vfs: make atomictempfile avoid ambiguity of file stat if needed This patch is a part of preparation for "Exact Cache Validation Plan": https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan

util: make atomictempfile avoid ambiguity of file stat if needed Ambiguity check is executed at close(), only if: - atomictempfile is created with checkambig=True, and - target file exists before renaming This restriction avoids performance decrement by needless examination of file stat (for example, filelog doesn't need exact cache validation, even though it uses atomictempfile to write changes out). See description of filestat class for detail about why the logic in this patch works as expected. This patch is a part of preparation for "Exact Cache Validation Plan": https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan

util: add filestat class to detect ambiguity of file stat Current posix.cachestat implementation might overlook change of a file, if changing keeps ctime, mtime and size of file. Comparison of inode number also overlooks changing in such situation, because inode number is rapidly reused. Contents of a file cached before changing isn't invalidated as expected, if change of a file is overlooked for this "ambiguity" of file stat. This patch adds filestat class to detect ambiguity of file stat. This patch is a part of preparation for "Exact Cache Validation Plan": https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan