/// The total virtual lifetime in words, denoted by L.
///
/// We have `L = (E * N + N - 1) * Q`.
/// We have L = (E × N + N - 1) × Q.
pub fn total_lifetime(&self) -> Position {
Position::new(self, self.max_page_erases(), self.num_pages() - 1, 0)
}
/// Returns the word position of the first entry of a page.
///
/// The init info of the page must be provided to know where the first entry of the page
/// starts.
pub fn page_head(&self, init: InitInfo, page: Nat) -> Position {
Position::new(self, init.cycle, page, init.prefix)
}
@@ -557,7 +549,7 @@ impl Format {
///
/// # Preconditions
///
/// - `bytes + self.word_size()` does not overflow.
/// - `bytes` + [`Self::word_size`] does not overflow.
pub fn bytes_to_words(&self, bytes: Nat) -> Nat {
div_ceil(bytes, self.word_size())
}
@@ -571,7 +563,7 @@ const COMPACT_WORD: Nat = 1;
/// The word index of the content of a page.
///
/// Since a page is at least 8 words, there is always at least 6 words of content.
/// This is also the length in words of the page header.
const CONTENT_WORD: Nat = 2;
/// The checksum for a single word.
@@ -718,21 +710,21 @@ bitfield! {
/// The position of a word in the virtual storage.
///
/// With the notations defined in `Format`, let:
/// - `w` a virtual word offset in a page which is between `0` and `Q - 1`
/// - `p` a page offset which is between `0` and `N - 1`
/// - `c` the number of erase cycles of a page which is between `0` and `E`
/// With the notations defined in [`Format`], let:
/// - w denote a word offset in a virtual page, thus between 0 and Q - 1
/// - p denote a page offset, thus between 0 and N - 1
/// - c denote the number of times a page was erased, thus between 0 and E
///
/// Then the position of a word is `(c*N + p)*Q + w`. This position monotonically increases and
/// The position of a word is (c × N + p) × Q + w. This position monotonically increases and
/// represents the consumed lifetime of the storage.
///
/// This type is kept abstract to avoid possible confusion with `Nat` and `Word` if they happen to
/// have the same representation. Here is an overview of their semantics:
/// This type is kept abstract to avoid possible confusion with [`Nat`] and [`Word`] if they happen
/// to have the same representation. Here is an overview of their semantics:
///
/// | Name | Semantics | Arithmetic operations | Bit-wise operations |
/// | ---------- | --------------------------- | --------------------- | ------------------- |
/// | `Nat` | Natural numbers | Yes (no overflow) | No |
/// | `Word` | Word in flash | No | Yes |
/// | [`Nat`] | Natural numbers | Yes (no overflow) | No |
/// | [`Word`] | Word in flash | No | Yes |
/// | `Position` | Position in virtual storage | Yes (no overflow) | No |
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub struct Position(Nat);
@@ -763,9 +755,9 @@ impl Position {
/// Create a word position given its coordinates.
///
/// The coordinates of a word are:
/// - Its word index in its page.
/// - Its word index in its virtual page.
/// - Its page index in the storage.
/// - The number of times that page was erased.
/// - The number of times its page was erased.
pub fn new(format: &Format, cycle: Nat, page: Nat, word: Nat) -> Position {
Position((cycle * format.num_pages() + page) * format.virt_page_size() + word)
}
@@ -928,11 +920,11 @@ pub fn is_erased(slice: &[u8]) -> bool {
/// Divides then takes ceiling.
///
/// Returns `ceil(x / m)` in mathematical notations (not Rust code).
/// Returns ⌈x / m⌉, i.e. the lowest natural number r such that r ≥ x / m.
///
/// # Preconditions
///
/// - `x + m` does not overflow.
/// - x + m does not overflow.
const fn div_ceil(x: Nat, m: Nat) -> Nat {
(x + m - 1) / m
}

4
libraries/persistent_store/src/fragment.rs
View File

@@ -12,7 +12,7 @@
// See the License for the specific language governing permissions and
// limitations under the License.
//! Helper functions for fragmented entries.
//! Support for fragmented entries.
//!
//! This module permits to handle entries larger than the [maximum value
//! length](Store::max_value_length) by storing ordered consecutive fragments in a sequence of keys.
@@ -36,7 +36,7 @@ pub trait Keys {
///
/// # Preconditions
///
/// The position must be within the length: `pos < len()`.
/// The position must be within the length: `pos` < [`Self::len`].
fn key(&self, pos: usize) -> usize;
}

467
libraries/persistent_store/src/lib.rs
View File

@@ -12,191 +12,191 @@
// See the License for the specific language governing permissions and
// limitations under the License.
// TODO(ia0): Add links once the code is complete.
// The documentation is easier to read from a browser:
// - Run: cargo doc --document-private-items --features=std
// - Open: target/doc/persistent_store/index.html
//! Store abstraction for flash storage
//!
//! # Specification
//!
//! The store provides a partial function from keys to values on top of a storage
//! interface. The store total capacity depends on the size of the storage. Store
//! updates may be bundled in transactions. Mutable operations are atomic, including
//! when interrupted.
//! The [store](Store) provides a partial function from keys to values on top of a
//! [storage](Storage) interface. The store total [capacity](Store::capacity) depends on the size of
//! the storage. Store [updates](StoreUpdate) may be bundled in [transactions](Store::transaction).
