rust-revlog: don't try to open the data file if the index is empty

This will cover the case where the data file is not present.

rust/hg-core/src/revlog/mod.rs

@@ -1,1419 +1,1422
 // Copyright 2018-2023 Georges Racinet <[e-mail]>
 //           and Mercurial contributors
 //
 // This software may be used and distributed according to the terms of the
 // GNU General Public License version 2 or any later version.
 //! Mercurial concepts for handling revision history
 
 pub mod node;
 pub mod nodemap;
 mod nodemap_docket;
 pub mod path_encode;
 pub use node::{FromHexError, Node, NodePrefix};
 pub mod changelog;
 pub mod filelog;
 pub mod index;
 pub mod manifest;
 pub mod patch;
 
 use std::borrow::Cow;
 use std::collections::HashSet;
 use std::io::Read;
 use std::ops::Deref;
 use std::path::Path;
 
 use flate2::read::ZlibDecoder;
 use sha1::{Digest, Sha1};
 use std::cell::RefCell;
 use zstd;
 
 use self::node::{NODE_BYTES_LENGTH, NULL_NODE};
 use self::nodemap_docket::NodeMapDocket;
 use super::index::Index;
 use super::index::INDEX_ENTRY_SIZE;
 use super::nodemap::{NodeMap, NodeMapError};
 use crate::config::{Config, ResourceProfileValue};
 use crate::errors::HgError;
 use crate::exit_codes;
 use crate::requirements::{
     GENERALDELTA_REQUIREMENT, NARROW_REQUIREMENT, SPARSEREVLOG_REQUIREMENT,
 };
 use crate::vfs::Vfs;
 
 /// As noted in revlog.c, revision numbers are actually encoded in
 /// 4 bytes, and are liberally converted to ints, whence the i32
 pub type BaseRevision = i32;
 
 /// Mercurial revision numbers
 /// In contrast to the more general [`UncheckedRevision`], these are "checked"
 /// in the sense that they should only be used for revisions that are
 /// valid for a given index (i.e. in bounds).
 #[derive(
     Debug,
     derive_more::Display,
     Clone,
     Copy,
     Hash,
     PartialEq,
     Eq,
     PartialOrd,
     Ord,
 )]
 pub struct Revision(pub BaseRevision);
 
 impl format_bytes::DisplayBytes for Revision {
     fn display_bytes(
         &self,
         output: &mut dyn std::io::Write,
     ) -> std::io::Result<()> {
         self.0.display_bytes(output)
     }
 }
 
 /// Unchecked Mercurial revision numbers.
 ///
 /// Values of this type have no guarantee of being a valid revision number
 /// in any context. Use method `check_revision` to get a valid revision within
 /// the appropriate index object.
 #[derive(
     Debug,
     derive_more::Display,
     Clone,
     Copy,
     Hash,
     PartialEq,
     Eq,
     PartialOrd,
     Ord,
 )]
 pub struct UncheckedRevision(pub BaseRevision);
 
 impl format_bytes::DisplayBytes for UncheckedRevision {
     fn display_bytes(
         &self,
         output: &mut dyn std::io::Write,
     ) -> std::io::Result<()> {
         self.0.display_bytes(output)
     }
 }
 
 impl From<Revision> for UncheckedRevision {
     fn from(value: Revision) -> Self {
         Self(value.0)
     }
 }
 
 impl From<BaseRevision> for UncheckedRevision {
     fn from(value: BaseRevision) -> Self {
         Self(value)
     }
 }
 
 /// Marker expressing the absence of a parent
 ///
 /// Independently of the actual representation, `NULL_REVISION` is guaranteed
 /// to be smaller than all existing revisions.
 pub const NULL_REVISION: Revision = Revision(-1);
 
 /// Same as `mercurial.node.wdirrev`
 ///
 /// This is also equal to `i32::max_value()`, but it's better to spell
 /// it out explicitely, same as in `mercurial.node`
 #[allow(clippy::unreadable_literal)]
 pub const WORKING_DIRECTORY_REVISION: UncheckedRevision =
     UncheckedRevision(0x7fffffff);
 
 pub const WORKING_DIRECTORY_HEX: &str =
     "ffffffffffffffffffffffffffffffffffffffff";
 
 /// The simplest expression of what we need of Mercurial DAGs.
 pub trait Graph {
     /// Return the two parents of the given `Revision`.
     ///
     /// Each of the parents can be independently `NULL_REVISION`
     fn parents(&self, rev: Revision) -> Result<[Revision; 2], GraphError>;
 }
 
 #[derive(Clone, Debug, PartialEq)]
 pub enum GraphError {
     ParentOutOfRange(Revision),
 }
 
 impl<T: Graph> Graph for &T {
     fn parents(&self, rev: Revision) -> Result<[Revision; 2], GraphError> {
         (*self).parents(rev)
     }
 }
 
 /// The Mercurial Revlog Index
 ///
 /// This is currently limited to the minimal interface that is needed for
 /// the [`nodemap`](nodemap/index.html) module
 pub trait RevlogIndex {
     /// Total number of Revisions referenced in this index
     fn len(&self) -> usize;
 
     fn is_empty(&self) -> bool {
         self.len() == 0
     }
 
     /// Return a reference to the Node or `None` for `NULL_REVISION`
     fn node(&self, rev: Revision) -> Option<&Node>;
 
     /// Return a [`Revision`] if `rev` is a valid revision number for this
     /// index.
     ///
     /// [`NULL_REVISION`] is considered to be valid.
     #[inline(always)]
     fn check_revision(&self, rev: UncheckedRevision) -> Option<Revision> {
         let rev = rev.0;
 
         if rev == NULL_REVISION.0 || (rev >= 0 && (rev as usize) < self.len())
         {
             Some(Revision(rev))
         } else {
             None
         }
     }
 }
 
 const REVISION_FLAG_CENSORED: u16 = 1 << 15;
 const REVISION_FLAG_ELLIPSIS: u16 = 1 << 14;
 const REVISION_FLAG_EXTSTORED: u16 = 1 << 13;
 const REVISION_FLAG_HASCOPIESINFO: u16 = 1 << 12;
 
