dirstate: Document Timestamp.second_ambiguous

Differential Revision: https://phab.mercurial-scm.org/D11888

mercurial/dirstateutils/timestamp.py

@@ -1,124 +1,128 b''
 # Copyright 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.
 
 from __future__ import absolute_import
 
 import functools
 import os
 import stat
 
 from .. import error
 
 
 rangemask = 0x7FFFFFFF
 
 
 @functools.total_ordering
 class timestamp(tuple):
     """
     A Unix timestamp with optional nanoseconds precision,
     modulo 2**31 seconds.
 
-    A 2-tuple containing:
+    A 3-tuple containing:
 
     `truncated_seconds`: seconds since the Unix epoch,
     truncated to its lower 31 bits
 
     `subsecond_nanoseconds`: number of nanoseconds since `truncated_seconds`.
     When this is zero, the sub-second precision is considered unknown.
+
+    `second_ambiguous`: whether this timestamp is still "reliable"
+    (see `reliable_mtime_of`) if we drop its sub-second component.
     """
 
     def __new__(cls, value):
         truncated_seconds, subsec_nanos, second_ambiguous = value
         value = (truncated_seconds & rangemask, subsec_nanos, second_ambiguous)
         return super(timestamp, cls).__new__(cls, value)
 
     def __eq__(self, other):
         raise error.ProgrammingError(
             'timestamp should never be compared directly'
         )
 
     def __gt__(self, other):
         raise error.ProgrammingError(
             'timestamp should never be compared directly'
         )
 
 
 def get_fs_now(vfs):
     """return a timestamp for "now" in the current vfs
 
     This will raise an exception if no temporary files could be created.
     """
     tmpfd, tmpname = vfs.mkstemp()
     try:
         return mtime_of(os.fstat(tmpfd))
     finally:
         os.close(tmpfd)
         vfs.unlink(tmpname)
 
 
 def zero():
     """
     Returns the `timestamp` at the Unix epoch.
     """
     return tuple.__new__(timestamp, (0, 0))
 
 
 def mtime_of(stat_result):
     """
     Takes an `os.stat_result`-like object and returns a `timestamp` object
     for its modification time.
     """
     try:
         # TODO: add this attribute to `osutil.stat` objects,
         # see `mercurial/cext/osutil.c`.
         #
         # This attribute is also not available on Python 2.
         nanos = stat_result.st_mtime_ns
     except AttributeError:
         # https://docs.python.org/2/library/os.html#os.stat_float_times
         # "For compatibility with older Python versions,
         #  accessing stat_result as a tuple always returns integers."
         secs = stat_result[stat.ST_MTIME]
 
         subsec_nanos = 0
     else:
         billion = int(1e9)
         secs = nanos // billion
         subsec_nanos = nanos % billion
 
     return timestamp((secs, subsec_nanos, False))
 
 
 def reliable_mtime_of(stat_result, present_mtime):
-    """same as `mtime_of`, but return None if the date might be ambiguous
+    """Same as `mtime_of`, but return `None` or a `Timestamp` with
+    `second_ambiguous` set if the date might be ambiguous.
 
     A modification time is reliable if it is older than "present_time" (or
     sufficiently in the future).
 
     Otherwise a concurrent modification might happens with the same mtime.
     """
     file_mtime = mtime_of(stat_result)
     file_second = file_mtime[0]
     file_ns = file_mtime[1]
     boundary_second = present_mtime[0]
     boundary_ns = present_mtime[1]
     # If the mtime of the ambiguous file is younger (or equal) to the starting
     # point of the `status` walk, we cannot garantee that another, racy, write
     # will not happen right after with the same mtime and we cannot cache the
     # information.
     #
     # However if the mtime is far away in the future, this is likely some
     # mismatch between the current clock and previous file system operation. So
     # mtime more than one days in the future are considered fine.
     if boundary_second == file_second:
         if file_ns and boundary_ns:
             if file_ns < boundary_ns:
                 return timestamp((file_second, file_ns, True))
         return None
     elif boundary_second < file_second < (3600 * 24 + boundary_second):
         return None
     else:
         return file_mtime