//! Mutable operations are atomic, including when interrupted.
//!
//! The store is flash-efficient in the sense that it uses the storage lifetime
//! efficiently. For each page, all words are written at least once between erase
//! cycles and all erase cycles are used. However, not all written words are user
//! content: lifetime is also consumed with metadata and compaction.
//! The store is flash-efficient in the sense that it uses the storage [lifetime](Store::lifetime)
//! efficiently. For each page, all words are written at least once between erase cycles and all
//! erase cycles are used. However, not all written words are user content: Lifetime is also
//! consumed with metadata and compaction.
//!
//! The store is extendable with other entries than key-values. It is essentially a
//! framework providing access to the storage lifetime. The partial function is
//! simply the most common usage and can be used to encode other usages.
//! The store is extendable with other entries than key-values. It is essentially a framework
//! providing access to the storage lifetime. The partial function is simply the most common usage
//! and can be used to encode other usages.
//!
//! ## Definitions
//!
//! An _entry_ is a pair of a key and a value. A _key_ is a number between 0
//! and 4095. A _value_ is a byte slice with a length between 0 and 1023 bytes (for
//! large enough pages).
//! An _entry_ is a pair of a key and a value. A _key_ is a number between 0 and
//! [4095](format::MAX_KEY_INDEX). A _value_ is a byte slice with a length between 0 and
//! [1023](format::Format::max_value_len) bytes (for large enough pages).
//!
//! The store provides the following _updates_:
//! - Given a key and a value, `Insert` updates the store such that the value is
//! - Given a key and a value, [`StoreUpdate::Insert`] updates the store such that the value is
//! associated with the key. The values for other keys are left unchanged.
//! - Given a key, `Remove` updates the store such that no value is associated with
//! the key. The values for other keys are left unchanged. Additionally, if there
//! was a value associated with the key, the value is wiped from the storage
//! (all its bits are set to 0).
//! - Given a key, [`StoreUpdate::Remove`] updates the store such that no value is associated with
//! the key. The values for other keys are left unchanged. Additionally, if there was a value
//! associated with the key, the value is wiped from the storage (all its bits are set to 0).
//!
//! The store provides the following _read-only operations_:
//! - `Iter` iterates through the store returning all entries exactly once. The
//! iteration order is not specified but stable between mutable operations.
//! - `Capacity` returns how many words can be stored before the store is full.
//! - `Lifetime` returns how many words can be written before the storage lifetime
//! is consumed.
//! - [`Store::iter`] iterates through the store returning all entries exactly once. The iteration
//! order is not specified but stable between mutable operations.
//! - [`Store::capacity`] returns how many words can be stored before the store is full.
//! - [`Store::lifetime`] returns how many words can be written before the storage lifetime is
//! consumed.
//!
//! The store provides the following _mutable operations_:
//! - Given a set of independent updates, `Transaction` applies the sequence of
//! updates.
//! - Given a threshold, `Clear` removes all entries with a key greater or equal
//! to the threshold.
//! - Given a length in words, `Prepare` makes one step of compaction unless that
//! many words can be written without compaction. This operation has no effect
//! on the store but may still mutate its storage. In particular, the store has
//! the same capacity but a possibly reduced lifetime.
//! - Given a set of independent updates, [`Store::transaction`] applies the sequence of updates.
//! - Given a threshold, [`Store::clear`] removes all entries with a key greater or equal to the
//! threshold.
//! - Given a length in words, [`Store::prepare`] makes one step of compaction unless that many
//! words can be written without compaction. This operation has no effect on the store but may
//! still mutate its storage. In particular, the store has the same capacity but a possibly
//! reduced lifetime.
//!
//! A mutable operation is _atomic_ if, when power is lost during the operation, the
//! store is either updated (as if the operation succeeded) or left unchanged (as if
//! the operation did not occur). If the store is left unchanged, lifetime may still
//! be consumed.
//! A mutable operation is _atomic_ if, when power is lost during the operation, the store is either
//! updated (as if the operation succeeded) or left unchanged (as if the operation did not occur).
//! If the store is left unchanged, lifetime may still be consumed.
//!
//! The store relies on the following _storage interface_:
//! - It is possible to read a byte slice. The slice won't span multiple pages.
//! - It is possible to write a word slice. The slice won't span multiple pages.
//! - It is possible to erase a page.
//! - The pages are sequentially indexed from 0. If the actual underlying storage
//! is segmented, then the storage layer should translate those indices to
//! actual page addresses.
//! - It is possible to [read](Storage::read_slice) a byte slice. The slice won't span multiple
//! pages.
//! - It is possible to [write](Storage::write_slice) a word slice. The slice won't span multiple
//! pages.
//! - It is possible to [erase](Storage::erase_page) a page.
//! - The pages are sequentially indexed from 0. If the actual underlying storage is segmented,
//! then the storage layer should translate those indices to actual page addresses.
//!
//! The store has a _total capacity_ of `C = (N - 1) * (P - 4) - M - 1` words, where
//! `P` is the number of words per page, `N` is the number of pages, and `M` is the
//! maximum length in words of a value (256 for large enough pages). The capacity
//! used by each mutable operation is given below (a transient word only uses
//! capacity during the operation):
//! - `Insert` uses `1 + ceil(len / 4)` words where `len` is the length of the
//! value in bytes. If an entry was replaced, the words used by its insertion
//! are freed.
