Use microsecond-resolution timestamps for outdated file detection (#1…

…1435)

Co-authored-by: Adam Turner <9087854+aa-turner@users.noreply.github.com>

sphinx/builders/__init__.py

@@ -496,11 +496,7 @@ def read_doc(self, docname: str, *, _cache: bool = True) -> None:
             doctree = publisher.document
 
         # store time of reading, for outdated files detection
-        # (Some filesystems have coarse timestamp resolution;
-        # therefore time.time() can be older than filesystem's timestamp.
-        # For example, FAT32 has 2sec timestamp resolution.)
-        self.env.all_docs[docname] = max(time.time(),
-                                         path.getmtime(self.env.doc2path(docname)))
+        self.env.all_docs[docname] = time.time_ns() // 1_000
 
         # cleanup
         self.env.temp_data.clear()

sphinx/environment/__init__.py

@@ -7,7 +7,7 @@
 import pickle
 from collections import defaultdict
 from copy import copy
-from datetime import datetime
+from datetime import datetime, timezone
 from os import path
 from typing import TYPE_CHECKING, Any, Callable, Generator, Iterator
 
@@ -55,7 +55,7 @@
 
 # This is increased every time an environment attribute is added
 # or changed to properly invalidate pickle files.
-ENV_VERSION = 57
+ENV_VERSION = 58
 
 # config status
 CONFIG_OK = 1
@@ -166,9 +166,9 @@ def __init__(self, app: Sphinx):
         # All "docnames" here are /-separated and relative and exclude
         # the source suffix.
 
-        # docname -> mtime at the time of reading
+        # docname -> time of reading (in integer microseconds)
         # contains all read docnames
-        self.all_docs: dict[str, float] = {}
+        self.all_docs: dict[str, int] = {}
         # docname -> set of dependent file
         # names, relative to documentation root
         self.dependencies: dict[str, set[str]] = defaultdict(set)
@@ -481,12 +481,14 @@ def get_outdated_files(self, config_changed: bool) -> tuple[set[str], set[str],
                     continue
                 # check the mtime of the document
                 mtime = self.all_docs[docname]
-                newmtime = path.getmtime(self.doc2path(docname))
+                newmtime = _last_modified_time(self.doc2path(docname))
                 if newmtime > mtime:
+                    # convert integer microseconds to floating-point seconds,
+                    # and then to timezone-aware datetime objects.
+                    mtime_dt = datetime.fromtimestamp(mtime / 1_000_000, tz=timezone.utc)
+                    newmtime_dt = datetime.fromtimestamp(mtime / 1_000_000, tz=timezone.utc)
                     logger.debug('[build target] outdated %r: %s -> %s',
-                                 docname,
-                                 datetime.utcfromtimestamp(mtime),
-                                 datetime.utcfromtimestamp(newmtime))
+                                 docname, mtime_dt, newmtime_dt)
                     changed.add(docname)
                     continue
                 # finally, check the mtime of dependencies
@@ -497,7 +499,7 @@ def get_outdated_files(self, config_changed: bool) -> tuple[set[str], set[str],
                         if not path.isfile(deppath):
                             changed.add(docname)
                             break
-                        depmtime = path.getmtime(deppath)
+                        depmtime = _last_modified_time(deppath)
                         if depmtime > mtime:
                             changed.add(docname)
                             break
@@ -728,3 +730,18 @@ def check_consistency(self) -> None:
         for domain in self.domains.values():
             domain.check_consistency()
         self.events.emit('env-check-consistency', self)
+
+
+def _last_modified_time(filename: str | os.PathLike[str]) -> int:
+    """Return the last modified time of ``filename``.
+
+    The time is returned as integer microseconds.
+    The lowest common denominator of modern file-systems seems to be
+    microsecond-level precision.
+
+    We prefer to err on the side of re-rendering a file,
+    so we round up to the nearest microsecond.
+    """
+
+    # upside-down floor division to get the ceiling
+    return -(os.stat(filename).st_mtime_ns // -1_000)