rust-status: query fs traversal metadata lazily

Currently, any time the status algorithm needs to read a directory from the
filesystem (because the stat-only optimization is not available), it also
stats each directory entry eagerly.

Stat'ing the entries is only needed in a few cases (like when checking
the mtime of a directory for caching): this patch creates a wrapper struct
`DirEntry` that only stats the directory entry it represents when needed.

Excerpt of an `strace` before this change on Mozilla Central:

```
openat(AT_FDCWD, ".", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 3
newfstatat(3, "", {st_mode=S_IFDIR|0755, st_size=3540, ...}, AT_EMPTY_PATH) = 0
getdents64(3, 0x55dc970bd440 /* 139 entries */, 32768) = 5072
statx(3, ".hg", AT_STATX_SYNC_AS_STAT|AT_SYMLINK_NOFOLLOW, STATX_ALL, {stx_mask=STATX_ALL|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFDIR|0755, stx_size=772, ...}) = 0

[... 135 other successful `statx` calls]

getdents64(3, 0x55dc970bd440 /* 0 entries */, 32768) = 0
close(3)                                = 0
```

After this change:

```
openat(AT_FDCWD, ".", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 3
newfstatat(3, "", {st_mode=S_IFDIR|0755, st_size=3540, ...}, AT_EMPTY_PATH) = 0
getdents64(3, 0x561567c10190 /* 139 entries */, 32768) = 5072
getdents64(3, 0x561567c10190 /* 0 entries */, 32768) = 0
close(3)                                = 0
```

rust/hg-core/src/dirstate_tree/status.rs

@@ -1,903 +1,975 b''
 use crate::dirstate::entry::TruncatedTimestamp;
 use crate::dirstate::status::IgnoreFnType;
 use crate::dirstate::status::StatusPath;
 use crate::dirstate_tree::dirstate_map::BorrowedPath;
 use crate::dirstate_tree::dirstate_map::ChildNodesRef;
 use crate::dirstate_tree::dirstate_map::DirstateMap;
 use crate::dirstate_tree::dirstate_map::DirstateVersion;
 use crate::dirstate_tree::dirstate_map::NodeRef;
 use crate::dirstate_tree::on_disk::DirstateV2ParseError;
 use crate::matchers::get_ignore_function;
 use crate::matchers::Matcher;
 use crate::utils::files::get_bytes_from_os_string;
 use crate::utils::files::get_path_from_bytes;
 use crate::utils::hg_path::HgPath;
 use crate::BadMatch;
 use crate::DirstateStatus;
-use crate::HgPathBuf;
 use crate::HgPathCow;
 use crate::PatternFileWarning;
 use crate::StatusError;
 use crate::StatusOptions;
 use micro_timer::timed;
 use once_cell::sync::OnceCell;
 use rayon::prelude::*;
 use sha1::{Digest, Sha1};
 use std::borrow::Cow;
+use std::convert::TryFrom;
+use std::convert::TryInto;
 use std::io;
 use std::path::Path;
 use std::path::PathBuf;
 use std::sync::Mutex;
 use std::time::SystemTime;
 
 /// Returns the status of the working directory compared to its parent
 /// changeset.
 ///
 /// This algorithm is based on traversing the filesystem tree (`fs` in function
 /// and variable names) and dirstate tree at the same time. The core of this
 /// traversal is the recursive `traverse_fs_directory_and_dirstate` function
 /// and its use of `itertools::merge_join_by`. When reaching a path that only
 /// exists in one of the two trees, depending on information requested by
 /// `options` we may need to traverse the remaining subtree.
 #[timed]
 pub fn status<'dirstate>(
     dmap: &'dirstate mut DirstateMap,
     matcher: &(dyn Matcher + Sync),
     root_dir: PathBuf,
     ignore_files: Vec<PathBuf>,
     options: StatusOptions,
 ) -> Result<(DirstateStatus<'dirstate>, Vec<PatternFileWarning>), StatusError>
 {
     // Force the global rayon threadpool to not exceed 16 concurrent threads.
     // This is a stop-gap measure until we figure out why using more than 16
     // threads makes `status` slower for each additional thread.
     // We use `ok()` in case the global threadpool has already been
     // instantiated in `rhg` or some other caller.
     // TODO find the underlying cause and fix it, then remove this.
     rayon::ThreadPoolBuilder::new()
         .num_threads(16)
         .build_global()
         .ok();
 
     let (ignore_fn, warnings, patterns_changed): (IgnoreFnType, _, _) =
         if options.list_ignored || options.list_unknown {
             let (ignore_fn, warnings, changed) = match dmap.dirstate_version {
                 DirstateVersion::V1 => {
                     let (ignore_fn, warnings) = get_ignore_function(
                         ignore_files,
                         &root_dir,
                         &mut |_pattern_bytes| {},
                     )?;
                     (ignore_fn, warnings, None)
                 }
                 DirstateVersion::V2 => {
                     let mut hasher = Sha1::new();
                     let (ignore_fn, warnings) = get_ignore_function(
                         ignore_files,
                         &root_dir,
                         &mut |pattern_bytes| hasher.update(pattern_bytes),
                     )?;
                     let new_hash = *hasher.finalize().as_ref();
                     let changed = new_hash != dmap.ignore_patterns_hash;
                     dmap.ignore_patterns_hash = new_hash;
                     (ignore_fn, warnings, Some(changed))
                 }
             };
             (ignore_fn, warnings, changed)
         } else {
             (Box::new(|&_| true), vec![], None)
         };
 
     let filesystem_time_at_status_start =
         filesystem_now(&root_dir).ok().map(TruncatedTimestamp::from);
 