//! - `Remove` doesn't use capacity if alone in the transaction and 1 transient
//! word otherwise. If an entry was deleted, the words used by its insertion are
//! freed.
//! - `Transaction` uses 1 transient word. In addition, the updates of the
//! transaction use and free words as described above.
//! - `Clear` doesn't use capacity and frees the words used by the insertion of
//! the deleted entries.
//! - `Prepare` doesn't use capacity.
//! The store has a _total capacity_ of C = (N - 1) × (P - 4) - M - 1 words, where:
//! - P is the number of words per page
//! - [N](format::Format::num_pages) is the number of pages
//! - [M](format::Format::max_prefix_len) is the maximum length in words of a value (256 for large
//! enough pages)
//!
//! The _total lifetime_ of the store is below `L = ((E + 1) * N - 1) * (P - 2)` and
//! above `L - M` words, where `E` is the maximum number of erase cycles. The
//! lifetime is used when capacity is used, including transiently, as well as when
//! compaction occurs. Compaction frequency and lifetime consumption are positively
//! correlated to the store load factor (the ratio of used capacity to total capacity).
//! The capacity used by each mutable operation is given below (a transient word only uses capacity
//! during the operation):
//!
//! It is possible to approximate the cost of transient words in terms of capacity:
//! `L` transient words are equivalent to `C - x` words of capacity where `x` is the
//! average capacity (including transient) of operations.
//! | Operation/Update | Used capacity | Freed capacity | Transient capacity |
//! | ----------------------- | ---------------- | ----------------- | ------------------ |
//! | [`StoreUpdate::Insert`] | 1 + value length | overwritten entry | 0 |
//! | [`StoreUpdate::Remove`] | 0 | deleted entry | see below\* |
//! | [`Store::transaction`] | 0 + updates | 0 + updates | 1 |
//! | [`Store::clear`] | 0 | deleted entries | 0 |
//! | [`Store::prepare`] | 0 | 0 | 0 |
//!
//! \*0 if the update is alone in the transaction, otherwise 1.
//!
//! The _total lifetime_ of the store is below L = ((E + 1) × N - 1) × (P - 2) and above L - M
//! words, where E is the maximum number of erase cycles. The lifetime is used when capacity is
//! used, including transiently, as well as when compaction occurs. Compaction frequency and
//! lifetime consumption are positively correlated to the store load factor (the ratio of used
//! capacity to total capacity).
//!
//! It is possible to approximate the cost of transient words in terms of capacity: L transient
//! words are equivalent to C - x words of capacity where x is the average capacity (including
//! transient) of operations.
//!
//! ## Preconditions
//!
//! The following assumptions need to hold, or the store may behave in unexpected ways:
//! - A word can be written twice between erase cycles.
//! - A page can be erased `E` times after the first boot of the store.
//! - When power is lost while writing a slice or erasing a page, the next read
//! returns a slice where a subset (possibly none or all) of the bits that
//! should have been modified have been modified.
//! - Reading a slice is deterministic. When power is lost while writing a slice
//! or erasing a slice (erasing a page containing that slice), reading that
//! slice repeatedly returns the same result (until it is overwritten or its
//! page is erased).
//! - To decide whether a page has been erased, it is enough to test if all its
//! bits are equal to 1.
//! - When power is lost while writing a slice or erasing a page, that operation
//! does not count towards the limits. However, completing that write or erase
//! operation would count towards the limits, as if the number of writes per
//! word and number of erase cycles could be fractional.
//! - The storage is only modified by the store. Note that completely erasing the
//! storage is supported, essentially losing all content and lifetime tracking.
//! It is preferred to use `Clear` with a threshold of 0 to keep the lifetime
//! tracking.
//! - A word can be written [twice](Storage::max_word_writes) between erase cycles.
//! - A page can be erased [E](Storage::max_page_erases) times after the first boot of the store.
//! - When power is lost while writing a slice or erasing a page, the next read returns a slice
//! where a subset (possibly none or all) of the bits that should have been modified have been
//! modified.
//! - Reading a slice is deterministic. When power is lost while writing a slice or erasing a
//! slice (erasing a page containing that slice), reading that slice repeatedly returns the same
//! result (until it is overwritten or its page is erased).
//! - To decide whether a page has been erased, it is enough to test if all its bits are equal
//! to 1.
//! - When power is lost while writing a slice or erasing a page, that operation does not count
//! towards the limits. However, completing that write or erase operation would count towards
//! the limits, as if the number of writes per word and number of erase cycles could be
//! fractional.
//! - The storage is only modified by the store. Note that completely erasing the storage is
//! supported, essentially losing all content and lifetime tracking. It is preferred to use
//! [`Store::clear`] with a threshold of 0 to keep the lifetime tracking.
//!
//! The store properties may still hold outside some of those assumptions, but with
//! an increasing chance of failure.
//! The store properties may still hold outside some of those assumptions, but with an increasing
//! chance of failure.
//!
//! # Implementation
//!
//! We define the following constants:
//! - `E < 65536` the number of times a page can be erased.
//! - `3 <= N < 64` the number of pages in the storage.