 // Keep this in sync with REVIDX_KNOWN_FLAGS in
 // mercurial/revlogutils/flagutil.py
 const REVIDX_KNOWN_FLAGS: u16 = REVISION_FLAG_CENSORED
     | REVISION_FLAG_ELLIPSIS
     | REVISION_FLAG_EXTSTORED
     | REVISION_FLAG_HASCOPIESINFO;
 
 const NULL_REVLOG_ENTRY_FLAGS: u16 = 0;
 
 #[derive(Debug, derive_more::From, derive_more::Display)]
 pub enum RevlogError {
     InvalidRevision,
     /// Working directory is not supported
     WDirUnsupported,
     /// Found more than one entry whose ID match the requested prefix
     AmbiguousPrefix,
     #[from]
     Other(HgError),
 }
 
 impl From<NodeMapError> for RevlogError {
     fn from(error: NodeMapError) -> Self {
         match error {
             NodeMapError::MultipleResults => RevlogError::AmbiguousPrefix,
             NodeMapError::RevisionNotInIndex(rev) => RevlogError::corrupted(
                 format!("nodemap point to revision {} not in index", rev),
             ),
         }
     }
 }
 
 fn corrupted<S: AsRef<str>>(context: S) -> HgError {
     HgError::corrupted(format!("corrupted revlog, {}", context.as_ref()))
 }
 
 impl RevlogError {
     fn corrupted<S: AsRef<str>>(context: S) -> Self {
         RevlogError::Other(corrupted(context))
     }
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub enum RevlogType {
     Changelog,
     Manifestlog,
     Filelog,
 }
 
 impl TryFrom<usize> for RevlogType {
     type Error = HgError;
 
     fn try_from(value: usize) -> Result<Self, Self::Error> {
         match value {
             1001 => Ok(Self::Changelog),
             1002 => Ok(Self::Manifestlog),
             1003 => Ok(Self::Filelog),
             t => Err(HgError::abort(
                 format!("Unknown revlog type {}", t),
                 exit_codes::ABORT,
                 None,
             )),
         }
     }
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub enum CompressionEngine {
     Zlib {
         /// Between 0 and 9 included
         level: u32,
     },
     Zstd {
         /// Between 0 and 22 included
         level: u32,
         /// Never used in practice for now
         threads: u32,
     },
     /// No compression is performed
     None,
 }
 impl CompressionEngine {
     pub fn set_level(&mut self, new_level: usize) -> Result<(), HgError> {
         match self {
             CompressionEngine::Zlib { level } => {
                 if new_level > 9 {
                     return Err(HgError::abort(
                         format!(
                             "invalid compression zlib compression level {}",
                             new_level
                         ),
                         exit_codes::ABORT,
                         None,
                     ));
                 }
                 *level = new_level as u32;
             }
             CompressionEngine::Zstd { level, .. } => {
                 if new_level > 22 {
                     return Err(HgError::abort(
                         format!(
                             "invalid compression zstd compression level {}",
                             new_level
                         ),
                         exit_codes::ABORT,
                         None,
                     ));
                 }
                 *level = new_level as u32;
             }
             CompressionEngine::None => {}
         }
         Ok(())
     }
 
     pub fn zstd(
         zstd_level: Option<u32>,
     ) -> Result<CompressionEngine, HgError> {
         let mut engine = CompressionEngine::Zstd {
             level: 3,
             threads: 0,
         };
         if let Some(level) = zstd_level {
             engine.set_level(level as usize)?;
         }
         Ok(engine)
     }
 }
 
 impl Default for CompressionEngine {
     fn default() -> Self {
         Self::Zlib { level: 6 }
     }
 }
 
 #[derive(Debug, Clone, Copy, PartialEq)]
 /// Holds configuration values about how the revlog data is read
 pub struct RevlogDataConfig {
     /// Should we try to open the "pending" version of the revlog
     pub try_pending: bool,
     /// Should we try to open the "split" version of the revlog
     pub try_split: bool,
     /// When True, `indexfile` should be opened with `checkambig=True` at
     /// writing time, to avoid file stat ambiguity
     pub check_ambig: bool,
     /// If true, use mmap instead of reading to deal with large indexes
     pub mmap_large_index: bool,
     /// How much data is considered large
     pub mmap_index_threshold: Option<u64>,
     /// How much data to read and cache into the raw revlog data cache
     pub chunk_cache_size: u64,
     /// The size of the uncompressed cache compared to the largest revision
     /// seen
     pub uncompressed_cache_factor: Option<f64>,
     /// The number of chunks cached
     pub uncompressed_cache_count: Option<u64>,
     /// Allow sparse reading of the revlog data
     pub with_sparse_read: bool,
     /// Minimal density of a sparse read chunk
     pub sr_density_threshold: f64,
     /// Minimal size of the data we skip when performing sparse reads
     pub sr_min_gap_size: u64,
     /// Whether deltas are encoded against arbitrary bases
     pub general_delta: bool,
 }
 
 impl RevlogDataConfig {
     pub fn new(
         config: &Config,
         requirements: &HashSet<String>,
     ) -> Result<Self, HgError> {
         let mut data_config = Self::default();
         if let Some(chunk_cache_size) =
             config.get_byte_size(b"format", b"chunkcachesize")?
         {
             data_config.chunk_cache_size = chunk_cache_size;
         }
 
         let memory_profile = config.get_resource_profile(Some("memory"));
         if memory_profile.value >= ResourceProfileValue::Medium {
             data_config.uncompressed_cache_count = Some(10_000);
             data_config.uncompressed_cache_factor = Some(4.0);
             if memory_profile.value >= ResourceProfileValue::High {
                 data_config.uncompressed_cache_factor = Some(10.0)
             }
         }
 