     // If the repository is under the current directory, prefer using a
     // relative path, so the kernel needs to traverse fewer directory in every
     // call to `read_dir` or `symlink_metadata`.
     // This is effective in the common case where the current directory is the
     // repository root.
 
     // TODO: Better yet would be to use libc functions like `openat` and
     // `fstatat` to remove such repeated traversals entirely, but the standard
     // library does not provide APIs based on those.
     // Maybe with a crate like https://crates.io/crates/openat instead?
     let root_dir = if let Some(relative) = std::env::current_dir()
         .ok()
         .and_then(|cwd| root_dir.strip_prefix(cwd).ok())
     {
         relative
     } else {
         &root_dir
     };
 
     let outcome = DirstateStatus {
         filesystem_time_at_status_start,
         ..Default::default()
     };
     let common = StatusCommon {
         dmap,
         options,
         matcher,
         ignore_fn,
         outcome: Mutex::new(outcome),
         ignore_patterns_have_changed: patterns_changed,
         new_cachable_directories: Default::default(),
         outated_cached_directories: Default::default(),
         filesystem_time_at_status_start,
     };
     let is_at_repo_root = true;
     let hg_path = &BorrowedPath::OnDisk(HgPath::new(""));
     let has_ignored_ancestor = HasIgnoredAncestor::create(None, hg_path);
     let root_cached_mtime = None;
-    let root_dir_metadata = None;
     // If the path we have for the repository root is a symlink, do follow it.
     // (As opposed to symlinks within the working directory which are not
     // followed, using `std::fs::symlink_metadata`.)
     common.traverse_fs_directory_and_dirstate(
         &has_ignored_ancestor,
         dmap.root.as_ref(),
         hg_path,
-        &root_dir,
-        root_dir_metadata,
+        &DirEntry {
+            hg_path: Cow::Borrowed(HgPath::new(b"")),
+            fs_path: Cow::Borrowed(&root_dir),
+            symlink_metadata: None,
+            file_type: FakeFileType::Directory,
+        },
         root_cached_mtime,
         is_at_repo_root,
     )?;
     let mut outcome = common.outcome.into_inner().unwrap();
     let new_cachable = common.new_cachable_directories.into_inner().unwrap();
     let outdated = common.outated_cached_directories.into_inner().unwrap();
 
     outcome.dirty = common.ignore_patterns_have_changed == Some(true)
         || !outdated.is_empty()
         || (!new_cachable.is_empty()
             && dmap.dirstate_version == DirstateVersion::V2);
 
     // Remove outdated mtimes before adding new mtimes, in case a given
     // directory is both
     for path in &outdated {
         dmap.clear_cached_mtime(path)?;
     }
     for (path, mtime) in &new_cachable {
         dmap.set_cached_mtime(path, *mtime)?;
     }
 
     Ok((outcome, warnings))
 }
 
 /// Bag of random things needed by various parts of the algorithm. Reduces the
 /// number of parameters passed to functions.
 struct StatusCommon<'a, 'tree, 'on_disk: 'tree> {
     dmap: &'tree DirstateMap<'on_disk>,
     options: StatusOptions,
     matcher: &'a (dyn Matcher + Sync),
     ignore_fn: IgnoreFnType<'a>,
     outcome: Mutex<DirstateStatus<'on_disk>>,
     new_cachable_directories:
         Mutex<Vec<(Cow<'on_disk, HgPath>, TruncatedTimestamp)>>,
     outated_cached_directories: Mutex<Vec<Cow<'on_disk, HgPath>>>,
 
     /// Whether ignore files like `.hgignore` have changed since the previous
     /// time a `status()` call wrote their hash to the dirstate. `None` means
     /// we don’t know as this run doesn’t list either ignored or uknown files
     /// and therefore isn’t reading `.hgignore`.
     ignore_patterns_have_changed: Option<bool>,
 
     /// The current time at the start of the `status()` algorithm, as measured
     /// and possibly truncated by the filesystem.
     filesystem_time_at_status_start: Option<TruncatedTimestamp>,
 }
 
 enum Outcome {
     Modified,
     Added,
     Removed,
     Deleted,
     Clean,
     Ignored,
     Unknown,
     Unsure,
 }
 
