stream-clone: factor computation of new clone requirement out
This gather code duplicated in multiple place and will make it easier to modify
it safely in the future.
Differential Revision: https://phab.mercurial-scm.org/D12027
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::NodeData;
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::EntryState;
use crate::HgPathBuf;
use crate::HgPathCow;
use crate::PatternFileWarning;
use crate::StatusError;
use crate::StatusOptions;
use micro_timer::timed;
use rayon::prelude::*;
use sha1::{Digest, Sha1};
use std::borrow::Cow;
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<'tree, 'on_disk: 'tree>(
dmap: &'tree mut DirstateMap<'on_disk>,
matcher: &(dyn Matcher + Sync),
root_dir: PathBuf,
ignore_files: Vec<PathBuf>,
options: StatusOptions,
) -> Result<(DirstateStatus<'on_disk>, Vec<PatternFileWarning>), StatusError> {
let (ignore_fn, warnings, patterns_changed): (IgnoreFnType, _, _) =
if options.list_ignored || options.list_unknown {
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))
} else {
(Box::new(|&_| true), vec![], None)
};
let filesystem_time_at_status_start =
filesystem_now(&root_dir).ok().map(TruncatedTimestamp::from);
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 = false;
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,
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();
// Remove outdated mtimes before adding new mtimes, in case a given
// directory is both
for path in &outdated {
let node = dmap.get_or_insert(path)?;
if let NodeData::CachedDirectory { .. } = &node.data {
node.data = NodeData::None
}
}
for (path, mtime) in &new_cachable {
let node = dmap.get_or_insert(path)?;
match &node.data {
NodeData::Entry(_) => {} // Don’t overwrite an entry
NodeData::CachedDirectory { .. } | NodeData::None => {
node.data = NodeData::CachedDirectory { mtime: *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,
}
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>,
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 cached_mtime
.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(
&self,
has_ignored_ancestor: bool,
dirstate_nodes: ChildNodesRef<'tree, 'on_disk>,
directory_hg_path: &BorrowedPath<'tree, 'on_disk>,
directory_fs_path: &Path,
directory_metadata: Option<&std::fs::Metadata>,
cached_directory_mtime: Option<TruncatedTimestamp>,
is_at_repo_root: bool,
) -> Result<bool, DirstateV2ParseError> {
if self.can_skip_fs_readdir(directory_metadata, cached_directory_mtime)
{
dirstate_nodes
.par_iter()
.map(|dirstate_node| {
let fs_path = directory_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,
),
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,
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.base_name.cmp(&e2.base_name));
// 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.base_name)
},
)
.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.full_path,
&fs_entry.metadata,
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,
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(
&self,
fs_path: &Path,
fs_metadata: &std::fs::Metadata,
dirstate_node: NodeRef<'tree, 'on_disk>,
has_ignored_ancestor: bool,
) -> 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();
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 self.options.collect_traversed_dirs {
self.outcome
.lock()
.unwrap()
.traversed
.push(hg_path.detach_from_tree())
}
let is_ignored = has_ignored_ancestor || (self.ignore_fn)(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),
dirstate_node.cached_directory_mtime()?,
is_at_repo_root,
)?;
self.maybe_save_directory_mtime(
children_all_have_dirstate_node_or_are_ignored,
fs_metadata,
dirstate_node,
)?
} else {
if file_or_symlink && self.matcher.matches(hg_path) {
if let Some(state) = dirstate_node.state()? {
match state {
EntryState::Added => {
self.push_outcome(Outcome::Added, &dirstate_node)?
}
EntryState::Removed => self
.push_outcome(Outcome::Removed, &dirstate_node)?,
EntryState::Merged => self
.push_outcome(Outcome::Modified, &dirstate_node)?,
EntryState::Normal => self
.handle_normal_file(&dirstate_node, fs_metadata)?,
}
} else {
// `node.entry.is_none()` indicates a "directory"
// node, but the filesystem has a file
self.mark_unknown_or_ignored(
has_ignored_ancestor,
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,
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 directory_mtime = if let Ok(option) =
TruncatedTimestamp::for_reliable_mtime_of(
directory_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 with `EntryState::Normal` in the dirstate was found in the
/// filesystem
fn handle_normal_file(
&self,
dirstate_node: &NodeRef<'tree, 'on_disk>,
fs_metadata: &std::fs::Metadata,
) -> Result<(), DirstateV2ParseError> {
// Keep the low 31 bits
fn truncate_u64(value: u64) -> i32 {
(value & 0x7FFF_FFFF) as i32
}
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);
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)
.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(state) = dirstate_node.state()? {
let path = dirstate_node.full_path(self.dmap.on_disk)?;
if self.matcher.matches(path) {
if let EntryState::Removed = state {
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.base_name);
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 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.full_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 {
base_name: HgPathBuf,
full_path: PathBuf,
metadata: std::fs::Metadata,
}
impl DirEntry {
/// Returns **unsorted** entries in the given directory, with name and
/// metadata.
///
/// 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>> {
let mut results = Vec::new();
for entry in path.read_dir()? {
let entry = entry?;
let metadata = entry.metadata()?;
let name = get_bytes_from_os_string(entry.file_name());
// FIXME don't do this when cached
if name == b".hg" {
if is_at_repo_root {
// Skip the repo’s own .hg (might be a symlink)
continue;
} else if metadata.is_dir() {
// A .hg sub-directory at another location means a subrepo,
// skip it entirely.
return Ok(Vec::new());
}
}
results.push(DirEntry {
base_name: name.into(),
full_path: entry.path(),
metadata,
})
}
Ok(results)
}
}
/// 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()
}