         if let Some(mmap_index_threshold) =
             config.get_byte_size(b"experimental", b"mmapindexthreshold")?
         {
             data_config.mmap_index_threshold = Some(mmap_index_threshold);
         }
 
         let with_sparse_read =
             config.get_bool(b"experimental", b"sparse-read")?;
         if let Some(sr_density_threshold) = config
             .get_f64(b"experimental", b"sparse-read.density-threshold")?
         {
             data_config.sr_density_threshold = sr_density_threshold;
         }
         data_config.with_sparse_read = with_sparse_read;
         if let Some(sr_min_gap_size) = config
             .get_byte_size(b"experimental", b"sparse-read.min-gap-size")?
         {
             data_config.sr_min_gap_size = sr_min_gap_size;
         }
 
         data_config.with_sparse_read =
             requirements.contains(SPARSEREVLOG_REQUIREMENT);
 
         Ok(data_config)
     }
 }
 
 impl Default for RevlogDataConfig {
     fn default() -> Self {
         Self {
             chunk_cache_size: 65536,
             sr_density_threshold: 0.50,
             sr_min_gap_size: 262144,
             try_pending: Default::default(),
             try_split: Default::default(),
             check_ambig: Default::default(),
             mmap_large_index: Default::default(),
             mmap_index_threshold: Default::default(),
             uncompressed_cache_factor: Default::default(),
             uncompressed_cache_count: Default::default(),
             with_sparse_read: Default::default(),
             general_delta: Default::default(),
         }
     }
 }
 
 #[derive(Debug, Clone, Copy, PartialEq)]
 /// Holds configuration values about how new deltas are computed.
 ///
 /// Some attributes are duplicated from [`RevlogDataConfig`] to help having
 /// each object self contained.
 pub struct RevlogDeltaConfig {
     /// Whether deltas can be encoded against arbitrary bases
     pub general_delta: bool,
     /// Allow sparse writing of the revlog data
     pub sparse_revlog: bool,
     /// Maximum length of a delta chain
     pub max_chain_len: Option<u64>,
     /// Maximum distance between a delta chain's start and end
     pub max_deltachain_span: Option<u64>,
     /// If `upper_bound_comp` is not None, this is the expected maximal
     /// gain from compression for the data content
     pub upper_bound_comp: Option<f64>,
     /// Should we try a delta against both parents
     pub delta_both_parents: bool,
     /// Test delta base candidate groups by chunks of this maximal size
     pub candidate_group_chunk_size: u64,
     /// Should we display debug information about delta computation
     pub debug_delta: bool,
     /// Trust incoming deltas by default
     pub lazy_delta: bool,
     /// Trust the base of incoming deltas by default
     pub lazy_delta_base: bool,
 }
 impl RevlogDeltaConfig {
     pub fn new(
         config: &Config,
         requirements: &HashSet<String>,
         revlog_type: RevlogType,
     ) -> Result<Self, HgError> {
         let mut delta_config = Self {
             delta_both_parents: config
                 .get_option_no_default(
                     b"storage",
                     b"revlog.optimize-delta-parent-choice",
                 )?
                 .unwrap_or(true),
             candidate_group_chunk_size: config
                 .get_u64(
                     b"storage",
                     b"revlog.delta-parent-search.candidate-group-chunk-size",
                 )?
                 .unwrap_or_default(),
             ..Default::default()
         };
 
         delta_config.debug_delta =
             config.get_bool(b"debug", b"revlog.debug-delta")?;
 
         delta_config.general_delta =
             requirements.contains(GENERALDELTA_REQUIREMENT);
 
         let lazy_delta =
             config.get_bool(b"storage", b"revlog.reuse-external-delta")?;
 
         if revlog_type == RevlogType::Manifestlog {
             // upper bound of what we expect from compression
             // (real life value seems to be 3)
             delta_config.upper_bound_comp = Some(3.0)
         }
 
         let mut lazy_delta_base = false;
         if lazy_delta {
             lazy_delta_base = match config.get_option_no_default(
                 b"storage",
                 b"revlog.reuse-external-delta-parent",
             )? {
                 Some(base) => base,
                 None => config.get_bool(b"format", b"generaldelta")?,
             };
         }
         delta_config.lazy_delta = lazy_delta;
         delta_config.lazy_delta_base = lazy_delta_base;
 
         delta_config.max_deltachain_span =
             match config.get_i64(b"experimental", b"maxdeltachainspan")? {
                 Some(span) => {
                     if span < 0 {
                         None
                     } else {
                         Some(span as u64)
                     }
                 }
                 None => None,
             };
 
         delta_config.sparse_revlog =
             requirements.contains(SPARSEREVLOG_REQUIREMENT);
 
         delta_config.max_chain_len =
             config.get_byte_size_no_default(b"format", b"maxchainlen")?;
 
         Ok(delta_config)
     }
 }
 
 impl Default for RevlogDeltaConfig {
     fn default() -> Self {
         Self {
             delta_both_parents: true,
             lazy_delta: true,
             general_delta: Default::default(),
             sparse_revlog: Default::default(),
             max_chain_len: Default::default(),
             max_deltachain_span: Default::default(),
             upper_bound_comp: Default::default(),
             candidate_group_chunk_size: Default::default(),
             debug_delta: Default::default(),
             lazy_delta_base: Default::default(),
         }
     }
 }
 