 /// Lazy computation of whether a given path has a hgignored
 /// ancestor.
 struct HasIgnoredAncestor<'a> {
     /// `path` and `parent` constitute the inputs to the computation,
     /// `cache` stores the outcome.
     path: &'a HgPath,
     parent: Option<&'a HasIgnoredAncestor<'a>>,
     cache: OnceCell<bool>,
 }
 
 impl<'a> HasIgnoredAncestor<'a> {
     fn create(
         parent: Option<&'a HasIgnoredAncestor<'a>>,
         path: &'a HgPath,
     ) -> HasIgnoredAncestor<'a> {
         Self {
             path,
             parent,
             cache: OnceCell::new(),
         }
     }
 
     fn force<'b>(&self, ignore_fn: &IgnoreFnType<'b>) -> bool {
         match self.parent {
             None => false,
             Some(parent) => {
                 *(parent.cache.get_or_init(|| {
                     parent.force(ignore_fn) || ignore_fn(&self.path)
                 }))
             }
         }
     }
 }
 
 impl<'a, 'tree, 'on_disk> StatusCommon<'a, 'tree, 'on_disk> {
     fn push_outcome(
         &self,
         which: Outcome,
         dirstate_node: &NodeRef<'tree, 'on_disk>,
     ) -> Result<(), DirstateV2ParseError> {
         let path = dirstate_node
             .full_path_borrowed(self.dmap.on_disk)?
             .detach_from_tree();
         let copy_source = if self.options.list_copies {
             dirstate_node
                 .copy_source_borrowed(self.dmap.on_disk)?
                 .map(|source| source.detach_from_tree())
         } else {
             None
         };
         self.push_outcome_common(which, path, copy_source);
         Ok(())
     }
 
     fn push_outcome_without_copy_source(
         &self,
         which: Outcome,
         path: &BorrowedPath<'_, 'on_disk>,
     ) {
         self.push_outcome_common(which, path.detach_from_tree(), None)
     }
 
     fn push_outcome_common(
         &self,
         which: Outcome,
         path: HgPathCow<'on_disk>,
         copy_source: Option<HgPathCow<'on_disk>>,
     ) {
         let mut outcome = self.outcome.lock().unwrap();
         let vec = match which {
             Outcome::Modified => &mut outcome.modified,
             Outcome::Added => &mut outcome.added,
             Outcome::Removed => &mut outcome.removed,
             Outcome::Deleted => &mut outcome.deleted,
             Outcome::Clean => &mut outcome.clean,
             Outcome::Ignored => &mut outcome.ignored,
             Outcome::Unknown => &mut outcome.unknown,
             Outcome::Unsure => &mut outcome.unsure,
         };
         vec.push(StatusPath { path, copy_source });
     }
 
     fn read_dir(
         &self,
         hg_path: &HgPath,
         fs_path: &Path,
         is_at_repo_root: bool,
     ) -> Result<Vec<DirEntry>, ()> {
         DirEntry::read_dir(fs_path, is_at_repo_root)
             .map_err(|error| self.io_error(error, hg_path))
     }
 
     fn io_error(&self, error: std::io::Error, hg_path: &HgPath) {
         let errno = error.raw_os_error().expect("expected real OS error");
         self.outcome
             .lock()
             .unwrap()
             .bad
             .push((hg_path.to_owned().into(), BadMatch::OsError(errno)))
     }
 
     fn check_for_outdated_directory_cache(
         &self,
         dirstate_node: &NodeRef<'tree, 'on_disk>,
     ) -> Result<(), DirstateV2ParseError> {
         if self.ignore_patterns_have_changed == Some(true)
             && dirstate_node.cached_directory_mtime()?.is_some()
         {
             self.outated_cached_directories.lock().unwrap().push(
                 dirstate_node
                     .full_path_borrowed(self.dmap.on_disk)?
                     .detach_from_tree(),
             )
         }
         Ok(())
     }
 
     /// If this returns true, we can get accurate results by only using
     /// `symlink_metadata` for child nodes that exist in the dirstate and don’t
     /// need to call `read_dir`.
     fn can_skip_fs_readdir(
         &self,
-        directory_metadata: Option<&std::fs::Metadata>,
+        directory_entry: &DirEntry,
         cached_directory_mtime: Option<TruncatedTimestamp>,
     ) -> bool {
         if !self.options.list_unknown && !self.options.list_ignored {
             // All states that we care about listing have corresponding
             // dirstate entries.
             // This happens for example with `hg status -mard`.
             return true;
         }
         if !self.options.list_ignored
             && self.ignore_patterns_have_changed == Some(false)
         {
             if let Some(cached_mtime) = cached_directory_mtime {
                 // The dirstate contains a cached mtime for this directory, set
                 // by a previous run of the `status` algorithm which found this
                 // directory eligible for `read_dir` caching.
-                if let Some(meta) = directory_metadata {
+                if let Ok(meta) = directory_entry.symlink_metadata() {
                     if cached_mtime
-                        .likely_equal_to_mtime_of(meta)
+                        .likely_equal_to_mtime_of(&meta)
                         .unwrap_or(false)
                     {
                         // The mtime of that directory has not changed
                         // since then, which means that the results of
                         // `read_dir` should also be unchanged.
                         return true;
                     }
                 }
             }
         }
         false
     }
 