//! - `8 <= P <= 1024` the number of words in a page.
//! - `Q = P - 2` the number of words in a virtual page.
//! - `K = 4096` the maximum number of keys.
//! - `M = min(Q - 1, 256)` the maximum length in words of a value.
//! - `V = (N - 1) * (Q - 1) - M` the virtual capacity.
//! - `C = V - N` the user capacity.
//! - [E](format::Format::max_page_erases) ≤ [65535](format::MAX_ERASE_CYCLE) the number of times
//! a page can be erased.
//! - 3 ≤ [N](format::Format::num_pages) < 64 the number of pages in the storage.
//! - 8 ≤ P ≤ 1024 the number of words in a page.
//! - [Q](format::Format::virt_page_size) = P - 2 the number of words in a virtual page.
//! - [M](format::Format::max_prefix_len) = min(Q - 1, 256) the maximum length in words of a
//! value.
//! - [V](format::Format::virt_size) = (N - 1) × (Q - 1) - M the virtual capacity.
//! - [C](format::Format::total_capacity) = V - N the user capacity.
//!
//! We build a virtual storage from the physical storage using the first 2 words of
//! each page:
//! We build a virtual storage from the physical storage using the first 2 words of each page:
//! - The first word contains the number of times the page has been erased.
//! - The second word contains the starting word to which this page is being moved
//! during compaction.
//! - The second word contains the starting word to which this page is being moved during
//! compaction.
//!
//! The virtual storage has a length of `(E + 1) * N * Q` words and represents the
//! lifetime of the store. (We reserve the last `Q + M` words to support adding
//! emergency lifetime.) This virtual storage has a linear address space.
//! The virtual storage has a length of (E + 1) × N × Q words and represents the lifetime of the
//! store. (We reserve the last Q + M words to support adding emergency lifetime.) This virtual
//! storage has a linear address space.
//!
//! We define a set of overlapping windows of `N * Q` words at each `Q`-aligned
//! boundary. We call `i` the window spanning from `i * Q` to `(i + N) * Q`. Only
//! those windows actually exist in the underlying storage. We use compaction to
//! shift the current window from `i` to `i + 1`, preserving the content of the
//! store.
//! We define a set of overlapping windows of N × Q words at each Q-aligned boundary. We call i the
//! window spanning from i × Q to (i + N) × Q. Only those windows actually exist in the underlying
//! storage. We use compaction to shift the current window from i to i + 1, preserving the content
//! of the store.
//!
//! For a given state of the virtual storage, we define `h_i` as the position of the
//! first entry of the window `i`. We call it the head of the window `i`. Because
//! entries are at most `M + 1` words, they can overlap on the next page only by `M`
//! words. So we have `i * Q <= h_i <= i * Q + M` . Since there are no entries
//! before the first page, we have `h_0 = 0`.
//! For a given state of the virtual storage, we define h\_i as the position of the first entry of
//! the window i. We call it the head of the window i. Because entries are at most M + 1 words, they
//! can overlap on the next page only by M words. So we have i × Q ≤ h_i ≤ i × Q + M . Since there
//! are no entries before the first page, we have h\_0 = 0.
//!
//! We define `t_i` as one past the last entry of the window `i`. If there are no
//! entries in that window, we have `t_i = h_i`. We call `t_i` the tail of the
//! window `i`. We define the compaction invariant as `t_i - h_i <= V`.
//! We define t\_i as one past the last entry of the window i. If there are no entries in that
//! window, we have t\_i = h\_i. We call t\_i the tail of the window i. We define the compaction
//! invariant as t\_i - h\_i ≤ V.
//!
//! We define `|x|` as the capacity used before position `x`. We have `|x| <= x`. We
//! define the capacity invariant as `|t_i| - |h_i| <= C`.
//! We define |x| as the capacity used before position x. We have |x| ≤ x. We define the capacity
//! invariant as |t\_i| - |h\_i| ≤ C.
//!
//! Using this virtual storage, entries are appended to the tail as long as there is
//! both virtual capacity to preserve the compaction invariant and capacity to
//! preserve the capacity invariant. When virtual capacity runs out, the first page
//! of the window is compacted and the window is shifted.
//! Using this virtual storage, entries are appended to the tail as long as there is both virtual
//! capacity to preserve the compaction invariant and capacity to preserve the capacity invariant.
//! When virtual capacity runs out, the first page of the window is compacted and the window is
//! shifted.
//!
//! Entries are identified by a prefix of bits. The prefix has to contain at least
//! one bit set to zero to differentiate from the tail. Entries can be one of:
//! - Padding: A word whose first bit is set to zero. The rest is arbitrary. This
//! entry is used to mark words partially written after an interrupted operation
//! as padding such that they are ignored by future operations.
//! - Header: A word whose second bit is set to zero. It contains the following fields:
//! - A bit indicating whether the entry is deleted.
//! - A bit indicating whether the value is word-aligned and has all bits set
//! to 1 in its last word. The last word of an entry is used to detect that
//! an entry has been fully written. As such it must contain at least one
//! bit equal to zero.
//! - The key of the entry.
//! - The length in bytes of the value. The value follows the header. The
//! entry is word-aligned if the value is not.
//! - The checksum of the first and last word of the entry.
//! - Erase: A word used during compaction. It contains the page to be erased and
//! a checksum.