 #[derive(Debug, Default, Clone, Copy, PartialEq)]
 /// Holds configuration values about the available revlog features
 pub struct RevlogFeatureConfig {
     /// The compression engine and its options
     pub compression_engine: CompressionEngine,
     /// Can we use censor on this revlog
     pub censorable: bool,
     /// Does this revlog use the "side data" feature
     pub has_side_data: bool,
     /// Might remove this configuration once the rank computation has no
     /// impact
     pub compute_rank: bool,
     /// Parent order is supposed to be semantically irrelevant, so we
     /// normally re-sort parents to ensure that the first parent is non-null,
     /// if there is a non-null parent at all.
     /// filelog abuses the parent order as a flag to mark some instances of
     /// meta-encoded files, so allow it to disable this behavior.
     pub canonical_parent_order: bool,
     /// Can ellipsis commit be used
     pub enable_ellipsis: bool,
 }
 impl RevlogFeatureConfig {
     pub fn new(
         config: &Config,
         requirements: &HashSet<String>,
     ) -> Result<Self, HgError> {
         let mut feature_config = Self::default();
 
         let zlib_level = config.get_u32(b"storage", b"revlog.zlib.level")?;
         let zstd_level = config.get_u32(b"storage", b"revlog.zstd.level")?;
 
         feature_config.compression_engine = CompressionEngine::default();
 
         for requirement in requirements {
             if requirement.starts_with("revlog-compression-")
                 || requirement.starts_with("exp-compression-")
             {
                 let split = &mut requirement.splitn(3, '-');
                 split.next();
                 split.next();
                 feature_config.compression_engine = match split.next().unwrap()
                 {
                     "zstd" => CompressionEngine::zstd(zstd_level)?,
                     e => {
                         return Err(HgError::UnsupportedFeature(format!(
                             "Unsupported compression engine '{e}'"
                         )))
                     }
                 };
             }
         }
         if let Some(level) = zlib_level {
             if matches!(
                 feature_config.compression_engine,
                 CompressionEngine::Zlib { .. }
             ) {
                 feature_config
                     .compression_engine
                     .set_level(level as usize)?;
             }
         }
 
         feature_config.enable_ellipsis =
             requirements.contains(NARROW_REQUIREMENT);
 
         Ok(feature_config)
     }
 }
 
 /// Read only implementation of revlog.
 pub struct Revlog {
     /// When index and data are not interleaved: bytes of the revlog index.
     /// When index and data are interleaved: bytes of the revlog index and
     /// data.
     index: Index,
     /// When index and data are not interleaved: bytes of the revlog data
     data_bytes: Option<Box<dyn Deref<Target = [u8]> + Send>>,
     /// When present on disk: the persistent nodemap for this revlog
     nodemap: Option<nodemap::NodeTree>,
 }
 
 impl Graph for Revlog {
     fn parents(&self, rev: Revision) -> Result<[Revision; 2], GraphError> {
         self.index.parents(rev)
     }
 }
 
 #[derive(Debug, Copy, Clone)]
 pub enum RevlogVersionOptions {
     V0,
     V1 { generaldelta: bool },
     V2,
     ChangelogV2 { compute_rank: bool },
 }
 
 /// Options to govern how a revlog should be opened, usually from the
 /// repository configuration or requirements.
 #[derive(Debug, Copy, Clone)]
 pub struct RevlogOpenOptions {
     /// The revlog version, along with any option specific to this version
     pub version: RevlogVersionOptions,
     /// Whether the revlog uses a persistent nodemap.
     pub use_nodemap: bool,
     // TODO other non-header/version options,
 }
 
 impl RevlogOpenOptions {
     pub fn new() -> Self {
         Self {
             version: RevlogVersionOptions::V1 { generaldelta: true },
             use_nodemap: false,
         }
     }
 
     fn default_index_header(&self) -> index::IndexHeader {
         index::IndexHeader {
             header_bytes: match self.version {
                 RevlogVersionOptions::V0 => [0, 0, 0, 0],
                 RevlogVersionOptions::V1 { generaldelta } => {
                     [0, if generaldelta { 3 } else { 1 }, 0, 1]
                 }
                 RevlogVersionOptions::V2 => 0xDEADu32.to_be_bytes(),
                 RevlogVersionOptions::ChangelogV2 { compute_rank: _ } => {
                     0xD34Du32.to_be_bytes()
                 }
             },
         }
     }
 }
 
 impl Default for RevlogOpenOptions {
     fn default() -> Self {
         Self::new()
     }
 }
 
 impl Revlog {
     /// Open a revlog index file.
     ///
     /// It will also open the associated data file if index and data are not
     /// interleaved.
     pub fn open(
         store_vfs: &Vfs,
         index_path: impl AsRef<Path>,
         data_path: Option<&Path>,
         options: RevlogOpenOptions,
     ) -> Result<Self, HgError> {
         Self::open_gen(store_vfs, index_path, data_path, options, None)
     }
 
     fn open_gen(
         store_vfs: &Vfs,
         index_path: impl AsRef<Path>,
         data_path: Option<&Path>,
         options: RevlogOpenOptions,
         nodemap_for_test: Option<nodemap::NodeTree>,
     ) -> Result<Self, HgError> {
         let index_path = index_path.as_ref();
         let index = {
             match store_vfs.mmap_open_opt(index_path)? {
                 None => Index::new(
                     Box::<Vec<_>>::default(),
                     options.default_index_header(),
                 ),
                 Some(index_mmap) => {
                     let index = Index::new(
                         Box::new(index_mmap),
                         options.default_index_header(),
                     )?;
                     Ok(index)
                 }
             }
         }?;
 
         let default_data_path = index_path.with_extension("d");
 
         // type annotation required
         // won't recognize Mmap as Deref<Target = [u8]>
         let data_bytes: Option<Box<dyn Deref<Target = [u8]> + Send>> =
             if index.is_inline() {
                 None
+            } else if index.is_empty() {
+                // No need to even try to open the data file then.
+                Some(Box::new(&[][..]))
             } else {
                 let data_path = data_path.unwrap_or(&default_data_path);
                 let data_mmap = store_vfs.mmap_open(data_path)?;
                 Some(Box::new(data_mmap))
             };
 
         let nodemap = if index.is_inline() || !options.use_nodemap {
             None
         } else {
             NodeMapDocket::read_from_file(store_vfs, index_path)?.map(
                 |(docket, data)| {
                     nodemap::NodeTree::load_bytes(
                         Box::new(data),
                         docket.data_length,
                     )
                 },
             )
         };
 
         let nodemap = nodemap_for_test.or(nodemap);
 
         Ok(Revlog {
             index,
             data_bytes,
             nodemap,
         })
     }
 
     /// Return number of entries of the `Revlog`.
     pub fn len(&self) -> usize {
         self.index.len()
     }
 
     /// Returns `true` if the `Revlog` has zero `entries`.
     pub fn is_empty(&self) -> bool {
         self.index.is_empty()
     }
 
     /// Returns the node ID for the given revision number, if it exists in this
     /// revlog
     pub fn node_from_rev(&self, rev: UncheckedRevision) -> Option<&Node> {
         if rev == NULL_REVISION.into() {
             return Some(&NULL_NODE);
         }
         let rev = self.index.check_revision(rev)?;
         Some(self.index.get_entry(rev)?.hash())
     }
 
     /// Return the revision number for the given node ID, if it exists in this
     /// revlog
     pub fn rev_from_node(
         &self,
         node: NodePrefix,
     ) -> Result<Revision, RevlogError> {
         if let Some(nodemap) = &self.nodemap {
             nodemap
                 .find_bin(&self.index, node)?
                 .ok_or(RevlogError::InvalidRevision)
         } else {
             self.rev_from_node_no_persistent_nodemap(node)
         }
     }
 
     /// Same as `rev_from_node`, without using a persistent nodemap
     ///
     /// This is used as fallback when a persistent nodemap is not present.
     /// This happens when the persistent-nodemap experimental feature is not
     /// enabled, or for small revlogs.
     fn rev_from_node_no_persistent_nodemap(
         &self,
         node: NodePrefix,
     ) -> Result<Revision, RevlogError> {
         // Linear scan of the revlog
         // TODO: consider building a non-persistent nodemap in memory to
         // optimize these cases.
         let mut found_by_prefix = None;
         for rev in (-1..self.len() as BaseRevision).rev() {
             let rev = Revision(rev as BaseRevision);
             let candidate_node = if rev == Revision(-1) {
                 NULL_NODE
             } else {
                 let index_entry =
                     self.index.get_entry(rev).ok_or_else(|| {
                         HgError::corrupted(
                             "revlog references a revision not in the index",
                         )
                     })?;
                 *index_entry.hash()
             };
             if node == candidate_node {
                 return Ok(rev);
             }
             if node.is_prefix_of(&candidate_node) {
                 if found_by_prefix.is_some() {
                     return Err(RevlogError::AmbiguousPrefix);
                 }
                 found_by_prefix = Some(rev)
             }
         }
         found_by_prefix.ok_or(RevlogError::InvalidRevision)
     }
 
     /// Returns whether the given revision exists in this revlog.
     pub fn has_rev(&self, rev: UncheckedRevision) -> bool {
         self.index.check_revision(rev).is_some()
     }
 
     /// Return the full data associated to a revision.
     ///
     /// All entries required to build the final data out of deltas will be
     /// retrieved as needed, and the deltas will be applied to the inital
     /// snapshot to rebuild the final data.
     pub fn get_rev_data(
         &self,
         rev: UncheckedRevision,
     ) -> Result<Cow<[u8]>, RevlogError> {
         if rev == NULL_REVISION.into() {
             return Ok(Cow::Borrowed(&[]));
         };
         self.get_entry(rev)?.data()
     }
 
     /// [`Self::get_rev_data`] for checked revisions.
     pub fn get_rev_data_for_checked_rev(
         &self,
         rev: Revision,
     ) -> Result<Cow<[u8]>, RevlogError> {
         if rev == NULL_REVISION {
             return Ok(Cow::Borrowed(&[]));
         };
         self.get_entry_for_checked_rev(rev)?.data()
     }
 
     /// Check the hash of some given data against the recorded hash.
     pub fn check_hash(
         &self,
         p1: Revision,
         p2: Revision,
         expected: &[u8],
         data: &[u8],
     ) -> bool {
         let e1 = self.index.get_entry(p1);
         let h1 = match e1 {
             Some(ref entry) => entry.hash(),
             None => &NULL_NODE,
         };
         let e2 = self.index.get_entry(p2);
         let h2 = match e2 {
             Some(ref entry) => entry.hash(),
             None => &NULL_NODE,
         };
 
         hash(data, h1.as_bytes(), h2.as_bytes()) == expected
     }
 
     /// Build the full data of a revision out its snapshot
     /// and its deltas.
     fn build_data_from_deltas(
         snapshot: RevlogEntry,
         deltas: &[RevlogEntry],
     ) -> Result<Vec<u8>, HgError> {
         let snapshot = snapshot.data_chunk()?;
         let deltas = deltas
             .iter()
             .rev()
             .map(RevlogEntry::data_chunk)
             .collect::<Result<Vec<_>, _>>()?;
         let patches: Vec<_> =
             deltas.iter().map(|d| patch::PatchList::new(d)).collect();
         let patch = patch::fold_patch_lists(&patches);
         Ok(patch.apply(&snapshot))
     }
 
     /// Return the revlog data.
     fn data(&self) -> &[u8] {
         match &self.data_bytes {
             Some(data_bytes) => data_bytes,
             None => panic!(
                 "forgot to load the data or trying to access inline data"
             ),
         }
     }
 
     pub fn make_null_entry(&self) -> RevlogEntry {
         RevlogEntry {
             revlog: self,
             rev: NULL_REVISION,
             bytes: b"",
             compressed_len: 0,
             uncompressed_len: 0,
             base_rev_or_base_of_delta_chain: None,
             p1: NULL_REVISION,
             p2: NULL_REVISION,
             flags: NULL_REVLOG_ENTRY_FLAGS,
             hash: NULL_NODE,
         }
     }
 
     fn get_entry_for_checked_rev(
         &self,
         rev: Revision,
     ) -> Result<RevlogEntry, RevlogError> {
         if rev == NULL_REVISION {
             return Ok(self.make_null_entry());
         }
         let index_entry = self
             .index
             .get_entry(rev)
             .ok_or(RevlogError::InvalidRevision)?;
         let offset = index_entry.offset();
         let start = if self.index.is_inline() {
             offset + ((rev.0 as usize + 1) * INDEX_ENTRY_SIZE)
         } else {
             offset
         };
         let end = start + index_entry.compressed_len() as usize;
         let data = if self.index.is_inline() {
             self.index.data(start, end)
         } else {
             &self.data()[start..end]
         };
         let base_rev = self
             .index
             .check_revision(index_entry.base_revision_or_base_of_delta_chain())
             .ok_or_else(|| {
                 RevlogError::corrupted(format!(
                     "base revision for rev {} is invalid",
                     rev
                 ))
             })?;
         let p1 =
             self.index.check_revision(index_entry.p1()).ok_or_else(|| {
                 RevlogError::corrupted(format!(
                     "p1 for rev {} is invalid",
                     rev
                 ))
             })?;
         let p2 =
             self.index.check_revision(index_entry.p2()).ok_or_else(|| {
                 RevlogError::corrupted(format!(
                     "p2 for rev {} is invalid",
                     rev
                 ))
             })?;
         let entry = RevlogEntry {
             revlog: self,
             rev,
             bytes: data,
             compressed_len: index_entry.compressed_len(),
             uncompressed_len: index_entry.uncompressed_len(),
             base_rev_or_base_of_delta_chain: if base_rev == rev {
                 None
             } else {
                 Some(base_rev)
             },
             p1,
             p2,
             flags: index_entry.flags(),
             hash: *index_entry.hash(),
         };
         Ok(entry)
     }
 
     /// Get an entry of the revlog.
     pub fn get_entry(
         &self,
         rev: UncheckedRevision,
     ) -> Result<RevlogEntry, RevlogError> {
         if rev == NULL_REVISION.into() {
             return Ok(self.make_null_entry());
         }
         let rev = self.index.check_revision(rev).ok_or_else(|| {
             RevlogError::corrupted(format!("rev {} is invalid", rev))
         })?;
         self.get_entry_for_checked_rev(rev)
     }
 }
 
 /// The revlog entry's bytes and the necessary informations to extract
 /// the entry's data.
 #[derive(Clone)]
 pub struct RevlogEntry<'revlog> {
     revlog: &'revlog Revlog,
     rev: Revision,
     bytes: &'revlog [u8],
     compressed_len: u32,
     uncompressed_len: i32,
     base_rev_or_base_of_delta_chain: Option<Revision>,
     p1: Revision,
     p2: Revision,
     flags: u16,
     hash: Node,
 }
 
 thread_local! {
   // seems fine to [unwrap] here: this can only fail due to memory allocation
   // failing, and it's normal for that to cause panic.
   static ZSTD_DECODER : RefCell<zstd::bulk::Decompressor<'static>> =
       RefCell::new(zstd::bulk::Decompressor::new().ok().unwrap());
 }
 
 fn zstd_decompress_to_buffer(
     bytes: &[u8],
     buf: &mut Vec<u8>,
 ) -> Result<usize, std::io::Error> {
     ZSTD_DECODER
         .with(|decoder| decoder.borrow_mut().decompress_to_buffer(bytes, buf))
 }
 
 impl<'revlog> RevlogEntry<'revlog> {
     pub fn revision(&self) -> Revision {
         self.rev
     }
 
     pub fn node(&self) -> &Node {
         &self.hash
     }
 
     pub fn uncompressed_len(&self) -> Option<u32> {
         u32::try_from(self.uncompressed_len).ok()
     }
 
     pub fn has_p1(&self) -> bool {
         self.p1 != NULL_REVISION
     }
 
     pub fn p1_entry(
         &self,
     ) -> Result<Option<RevlogEntry<'revlog>>, RevlogError> {
         if self.p1 == NULL_REVISION {
             Ok(None)
         } else {
             Ok(Some(self.revlog.get_entry_for_checked_rev(self.p1)?))
         }
     }
 
     pub fn p2_entry(
         &self,
     ) -> Result<Option<RevlogEntry<'revlog>>, RevlogError> {
         if self.p2 == NULL_REVISION {
             Ok(None)
         } else {
             Ok(Some(self.revlog.get_entry_for_checked_rev(self.p2)?))
         }
     }
 
     pub fn p1(&self) -> Option<Revision> {
         if self.p1 == NULL_REVISION {
             None
         } else {
             Some(self.p1)
         }
     }
 
     pub fn p2(&self) -> Option<Revision> {
         if self.p2 == NULL_REVISION {
             None
         } else {
             Some(self.p2)
         }
     }
 
     pub fn is_censored(&self) -> bool {
         (self.flags & REVISION_FLAG_CENSORED) != 0
     }
 
     pub fn has_length_affecting_flag_processor(&self) -> bool {
         // Relevant Python code: revlog.size()
         // note: ELLIPSIS is known to not change the content
         (self.flags & (REVIDX_KNOWN_FLAGS ^ REVISION_FLAG_ELLIPSIS)) != 0
     }
 
     /// The data for this entry, after resolving deltas if any.
     pub fn rawdata(&self) -> Result<Cow<'revlog, [u8]>, RevlogError> {
         let mut entry = self.clone();
         let mut delta_chain = vec![];
 