     /// Returns whether all child entries of the filesystem directory have a
     /// corresponding dirstate node or are ignored.
     fn traverse_fs_directory_and_dirstate<'ancestor>(
         &self,
         has_ignored_ancestor: &'ancestor HasIgnoredAncestor<'ancestor>,
         dirstate_nodes: ChildNodesRef<'tree, 'on_disk>,
         directory_hg_path: &BorrowedPath<'tree, 'on_disk>,
-        directory_fs_path: &Path,
-        directory_metadata: Option<&std::fs::Metadata>,
+        directory_entry: &DirEntry,
         cached_directory_mtime: Option<TruncatedTimestamp>,
         is_at_repo_root: bool,
     ) -> Result<bool, DirstateV2ParseError> {
-        if self.can_skip_fs_readdir(directory_metadata, cached_directory_mtime)
-        {
+        if self.can_skip_fs_readdir(directory_entry, cached_directory_mtime) {
             dirstate_nodes
                 .par_iter()
                 .map(|dirstate_node| {
-                    let fs_path = directory_fs_path.join(get_path_from_bytes(
+                    let fs_path = &directory_entry.fs_path;
+                    let fs_path = fs_path.join(get_path_from_bytes(
                         dirstate_node.base_name(self.dmap.on_disk)?.as_bytes(),
                     ));
                     match std::fs::symlink_metadata(&fs_path) {
-                        Ok(fs_metadata) => self.traverse_fs_and_dirstate(
-                            &fs_path,
-                            &fs_metadata,
-                            dirstate_node,
-                            has_ignored_ancestor,
-                        ),
+                        Ok(fs_metadata) => {
+                            let file_type =
+                                match fs_metadata.file_type().try_into() {
+                                    Ok(file_type) => file_type,
+                                    Err(_) => return Ok(()),
+                                };
+                            let entry = DirEntry {
+                                hg_path: Cow::Borrowed(
+                                    dirstate_node
+                                        .full_path(&self.dmap.on_disk)?,
+                                ),
+                                fs_path: Cow::Borrowed(&fs_path),
+                                symlink_metadata: Some(fs_metadata),
+                                file_type,
+                            };
+                            self.traverse_fs_and_dirstate(
+                                &entry,
+                                dirstate_node,
+                                has_ignored_ancestor,
+                            )
+                        }
                         Err(e) if e.kind() == std::io::ErrorKind::NotFound => {
                             self.traverse_dirstate_only(dirstate_node)
                         }
                         Err(error) => {
                             let hg_path =
                                 dirstate_node.full_path(self.dmap.on_disk)?;
                             Ok(self.io_error(error, hg_path))
                         }
                     }
                 })
                 .collect::<Result<_, _>>()?;
 
             // We don’t know, so conservatively say this isn’t the case
             let children_all_have_dirstate_node_or_are_ignored = false;
 
             return Ok(children_all_have_dirstate_node_or_are_ignored);
         }
 
         let mut fs_entries = if let Ok(entries) = self.read_dir(
             directory_hg_path,
-            directory_fs_path,
+            &directory_entry.fs_path,
             is_at_repo_root,
         ) {
             entries
         } else {
             // Treat an unreadable directory (typically because of insufficient
             // permissions) like an empty directory. `self.read_dir` has
             // already called `self.io_error` so a warning will be emitted.
             Vec::new()
         };
 
         // `merge_join_by` requires both its input iterators to be sorted:
 
         let dirstate_nodes = dirstate_nodes.sorted();
         // `sort_unstable_by_key` doesn’t allow keys borrowing from the value:
         // https://github.com/rust-lang/rust/issues/34162
         fs_entries.sort_unstable_by(|e1, e2| e1.hg_path.cmp(&e2.hg_path));
 
         // Propagate here any error that would happen inside the comparison
         // callback below
         for dirstate_node in &dirstate_nodes {
             dirstate_node.base_name(self.dmap.on_disk)?;
         }
         itertools::merge_join_by(
             dirstate_nodes,
             &fs_entries,
             |dirstate_node, fs_entry| {
                 // This `unwrap` never panics because we already propagated
                 // those errors above
                 dirstate_node
                     .base_name(self.dmap.on_disk)
                     .unwrap()
                     .cmp(&fs_entry.hg_path)
             },
         )
         .par_bridge()
         .map(|pair| {
             use itertools::EitherOrBoth::*;
             let has_dirstate_node_or_is_ignored;
             match pair {
                 Both(dirstate_node, fs_entry) => {
                     self.traverse_fs_and_dirstate(
-                        &fs_entry.fs_path,
-                        &fs_entry.metadata,
+                        &fs_entry,
                         dirstate_node,
                         has_ignored_ancestor,
                     )?;
                     has_dirstate_node_or_is_ignored = true
                 }
                 Left(dirstate_node) => {
                     self.traverse_dirstate_only(dirstate_node)?;
                     has_dirstate_node_or_is_ignored = true;
                 }
                 Right(fs_entry) => {
                     has_dirstate_node_or_is_ignored = self.traverse_fs_only(
                         has_ignored_ancestor.force(&self.ignore_fn),
                         directory_hg_path,
                         fs_entry,
                     )
                 }
             }
             Ok(has_dirstate_node_or_is_ignored)
         })
         .try_reduce(|| true, |a, b| Ok(a && b))
     }
 
     fn traverse_fs_and_dirstate<'ancestor>(
         &self,
-        fs_path: &Path,
-        fs_metadata: &std::fs::Metadata,
+        fs_entry: &DirEntry,
         dirstate_node: NodeRef<'tree, 'on_disk>,
         has_ignored_ancestor: &'ancestor HasIgnoredAncestor<'ancestor>,
     ) -> Result<(), DirstateV2ParseError> {
         self.check_for_outdated_directory_cache(&dirstate_node)?;
         let hg_path = &dirstate_node.full_path_borrowed(self.dmap.on_disk)?;
-        let file_type = fs_metadata.file_type();
-        let file_or_symlink = file_type.is_file() || file_type.is_symlink();
+        let file_or_symlink = fs_entry.is_file() || fs_entry.is_symlink();
         if !file_or_symlink {
             // If we previously had a file here, it was removed (with
             // `hg rm` or similar) or deleted before it could be
             // replaced by a directory or something else.
             self.mark_removed_or_deleted_if_file(&dirstate_node)?;
         }
-        if file_type.is_dir() {
+        if fs_entry.is_dir() {
             if self.options.collect_traversed_dirs {
                 self.outcome
                     .lock()
                     .unwrap()
                     .traversed
                     .push(hg_path.detach_from_tree())
             }
             let is_ignored = HasIgnoredAncestor::create(
                 Some(&has_ignored_ancestor),
                 hg_path,
             );
             let is_at_repo_root = false;
             let children_all_have_dirstate_node_or_are_ignored = self
                 .traverse_fs_directory_and_dirstate(
                     &is_ignored,
                     dirstate_node.children(self.dmap.on_disk)?,
                     hg_path,
-                    fs_path,
-                    Some(fs_metadata),
+                    fs_entry,
                     dirstate_node.cached_directory_mtime()?,
                     is_at_repo_root,
                 )?;
             self.maybe_save_directory_mtime(
                 children_all_have_dirstate_node_or_are_ignored,
-                fs_metadata,
+                fs_entry,
                 dirstate_node,
             )?
         } else {
             if file_or_symlink && self.matcher.matches(&hg_path) {
                 if let Some(entry) = dirstate_node.entry()? {
                     if !entry.any_tracked() {
                         // Forward-compat if we start tracking unknown/ignored
                         // files for caching reasons
                         self.mark_unknown_or_ignored(
                             has_ignored_ancestor.force(&self.ignore_fn),
                             &hg_path,
                         );
                     }
                     if entry.added() {
                         self.push_outcome(Outcome::Added, &dirstate_node)?;
                     } else if entry.removed() {
                         self.push_outcome(Outcome::Removed, &dirstate_node)?;
                     } else if entry.modified() {
                         self.push_outcome(Outcome::Modified, &dirstate_node)?;
                     } else {
-                        self.handle_normal_file(&dirstate_node, fs_metadata)?;
+                        self.handle_normal_file(&dirstate_node, fs_entry)?;
                     }
                 } else {
                     // `node.entry.is_none()` indicates a "directory"
                     // node, but the filesystem has a file
                     self.mark_unknown_or_ignored(
                         has_ignored_ancestor.force(&self.ignore_fn),
                         hg_path,
                     );
                 }
             }
 
             for child_node in dirstate_node.children(self.dmap.on_disk)?.iter()
             {
                 self.traverse_dirstate_only(child_node)?
             }
         }
         Ok(())
     }
 
     fn maybe_save_directory_mtime(
         &self,
         children_all_have_dirstate_node_or_are_ignored: bool,
-        directory_metadata: &std::fs::Metadata,
+        directory_entry: &DirEntry,
         dirstate_node: NodeRef<'tree, 'on_disk>,
     ) -> Result<(), DirstateV2ParseError> {
         if !children_all_have_dirstate_node_or_are_ignored {
             return Ok(());
         }
         // All filesystem directory entries from `read_dir` have a
         // corresponding node in the dirstate, so we can reconstitute the
         // names of those entries without calling `read_dir` again.
 
         // TODO: use let-else here and below when available:
         // https://github.com/rust-lang/rust/issues/87335
         let status_start = if let Some(status_start) =
             &self.filesystem_time_at_status_start
         {
             status_start
         } else {
             return Ok(());
         };
 
         // Although the Rust standard library’s `SystemTime` type
         // has nanosecond precision, the times reported for a
         // directory’s (or file’s) modified time may have lower
         // resolution based on the filesystem (for example ext3
         // only stores integer seconds), kernel (see
         // https://stackoverflow.com/a/14393315/1162888), etc.
+        let metadata = match directory_entry.symlink_metadata() {
+            Ok(meta) => meta,
+            Err(_) => return Ok(()),
+        };
         let directory_mtime = if let Ok(option) =
-            TruncatedTimestamp::for_reliable_mtime_of(
-                directory_metadata,
-                status_start,
-            ) {
+            TruncatedTimestamp::for_reliable_mtime_of(&metadata, status_start)
+        {
             if let Some(directory_mtime) = option {
                 directory_mtime
             } else {
                 // The directory was modified too recently,
                 // don’t cache its `read_dir` results.
                 //
                 // 1. A change to this directory (direct child was
                 //    added or removed) cause its mtime to be set
                 //    (possibly truncated) to `directory_mtime`
                 // 2. This `status` algorithm calls `read_dir`
                 // 3. An other change is made to the same directory is
                 //    made so that calling `read_dir` agin would give
                 //    different results, but soon enough after 1. that
                 //    the mtime stays the same
                 //
                 // On a system where the time resolution poor, this
                 // scenario is not unlikely if all three steps are caused
                 // by the same script.
                 return Ok(());
             }
         } else {
             // OS/libc does not support mtime?
             return Ok(());
         };
         // We’ve observed (through `status_start`) that time has
         // “progressed” since `directory_mtime`, so any further
         // change to this directory is extremely likely to cause a
         // different mtime.
         //
         // Having the same mtime again is not entirely impossible
         // since the system clock is not monotonous. It could jump
         // backward to some point before `directory_mtime`, then a
         // directory change could potentially happen during exactly
         // the wrong tick.
         //
         // We deem this scenario (unlike the previous one) to be
         // unlikely enough in practice.
 
         let is_up_to_date =
             if let Some(cached) = dirstate_node.cached_directory_mtime()? {
                 cached.likely_equal(directory_mtime)
             } else {
                 false
             };
         if !is_up_to_date {
             let hg_path = dirstate_node
                 .full_path_borrowed(self.dmap.on_disk)?
                 .detach_from_tree();
             self.new_cachable_directories
                 .lock()
                 .unwrap()
                 .push((hg_path, directory_mtime))
         }
         Ok(())
     }
 
     /// A file that is clean in the dirstate was found in the filesystem
     fn handle_normal_file(
         &self,
         dirstate_node: &NodeRef<'tree, 'on_disk>,
-        fs_metadata: &std::fs::Metadata,
+        fs_entry: &DirEntry,
     ) -> Result<(), DirstateV2ParseError> {
         // Keep the low 31 bits
         fn truncate_u64(value: u64) -> i32 {
             (value & 0x7FFF_FFFF) as i32
         }
 
+        let fs_metadata = match fs_entry.symlink_metadata() {
+            Ok(meta) => meta,
+            Err(_) => return Ok(()),
+        };
+
         let entry = dirstate_node
             .entry()?
             .expect("handle_normal_file called with entry-less node");
         let mode_changed =
-            || self.options.check_exec && entry.mode_changed(fs_metadata);
+            || self.options.check_exec && entry.mode_changed(&fs_metadata);
         let size = entry.size();
         let size_changed = size != truncate_u64(fs_metadata.len());
         if size >= 0 && size_changed && fs_metadata.file_type().is_symlink() {
             // issue6456: Size returned may be longer due to encryption
             // on EXT-4 fscrypt. TODO maybe only do it on EXT4?
             self.push_outcome(Outcome::Unsure, dirstate_node)?
         } else if dirstate_node.has_copy_source()
             || entry.is_from_other_parent()
             || (size >= 0 && (size_changed || mode_changed()))
         {
             self.push_outcome(Outcome::Modified, dirstate_node)?
         } else {
             let mtime_looks_clean;
             if let Some(dirstate_mtime) = entry.truncated_mtime() {
-                let fs_mtime = TruncatedTimestamp::for_mtime_of(fs_metadata)
+                let fs_mtime = TruncatedTimestamp::for_mtime_of(&fs_metadata)
                     .expect("OS/libc does not support mtime?");
                 // There might be a change in the future if for example the
                 // internal clock become off while process run, but this is a
                 // case where the issues the user would face
                 // would be a lot worse and there is nothing we
                 // can really do.
                 mtime_looks_clean = fs_mtime.likely_equal(dirstate_mtime)
             } else {
                 // No mtime in the dirstate entry
                 mtime_looks_clean = false
             };
             if !mtime_looks_clean {
                 self.push_outcome(Outcome::Unsure, dirstate_node)?
             } else if self.options.list_clean {
                 self.push_outcome(Outcome::Clean, dirstate_node)?
             }
         }
         Ok(())
     }
 
     /// A node in the dirstate tree has no corresponding filesystem entry
     fn traverse_dirstate_only(
         &self,
         dirstate_node: NodeRef<'tree, 'on_disk>,
     ) -> Result<(), DirstateV2ParseError> {
         self.check_for_outdated_directory_cache(&dirstate_node)?;
         self.mark_removed_or_deleted_if_file(&dirstate_node)?;
         dirstate_node
             .children(self.dmap.on_disk)?
             .par_iter()
             .map(|child_node| self.traverse_dirstate_only(child_node))
             .collect()
     }
 
     /// A node in the dirstate tree has no corresponding *file* on the
     /// filesystem
     ///
     /// Does nothing on a "directory" node
     fn mark_removed_or_deleted_if_file(
         &self,
         dirstate_node: &NodeRef<'tree, 'on_disk>,
     ) -> Result<(), DirstateV2ParseError> {
         if let Some(entry) = dirstate_node.entry()? {
             if !entry.any_tracked() {
                 // Future-compat for when we start storing ignored and unknown
                 // files for caching reasons
                 return Ok(());
             }
             let path = dirstate_node.full_path(self.dmap.on_disk)?;
             if self.matcher.matches(path) {
                 if entry.removed() {
                     self.push_outcome(Outcome::Removed, dirstate_node)?
                 } else {
                     self.push_outcome(Outcome::Deleted, &dirstate_node)?
                 }
             }
         }
         Ok(())
     }
 
     /// Something in the filesystem has no corresponding dirstate node
     ///
     /// Returns whether that path is ignored
     fn traverse_fs_only(
         &self,
         has_ignored_ancestor: bool,
         directory_hg_path: &HgPath,
         fs_entry: &DirEntry,
     ) -> bool {
         let hg_path = directory_hg_path.join(&fs_entry.hg_path);
-        let file_type = fs_entry.metadata.file_type();
-        let file_or_symlink = file_type.is_file() || file_type.is_symlink();
-        if file_type.is_dir() {
+        let file_or_symlink = fs_entry.is_file() || fs_entry.is_symlink();
+        if fs_entry.is_dir() {
             let is_ignored =
                 has_ignored_ancestor || (self.ignore_fn)(&hg_path);
             let traverse_children = if is_ignored {
                 // Descendants of an ignored directory are all ignored
                 self.options.list_ignored
             } else {
                 // Descendants of an unknown directory may be either unknown or
                 // ignored
                 self.options.list_unknown || self.options.list_ignored
             };
             if traverse_children {
                 let is_at_repo_root = false;
-                if let Ok(children_fs_entries) = self.read_dir(
-                    &hg_path,
-                    &fs_entry.fs_path,
-                    is_at_repo_root,
-                ) {
+                if let Ok(children_fs_entries) =
+                    self.read_dir(&hg_path, &fs_entry.fs_path, is_at_repo_root)
+                {
                     children_fs_entries.par_iter().for_each(|child_fs_entry| {
                         self.traverse_fs_only(
                             is_ignored,
                             &hg_path,
                             child_fs_entry,
                         );
                     })
                 }
                 if self.options.collect_traversed_dirs {
                     self.outcome.lock().unwrap().traversed.push(hg_path.into())
                 }
             }
             is_ignored
         } else {
             if file_or_symlink {
                 if self.matcher.matches(&hg_path) {
                     self.mark_unknown_or_ignored(
                         has_ignored_ancestor,
                         &BorrowedPath::InMemory(&hg_path),
                     )
                 } else {
                     // We haven’t computed whether this path is ignored. It
                     // might not be, and a future run of status might have a
                     // different matcher that matches it. So treat it as not
                     // ignored. That is, inhibit readdir caching of the parent
                     // directory.
                     false
                 }
             } else {
                 // This is neither a directory, a plain file, or a symlink.
                 // Treat it like an ignored file.
                 true
             }
         }
     }
 
     /// Returns whether that path is ignored
     fn mark_unknown_or_ignored(
         &self,
         has_ignored_ancestor: bool,
         hg_path: &BorrowedPath<'_, 'on_disk>,
     ) -> bool {
         let is_ignored = has_ignored_ancestor || (self.ignore_fn)(&hg_path);
         if is_ignored {
             if self.options.list_ignored {
                 self.push_outcome_without_copy_source(
                     Outcome::Ignored,
                     hg_path,
                 )
             }
         } else {
             if self.options.list_unknown {
                 self.push_outcome_without_copy_source(
                     Outcome::Unknown,
                     hg_path,
                 )
             }
         }
         is_ignored
     }
 }
 
