util: make copyfile avoid ambiguity of file stat if needed
In some cases below, copying from backup is used to restore original
contents of a file. If copying keeps ctime, mtime and size of a file,
restoring is overlooked, and old contents cached before restoring
isn't invalidated as expected.
- failure of transaction before closing (from '.hg/journal.backup.*')
- rollback of previous transaction (from '.hg/undo.backup.*')
To avoid such problem, this patch makes copyfile() avoid ambiguity of
file stat, if needed.
Ambiguity check is executed, only if:
- checkambig=True is specified (not all copying needs ambiguity check), and
- destination file exists before copying
This patch also adds 'not (copystat and checkambig)' assertion,
because combination of copystat and checkambig is meaningless.
This patch is a part of preparation for "Exact Cache Validation Plan":
https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan
Mercurial
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$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing:
$ make local # build for inplace usage
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See https://mercurial-scm.org/ for detailed installation
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