//! - Clear: A word used during the `Clear` operation. It contains the threshold
//! and a checksum.
//! - Marker: A word used during the `Transaction` operation. It contains the
//! number of updates following the marker and a checksum.
//! - Remove: A word used during the `Transaction` operation. It contains the key
//! of the entry to be removed and a checksum.
//! Entries are identified by a prefix of bits. The prefix has to contain at least one bit set to
//! zero to differentiate from the tail. Entries can be one of:
//! - [Padding](format::ID_PADDING): A word whose first bit is set to zero. The rest is arbitrary.
//! This entry is used to mark words partially written after an interrupted operation as padding
//! such that they are ignored by future operations.
//! - [Header](format::ID_HEADER): A word whose second bit is set to zero. It contains the
//! following fields:
//! - A [bit](format::HEADER_DELETED) indicating whether the entry is deleted.
//! - A [bit](format::HEADER_FLIPPED) indicating whether the value is word-aligned and has all
//! bits set to 1 in its last word. The last word of an entry is used to detect that an
//! entry has been fully written. As such it must contain at least one bit equal to zero.
//! - The [key](format::HEADER_KEY) of the entry.
//! - The [length](format::HEADER_LENGTH) in bytes of the value. The value follows the header.
//! The entry is word-aligned if the value is not.
//! - The [checksum](format::HEADER_CHECKSUM) of the first and last word of the entry.
//! - [Erase](format::ID_ERASE): A word used during compaction. It contains the
//! [page](format::ERASE_PAGE) to be erased and a [checksum](format::WORD_CHECKSUM).
//! - [Clear](format::ID_CLEAR): A word used during the clear operation. It contains the
//! [threshold](format::CLEAR_MIN_KEY) and a [checksum](format::WORD_CHECKSUM).
//! - [Marker](format::ID_MARKER): A word used during a transaction. It contains the [number of
//! updates](format::MARKER_COUNT) following the marker and a [checksum](format::WORD_CHECKSUM).
//! - [Remove](format::ID_REMOVE): A word used inside a transaction. It contains the
//! [key](format::REMOVE_KEY) of the entry to be removed and a
//! [checksum](format::WORD_CHECKSUM).
//!
//! Checksums are the number of bits equal to 0.
//!
@@ -204,107 +204,105 @@
//!
//! ## Compaction
//!
//! It should always be possible to fully compact the store, after what the
//! remaining capacity should be available in the current window (restoring the
//! compaction invariant). We consider all notations on the virtual storage after
//! the full compaction. We will use the `|x|` notation although we update the state
//! of the virtual storage. This is fine because compaction doesn't change the
//! status of an existing word.
//! It should always be possible to fully compact the store, after what the remaining capacity
//! should be available in the current window (restoring the compaction invariant). We consider all
//! notations on the virtual storage after the full compaction. We will use the |x| notation
//! although we update the state of the virtual storage. This is fine because compaction doesn't
//! change the status of an existing word.
//!
//! We want to show that the next `N - 1` compactions won't move the tail past the
//! last page of their window, with `I` the initial window:
//! We want to show that the next N - 1 compactions won't move the tail past the last page of their
//! window, with I the initial window:
//!
//! ```text
//! forall 1 <= i <= N - 1, t_{I + i} <= (I + i + N - 1) * Q
//! ```
//! | | | | |
//! | ----------------:| ----------:|:-:|:------------------- |
//! | ∀(1 ≤ i ≤ N - 1) | t\_{I + i} | ≤ | (I + i + N - 1) × Q |
//!
//! We assume `i` between `1` and `N - 1`.
//! We assume i between 1 and N - 1.
//!
//! One step of compaction advances the tail by how many words were used in the
//! first page of the window with the last entry possibly overlapping on the next
//! page.
//! One step of compaction advances the tail by how many words were used in the first page of the
//! window with the last entry possibly overlapping on the next page.
//!
//! ```text
//! forall j, t_{j + 1} = t_j + |h_{j + 1}| - |h_j| + 1
//! ```
//! | | | | |
//! | --:| ----------:|:-:|:------------------------------------ |
//! | ∀j | t\_{j + 1} | = | t\_j + \|h\_{j + 1}\| - \|h\_j\| + 1 |
//!
//! By induction, we have:
//!
//! ```text
//! t_{I + i} <= t_I + |h_{I + i}| - |h_I| + i
//! ```
//! | | | |
//! | ----------:|:-:|:------------------------------------ |
//! | t\_{I + i} | ≤ | t\_I + \|h\_{I + i}\| - \|h\_I\| + i |
//!
//! We have the following properties:
//!
//! ```text
//! t_I <= h_I + V
//! |h_{I + i}| - |h_I| <= h_{I + i} - h_I
//! h_{I + i} <= (I + i) * Q + M
//! ```
//! | | | |
//! | -------------------------:|:-:|:----------------- |
//! | t\_I | ≤ | h\_I + V |
//! | \|h\_{I + i}\| - \|h\_I\| | ≤ | h\_{I + i} - h\_I |
//! | h\_{I + i} | ≤ | (I + i) × Q + M |
//!
//! Replacing into our previous equality, we can conclude:
//!
//! ```text
//! t_{I + i} = t_I + |h_{I + i}| - |h_I| + i
//! <= h_I + V + (I + i) * Q + M - h_I + i
//! = (N - 1) * (Q - 1) - M + (I + i) * Q + M + i
//! = (N - 1) * (Q - 1) + (I + i) * Q + i
//! = (I + i + N - 1) * Q + i - (N - 1)
//! <= (I + i + N - 1) * Q
//! ```
//! | | | |
//! | ----------:|:-:| ------------------------------------------- |
//! | t\_{I + i} | = | t_I + \|h_{I + i}\| - \|h_I\| + i |
//! | | ≤ | h\_I + V + (I + i) * Q + M - h\_I + i |
//! | | = | (N - 1) × (Q - 1) - M + (I + i) × Q + M + i |
//! | | = | (N - 1) × (Q - 1) + (I + i) × Q + i |
//! | | = | (I + i + N - 1) × Q + i - (N - 1) |
//! | | ≤ | (I + i + N - 1) × Q |
//!
//! We also want to show that after `N - 1` compactions, the remaining capacity is
//! available without compaction.
//! We also want to show that after N - 1 compactions, the remaining capacity is available without
//! compaction.
//!
//! ```text
//! V - (t_{I + N - 1} - h_{I + N - 1}) >= // The available words in the window.
//! C - (|t_{I + N - 1}| - |h_{I + N - 1}|) // The remaining capacity.
//! + 1 // Reserved for Clear.
//! ```
//! | | | |
//! | -:| --------------------------------------------- | --------------------------------- |
//! | | V - (t\_{I + N - 1} - h\_{I + N - 1}) | The available words in the window |
//! | ≥ | C - (\|t\_{I + N - 1}\| - \|h\_{I + N - 1}\|) | The remaining capacity |
//! | + | 1 | Reserved for clear |
//!
//! We can replace the definition of `C` and simplify:
//! We can replace the definition of C and simplify:
//!
//! ```text
//! V - (t_{I + N - 1} - h_{I + N - 1}) >= V - N - (|t_{I + N - 1}| - |h_{I + N - 1}|) + 1
//! iff t_{I + N - 1} - h_{I + N - 1} <= |t_{I + N - 1}| - |h_{I + N - 1}| + N - 1
//! ```
//! | | | | |
//! | ---:| -------------------------------------:|:-:|:----------------------------------------------------- |
//! | | V - (t\_{I + N - 1} - h\_{I + N - 1}) | ≥ | V - N - (\|t\_{I + N - 1}\| - \|h\_{I + N - 1}\|) + 1 |
//! | iff | t\_{I + N - 1} - h\_{I + N - 1} | ≤ | \|t\_{I + N - 1}\| - \|h\_{I + N - 1}\| + N - 1 |
//!
//! We have the following properties:
//!
//! ```text
//! t_{I + N - 1} = t_I + |h_{I + N - 1}| - |h_I| + N - 1
//! |t_{I + N - 1}| - |h_{I + N - 1}| = |t_I| - |h_I| // Compaction preserves capacity.
//! |h_{I + N - 1}| - |t_I| <= h_{I + N - 1} - t_I
//! ```
//!
//! | | | | |
//! | ---------------------------------------:|:-:|:-------------------------------------------- |:------ |
//! | t\_{I + N - 1} | = | t\_I + \|h\_{I + N - 1}\| - \|h\_I\| + N - 1 | |
//! | \|t\_{I + N - 1}\| - \|h\_{I + N - 1}\| | = | \|t\_I\| - \|h\_I\| | Compaction preserves capacity |
//! | \|h\_{I + N - 1}\| - \|t\_I\| | ≤ | h\_{I + N - 1} - t\_I | |
//!
//! From which we conclude:
//!
//! ```text
//! t_{I + N - 1} - h_{I + N - 1} <= |t_{I + N - 1}| - |h_{I + N - 1}| + N - 1
//! iff t_I + |h_{I + N - 1}| - |h_I| + N - 1 - h_{I + N - 1} <= |t_I| - |h_I| + N - 1
//! iff t_I + |h_{I + N - 1}| - h_{I + N - 1} <= |t_I|
//! iff |h_{I + N - 1}| - |t_I| <= h_{I + N - 1} - t_I
//! ```
//! | | | | |
//! | ---:| -------------------------------:|:-:|:----------------------------------------------- |
//! | | t\_{I + N - 1} - h\_{I + N - 1} | ≤ | \|t\_{I + N - 1}\| - \|h\_{I + N - 1}\| + N - 1 |
//! | iff | t\_I + \|h\_{I + N - 1}\| - \|h\_I\| + N - 1 - h\_{I + N - 1} | ≤ | \|t\_I\| - \|h\_I\| + N - 1 |
//! | iff | t\_I + \|h\_{I + N - 1}\| - h\_{I + N - 1} | ≤ | \|t\_I\| |
//! | iff | \|h\_{I + N - 1}\| - \|t\_I\| | ≤ | h\_{I + N - 1} - t\_I |
//!
//!
//! ## Checksum
//!
//! The main property we want is that all partially written/erased words are either
//! the initial word, the final word, or invalid.
//! The main property we want is that all partially written/erased words are either the initial
//! word, the final word, or invalid.
//!