         // The meaning of `base_rev_or_base_of_delta_chain` depends on
         // generaldelta. See the doc on `ENTRY_DELTA_BASE` in
         // `mercurial/revlogutils/constants.py` and the code in
         // [_chaininfo] and in [index_deltachain].
         let uses_generaldelta = self.revlog.index.uses_generaldelta();
         while let Some(base_rev) = entry.base_rev_or_base_of_delta_chain {
             entry = if uses_generaldelta {
                 delta_chain.push(entry);
                 self.revlog.get_entry_for_checked_rev(base_rev)?
             } else {
                 let base_rev = UncheckedRevision(entry.rev.0 - 1);
                 delta_chain.push(entry);
                 self.revlog.get_entry(base_rev)?
             };
         }
 
         let data = if delta_chain.is_empty() {
             entry.data_chunk()?
         } else {
             Revlog::build_data_from_deltas(entry, &delta_chain)?.into()
         };
 
         Ok(data)
     }
 
     fn check_data(
         &self,
         data: Cow<'revlog, [u8]>,
     ) -> Result<Cow<'revlog, [u8]>, RevlogError> {
         if self.revlog.check_hash(
             self.p1,
             self.p2,
             self.hash.as_bytes(),
             &data,
         ) {
             Ok(data)
         } else {
             if (self.flags & REVISION_FLAG_ELLIPSIS) != 0 {
                 return Err(HgError::unsupported(
                     "ellipsis revisions are not supported by rhg",
                 )
                 .into());
             }
             Err(corrupted(format!(
                 "hash check failed for revision {}",
                 self.rev
             ))
             .into())
         }
     }
 
     pub fn data(&self) -> Result<Cow<'revlog, [u8]>, RevlogError> {
         let data = self.rawdata()?;
         if self.rev == NULL_REVISION {
             return Ok(data);
         }
         if self.is_censored() {
             return Err(HgError::CensoredNodeError.into());
         }
         self.check_data(data)
     }
 
     /// Extract the data contained in the entry.
     /// This may be a delta. (See `is_delta`.)
     fn data_chunk(&self) -> Result<Cow<'revlog, [u8]>, HgError> {
         if self.bytes.is_empty() {
             return Ok(Cow::Borrowed(&[]));
         }
         match self.bytes[0] {
             // Revision data is the entirety of the entry, including this
             // header.
             b'\0' => Ok(Cow::Borrowed(self.bytes)),
             // Raw revision data follows.
             b'u' => Ok(Cow::Borrowed(&self.bytes[1..])),
             // zlib (RFC 1950) data.
             b'x' => Ok(Cow::Owned(self.uncompressed_zlib_data()?)),
             // zstd data.
             b'\x28' => Ok(Cow::Owned(self.uncompressed_zstd_data()?)),
             // A proper new format should have had a repo/store requirement.
             format_type => Err(corrupted(format!(
                 "unknown compression header '{}'",
                 format_type
             ))),
         }
     }
 
     fn uncompressed_zlib_data(&self) -> Result<Vec<u8>, HgError> {
         let mut decoder = ZlibDecoder::new(self.bytes);
         if self.is_delta() {
             let mut buf = Vec::with_capacity(self.compressed_len as usize);
             decoder
                 .read_to_end(&mut buf)
                 .map_err(|e| corrupted(e.to_string()))?;
             Ok(buf)
         } else {
             let cap = self.uncompressed_len.max(0) as usize;
             let mut buf = vec![0; cap];
             decoder
                 .read_exact(&mut buf)
                 .map_err(|e| corrupted(e.to_string()))?;
             Ok(buf)
         }
     }
 
     fn uncompressed_zstd_data(&self) -> Result<Vec<u8>, HgError> {
         let cap = self.uncompressed_len.max(0) as usize;
         if self.is_delta() {
             // [cap] is usually an over-estimate of the space needed because
             // it's the length of delta-decoded data, but we're interested
             // in the size of the delta.
             // This means we have to [shrink_to_fit] to avoid holding on
             // to a large chunk of memory, but it also means we must have a
             // fallback branch, for the case when the delta is longer than
             // the original data (surprisingly, this does happen in practice)
             let mut buf = Vec::with_capacity(cap);
             match zstd_decompress_to_buffer(self.bytes, &mut buf) {
                 Ok(_) => buf.shrink_to_fit(),
                 Err(_) => {
                     buf.clear();
                     zstd::stream::copy_decode(self.bytes, &mut buf)
                         .map_err(|e| corrupted(e.to_string()))?;
                 }
             };
             Ok(buf)
         } else {
             let mut buf = Vec::with_capacity(cap);
             let len = zstd_decompress_to_buffer(self.bytes, &mut buf)
                 .map_err(|e| corrupted(e.to_string()))?;
             if len != self.uncompressed_len as usize {
                 Err(corrupted("uncompressed length does not match"))
             } else {
                 Ok(buf)
             }
         }
     }
 
     /// Tell if the entry is a snapshot or a delta
     /// (influences on decompression).
     fn is_delta(&self) -> bool {
         self.base_rev_or_base_of_delta_chain.is_some()
     }
 }
 
 /// Calculate the hash of a revision given its data and its parents.
 fn hash(
     data: &[u8],
     p1_hash: &[u8],
     p2_hash: &[u8],
 ) -> [u8; NODE_BYTES_LENGTH] {
     let mut hasher = Sha1::new();
     let (a, b) = (p1_hash, p2_hash);
     if a > b {
         hasher.update(b);
         hasher.update(a);
     } else {
         hasher.update(a);
         hasher.update(b);
     }
     hasher.update(data);
     *hasher.finalize().as_ref()
 }
 
 #[cfg(test)]
 mod tests {
     use super::*;
     use crate::index::IndexEntryBuilder;
     use itertools::Itertools;
 
     #[test]
     fn test_empty() {
         let temp = tempfile::tempdir().unwrap();
         let vfs = Vfs { base: temp.path() };
         std::fs::write(temp.path().join("foo.i"), b"").unwrap();
         let revlog =
             Revlog::open(&vfs, "foo.i", None, RevlogOpenOptions::new())
                 .unwrap();
         assert!(revlog.is_empty());
         assert_eq!(revlog.len(), 0);
         assert!(revlog.get_entry(0.into()).is_err());
         assert!(!revlog.has_rev(0.into()));
         assert_eq!(
             revlog.rev_from_node(NULL_NODE.into()).unwrap(),
             NULL_REVISION
         );
         let null_entry = revlog.get_entry(NULL_REVISION.into()).ok().unwrap();
         assert_eq!(null_entry.revision(), NULL_REVISION);
         assert!(null_entry.data().unwrap().is_empty());
     }
 