-struct DirEntry {
-    /// Path as stored in the dirstate
-    hg_path: HgPathBuf,
-    /// Filesystem path
-    fs_path: PathBuf,
-    metadata: std::fs::Metadata,
+/// Since [`std::fs::FileType`] cannot be built directly, we emulate what we
+/// care about.
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+enum FakeFileType {
+    File,
+    Directory,
+    Symlink,
 }
 
-impl DirEntry {
-    /// Returns **unsorted** entries in the given directory, with name and
-    /// metadata.
+impl TryFrom<std::fs::FileType> for FakeFileType {
+    type Error = ();
+
+    fn try_from(f: std::fs::FileType) -> Result<Self, Self::Error> {
+        if f.is_dir() {
+            Ok(Self::Directory)
+        } else if f.is_file() {
+            Ok(Self::File)
+        } else if f.is_symlink() {
+            Ok(Self::Symlink)
+        } else {
+            // Things like FIFO etc.
+            Err(())
+        }
+    }
+}
+
+struct DirEntry<'a> {
+    /// Path as stored in the dirstate, or just the filename for optimization.
+    hg_path: HgPathCow<'a>,
+    /// Filesystem path
+    fs_path: Cow<'a, Path>,
+    /// Lazily computed
+    symlink_metadata: Option<std::fs::Metadata>,
+    /// Already computed for ergonomics.
+    file_type: FakeFileType,
+}
+
+impl<'a> DirEntry<'a> {
+    /// Returns **unsorted** entries in the given directory, with name,
+    /// metadata and file type.
     ///
     /// If a `.hg` sub-directory is encountered:
     ///
     /// * At the repository root, ignore that sub-directory
     /// * Elsewhere, we’re listing the content of a sub-repo. Return an empty
     ///   list instead.
     fn read_dir(path: &Path, is_at_repo_root: bool) -> io::Result<Vec<Self>> {
         // `read_dir` returns a "not found" error for the empty path
         let at_cwd = path == Path::new("");
         let read_dir_path = if at_cwd { Path::new(".") } else { path };
         let mut results = Vec::new();
         for entry in read_dir_path.read_dir()? {
             let entry = entry?;
-            let metadata = match entry.metadata() {
+            let file_type = match entry.file_type() {
                 Ok(v) => v,
                 Err(e) => {
                     // race with file deletion?
                     if e.kind() == std::io::ErrorKind::NotFound {
                         continue;
                     } else {
                         return Err(e);
                     }
                 }
             };
             let file_name = entry.file_name();
             // FIXME don't do this when cached
             if file_name == ".hg" {
                 if is_at_repo_root {
                     // Skip the repo’s own .hg (might be a symlink)
                     continue;
-                } else if metadata.is_dir() {
+                } else if file_type.is_dir() {
                     // A .hg sub-directory at another location means a subrepo,
                     // skip it entirely.
                     return Ok(Vec::new());
                 }
             }
             let full_path = if at_cwd {
                 file_name.clone().into()
             } else {
                 entry.path()
             };
-            let base_name = get_bytes_from_os_string(file_name).into();
+            let filename =
+                Cow::Owned(get_bytes_from_os_string(file_name).into());
+            let file_type = match FakeFileType::try_from(file_type) {
+                Ok(file_type) => file_type,
+                Err(_) => continue,
+            };
             results.push(DirEntry {
-                hg_path: base_name,
-                fs_path: full_path,
-                metadata,
+                hg_path: filename,
+                fs_path: Cow::Owned(full_path.to_path_buf()),
+                symlink_metadata: None,
+                file_type,
             })
         }
         Ok(results)
     }
+
+    fn symlink_metadata(&self) -> Result<std::fs::Metadata, std::io::Error> {
+        match &self.symlink_metadata {
+            Some(meta) => Ok(meta.clone()),
+            None => std::fs::symlink_metadata(&self.fs_path),
+        }
+    }
+
+    fn is_dir(&self) -> bool {
+        self.file_type == FakeFileType::Directory
+    }
+
+    fn is_file(&self) -> bool {
+        self.file_type == FakeFileType::File
+    }
+
+    fn is_symlink(&self) -> bool {
+        self.file_type == FakeFileType::Symlink
+    }
 }
 
 /// Return the `mtime` of a temporary file newly-created in the `.hg` directory
 /// of the give repository.
 ///
 /// This is similar to `SystemTime::now()`, with the result truncated to the
 /// same time resolution as other files’ modification times. Using `.hg`
 /// instead of the system’s default temporary directory (such as `/tmp`) makes
 /// it more likely the temporary file is in the same disk partition as contents
 /// of the working directory, which can matter since different filesystems may
 /// store timestamps with different resolutions.
 ///
 /// This may fail, typically if we lack write permissions. In that case we
 /// should continue the `status()` algoritm anyway and consider the current
 /// date/time to be unknown.
 fn filesystem_now(repo_root: &Path) -> Result<SystemTime, io::Error> {
     tempfile::tempfile_in(repo_root.join(".hg"))?
         .metadata()?
         .modified()
 }