//! We say that a bit sequence `TARGET` is reachable from a bit sequence `SOURCE` if
//! both have the same length and `SOURCE & TARGET == TARGET` where `&` is the
//! bitwise AND operation on bit sequences of that length. In other words, when
//! `SOURCE` has a bit equal to 0 then `TARGET` also has that bit equal to 0.
//! We say that a bit sequence `TARGET` is reachable from a bit sequence `SOURCE` if both have the
//! same length and `SOURCE & TARGET == TARGET` where `&` is the bitwise AND operation on bit
//! sequences of that length. In other words, when `SOURCE` has a bit equal to 0 then `TARGET` also
//! has that bit equal to 0.
//!
//! The only written entries start with `101` or `110` and are written from an
//! erased word. Marking an entry as padding or deleted is a single bit operation,
//! so the property trivially holds. For those cases, the proof relies on the fact
//! that there is exactly one bit equal to 0 in the 3 first bits. Either the 3 first
//! bits are still `111` in which case we expect the remaining bits to be equal
//! to 1. Otherwise we can use the checksum of the given type of entry because those
//! 2 types of entries are not reachable from each other. Here is a visualization of
//! the partitioning based on the first 3 bits:
//! The only written entries start with `101` or `110` and are written from an erased word. Marking
//! an entry as padding or deleted is a single bit operation, so the property trivially holds. For
//! those cases, the proof relies on the fact that there is exactly one bit equal to 0 in the 3
//! first bits. Either the 3 first bits are still `111` in which case we expect the remaining bits
//! to be equal to 1. Otherwise we can use the checksum of the given type of entry because those 2
//! types of entries are not reachable from each other. Here is a visualization of the partitioning
//! based on the first 3 bits:
//!
//! | First 3 bits | Description | How to check |
//! | ------------:| ------------------ | ---------------------------- |
@@ -314,34 +312,27 @@
//! | `100` | Deleted user entry | No check, atomically written |
//! | `0??` | Padding entry | No check, atomically written |
//!
//! To show that valid entries of a given type are not reachable from each other, we
//! show 3 lemmas:
//! To show that valid entries of a given type are not reachable from each other, we show 3 lemmas:
//!
//! 1. A bit sequence is not reachable from another if its number of bits equal to
//! 0 is smaller.
//! 1. A bit sequence is not reachable from another if its number of bits equal to 0 is smaller.
//! 2. A bit sequence is not reachable from another if they have the same number of bits equals to
//! 0 and are different.
//! 3. A bit sequence is not reachable from another if it is bigger when they are interpreted as
//! numbers in binary representation.
//!
//! 2. A bit sequence is not reachable from another if they have the same number of
//! bits equals to 0 and are different.
//!
//! 3. A bit sequence is not reachable from another if it is bigger when they are
//! interpreted as numbers in binary representation.
//!
//! From those lemmas we consider the 2 cases. If both entries have the same number
//! of bits equal to 0, they are either equal or not reachable from each other
//! because of the second lemma. If they don't have the same number of bits equal to
//! 0, then the one with less bits equal to 0 is not reachable from the other
//! because of the first lemma and the one with more bits equal to 0 is not
//! reachable from the other because of the third lemma and the definition of the
//! checksum.
//! From those lemmas we consider the 2 cases. If both entries have the same number of bits equal to
//! 0, they are either equal or not reachable from each other because of the second lemma. If they
//! don't have the same number of bits equal to 0, then the one with less bits equal to 0 is not
//! reachable from the other because of the first lemma and the one with more bits equal to 0 is not
//! reachable from the other because of the third lemma and the definition of the checksum.
//!
//! # Fuzzing
//!
//! For any sequence of operations and interruptions starting from an erased
//! storage, the store is checked against its model and some internal invariant at
//! each step.
//! For any sequence of operations and interruptions starting from an erased storage, the store is
//! checked against its model and some internal invariant at each step.
//!
//! For any sequence of operations and interruptions starting from an arbitrary
//! storage, the store is checked not to crash.
//! For any sequence of operations and interruptions starting from an arbitrary storage, the store
//! is checked not to crash.
#![cfg_attr(not(feature = "std"), no_std)]
#![feature(try_trait)]

5
libraries/persistent_store/src/model.rs
View File

@@ -12,13 +12,16 @@
// See the License for the specific language governing permissions and
// limitations under the License.
//! Store specification.
use crate::format::Format;
use crate::{usize_to_nat, StoreError, StoreRatio, StoreResult, StoreUpdate};
use std::collections::HashMap;
/// Models the mutable operations of a store.
///
/// The model doesn't model the storage and read-only operations. This is done by the driver.
/// The model doesn't model the storage and read-only operations. This is done by the
/// [driver](crate::StoreDriver).
#[derive(Clone, Debug)]
pub struct StoreModel {
/// Represents the content of the store.

8
libraries/persistent_store/src/storage.rs
View File

@@ -12,6 +12,8 @@
// See the License for the specific language governing permissions and
// limitations under the License.
//! Flash storage abstraction.
/// Represents a byte position in a storage.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub struct StorageIndex {
@@ -65,12 +67,14 @@ pub trait Storage {
/// The following pre-conditions must hold:
/// - The `index` must designate `value.len()` bytes in the storage.
/// - Both `index` and `value.len()` must be word-aligned.
/// - The written words should not have been written too many times since last page erasure.
/// - The written words should not have been written [too many](Self::max_word_writes) times
/// since the last page erasure.
fn write_slice(&mut self, index: StorageIndex, value: &[u8]) -> StorageResult<()>;
/// Erases a page of the storage.
///
/// The `page` must be in the storage.
/// The `page` must be in the storage, i.e. less than [`Storage::num_pages`]. And the page
/// should not have been erased [too many](Self::max_page_erases) times.
fn erase_page(&mut self, page: usize) -> StorageResult<()>;
}

43
libraries/persistent_store/src/store.rs
View File

@@ -12,6 +12,8 @@
// See the License for the specific language governing permissions and
// limitations under the License.
//! Store implementation.
use crate::format::{
is_erased, CompactInfo, Format, Header, InitInfo, InternalEntry, Padding, ParsedWord, Position,
Word, WordState,
@@ -55,17 +57,14 @@ pub enum StoreError {
///
/// The consequences depend on the storage failure. In particular, the operation may or may not
/// have succeeded, and the storage may have become invalid. Before doing any other operation,
/// the store should be [recovered]. The operation may then be retried if idempotent.
///
/// [recovered]: struct.Store.html#method.recover
/// the store should be [recovered](Store::recover). The operation may then be retried if
/// idempotent.
StorageError,
/// Storage is invalid.
///
/// The storage should be erased and the store [recovered]. The store would be empty and have
/// lost track of lifetime.
///
/// [recovered]: struct.Store.html#method.recover
/// The storage should be erased and the store [recovered](Store::recover). The store would be
/// empty and have lost track of lifetime.
InvalidStorage,
}
@@ -92,14 +91,12 @@ pub type StoreResult<T> = Result<T, StoreError>;
/// Progression ratio for store metrics.
///
/// This is used for the [capacity] and [lifetime] metrics. Those metrics are measured in words.
/// This is used for the [`Store::capacity`] and [`Store::lifetime`] metrics. Those metrics are
/// measured in words.
///
/// # Invariant
///
/// - The used value does not exceed the total: `used <= total`.
///
/// [capacity]: struct.Store.html#method.capacity
/// [lifetime]: struct.Store.html#method.lifetime
/// - The used value does not exceed the total: `used` ≤ `total`.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub struct StoreRatio {
/// How much of the metric is used.
@@ -148,11 +145,11 @@ impl StoreHandle {
self.key as usize
}
/// Returns the length of value of the entry.
/// Returns the value length of the entry.
///
/// # Errors
///
/// Returns `InvalidArgument` if the entry has been deleted or compacted.
/// Returns [`StoreError::InvalidArgument`] if the entry has been deleted or compacted.
pub fn get_length<S: Storage>(&self, store: &Store<S>) -> StoreResult<usize> {
store.get_length(self)
}
@@ -161,7 +158,7 @@ impl StoreHandle {
///
/// # Errors
///
/// Returns `InvalidArgument` if the entry has been deleted or compacted.
/// Returns [`StoreError::InvalidArgument`] if the entry has been deleted or compacted.
pub fn get_value<S: Storage>(&self, store: &Store<S>) -> StoreResult<Vec<u8>> {
store.get_value(self)
}
@@ -211,7 +208,7 @@ pub struct Store<S: Storage> {
/// The list of the position of the user entries.
///
/// The position is encoded as the word offset from the [head](Store#structfield.head).
/// The position is encoded as the word offset from the [head](Store::head).
entries: Option<Vec<u16>>,
}
@@ -224,7 +221,8 @@ impl<S: Storage> Store<S> {
///
/// # Errors
///
/// Returns `InvalidArgument` if the storage is not supported.
/// Returns [`StoreError::InvalidArgument`] if the storage is not
/// [supported](Format::is_storage_supported).
pub fn new(storage: S) -> Result<Store<S>, (StoreError, S)> {
let format = match Format::new(&storage) {
None => return Err((StoreError::InvalidArgument, storage)),
@@ -258,7 +256,7 @@ impl<S: Storage> Store<S> {
)))
}
/// Returns the current capacity in words.
/// Returns the current and total capacity in words.
///
/// The capacity represents the size of what is stored.
pub fn capacity(&self) -> StoreResult<StoreRatio> {
@@ -271,7 +269,7 @@ impl<S: Storage> Store<S> {
Ok(StoreRatio { used, total })
}
/// Returns the current lifetime in words.
/// Returns the current and total lifetime in words.
///
/// The lifetime represents the age of the storage. The limit is an over-approximation by at
/// most the maximum length of a value (the actual limit depends on the length of the prefix of
@@ -286,10 +284,11 @@ impl<S: Storage> Store<S> {
///
/// # Errors
///
/// Returns `InvalidArgument` in the following circumstances:
/// - There are too many updates.
/// Returns [`StoreError::InvalidArgument`] in the following circumstances:
/// - There are [too many](Format::max_updates) updates.
/// - The updates overlap, i.e. their keys are not disjoint.
/// - The updates are invalid, e.g. key out of bound or value too long.
/// - The updates are invalid, e.g. key [out of bound](Format::max_key) or value [too
/// long](Format::max_value_len).
pub fn transaction<ByteSlice: Borrow<[u8]>>(
&mut self,
updates: &[StoreUpdate<ByteSlice>],