     #[test]
     fn test_inline() {
         let temp = tempfile::tempdir().unwrap();
         let vfs = Vfs { base: temp.path() };
         let node0 = Node::from_hex("2ed2a3912a0b24502043eae84ee4b279c18b90dd")
             .unwrap();
         let node1 = Node::from_hex("b004912a8510032a0350a74daa2803dadfb00e12")
             .unwrap();
         let node2 = Node::from_hex("dd6ad206e907be60927b5a3117b97dffb2590582")
             .unwrap();
         let entry0_bytes = IndexEntryBuilder::new()
             .is_first(true)
             .with_version(1)
             .with_inline(true)
             .with_node(node0)
             .build();
         let entry1_bytes = IndexEntryBuilder::new().with_node(node1).build();
         let entry2_bytes = IndexEntryBuilder::new()
             .with_p1(Revision(0))
             .with_p2(Revision(1))
             .with_node(node2)
             .build();
         let contents = vec![entry0_bytes, entry1_bytes, entry2_bytes]
             .into_iter()
             .flatten()
             .collect_vec();
         std::fs::write(temp.path().join("foo.i"), contents).unwrap();
         let revlog =
             Revlog::open(&vfs, "foo.i", None, RevlogOpenOptions::new())
                 .unwrap();
 
         let entry0 = revlog.get_entry(0.into()).ok().unwrap();
         assert_eq!(entry0.revision(), Revision(0));
         assert_eq!(*entry0.node(), node0);
         assert!(!entry0.has_p1());
         assert_eq!(entry0.p1(), None);
         assert_eq!(entry0.p2(), None);
         let p1_entry = entry0.p1_entry().unwrap();
         assert!(p1_entry.is_none());
         let p2_entry = entry0.p2_entry().unwrap();
         assert!(p2_entry.is_none());
 
         let entry1 = revlog.get_entry(1.into()).ok().unwrap();
         assert_eq!(entry1.revision(), Revision(1));
         assert_eq!(*entry1.node(), node1);
         assert!(!entry1.has_p1());
         assert_eq!(entry1.p1(), None);
         assert_eq!(entry1.p2(), None);
         let p1_entry = entry1.p1_entry().unwrap();
         assert!(p1_entry.is_none());
         let p2_entry = entry1.p2_entry().unwrap();
         assert!(p2_entry.is_none());
 
         let entry2 = revlog.get_entry(2.into()).ok().unwrap();
         assert_eq!(entry2.revision(), Revision(2));
         assert_eq!(*entry2.node(), node2);
         assert!(entry2.has_p1());
         assert_eq!(entry2.p1(), Some(Revision(0)));
         assert_eq!(entry2.p2(), Some(Revision(1)));
         let p1_entry = entry2.p1_entry().unwrap();
         assert!(p1_entry.is_some());
         assert_eq!(p1_entry.unwrap().revision(), Revision(0));
         let p2_entry = entry2.p2_entry().unwrap();
         assert!(p2_entry.is_some());
         assert_eq!(p2_entry.unwrap().revision(), Revision(1));
     }
 
     #[test]
     fn test_nodemap() {
         let temp = tempfile::tempdir().unwrap();
         let vfs = Vfs { base: temp.path() };
 
         // building a revlog with a forced Node starting with zeros
         // This is a corruption, but it does not preclude using the nodemap
         // if we don't try and access the data
         let node0 = Node::from_hex("00d2a3912a0b24502043eae84ee4b279c18b90dd")
             .unwrap();
         let node1 = Node::from_hex("b004912a8510032a0350a74daa2803dadfb00e12")
             .unwrap();
         let entry0_bytes = IndexEntryBuilder::new()
             .is_first(true)
             .with_version(1)
             .with_inline(true)
             .with_node(node0)
             .build();
         let entry1_bytes = IndexEntryBuilder::new().with_node(node1).build();
         let contents = vec![entry0_bytes, entry1_bytes]
             .into_iter()
             .flatten()
             .collect_vec();
         std::fs::write(temp.path().join("foo.i"), contents).unwrap();
 
         let mut idx = nodemap::tests::TestNtIndex::new();
         idx.insert_node(Revision(0), node0).unwrap();
         idx.insert_node(Revision(1), node1).unwrap();
 
         let revlog = Revlog::open_gen(
             &vfs,
             "foo.i",
             None,
             RevlogOpenOptions::new(),
             Some(idx.nt),
         )
         .unwrap();
 
         // accessing the data shows the corruption
         revlog.get_entry(0.into()).unwrap().data().unwrap_err();
 
         assert_eq!(
             revlog.rev_from_node(NULL_NODE.into()).unwrap(),
             Revision(-1)
         );
         assert_eq!(revlog.rev_from_node(node0.into()).unwrap(), Revision(0));
         assert_eq!(revlog.rev_from_node(node1.into()).unwrap(), Revision(1));
         assert_eq!(
             revlog
                 .rev_from_node(NodePrefix::from_hex("000").unwrap())
                 .unwrap(),
             Revision(-1)
         );
         assert_eq!(
             revlog
                 .rev_from_node(NodePrefix::from_hex("b00").unwrap())
                 .unwrap(),
             Revision(1)
         );
         // RevlogError does not implement PartialEq
         // (ultimately because io::Error does not)
         match revlog
             .rev_from_node(NodePrefix::from_hex("00").unwrap())
             .expect_err("Expected to give AmbiguousPrefix error")
         {
             RevlogError::AmbiguousPrefix => (),
             e => {
                 panic!("Got another error than AmbiguousPrefix: {:?}", e);
             }
         };
     }
 }