update-index: refresh should rewrite index in case of racy timestamps

This patch makes update-index --refresh write the index if it contains racy timestamps, as discussed at [1].

Changes since v3:

* test-lib: improve API for verifying file mtime
* fix quoting around "$1"
* use "rm -f" for cleanup of auxiliary files
* improve API description comments
* Note that gitgitgadget's "freebsd_12" check is failing since a couple of days (unrelated to this pull request); hence, this check hasn't been applied to this patch series

Changes since v2:

* new patch: test-lib: introduce API for verifying file mtime
* new patch: t7508: fix bogus mtime verification for test "--no-optional-locks prevents index update"
* change new tests in t2108 and t7508 to use new test-lib mtime API
* fix "--refresh has no racy timestamps to fix" to use +60s mtime to be save on VFAT

Changes since v1:

* main commit message now uses 'git update-index' and the paragraph was dropped
* t/t7508-status.sh: two tests added which capture status racy handling
* builtin/update-index.c: comment improved
* t/t2108-update-index-refresh-racy.sh: major overhaul
* one test case added
* mtime-manipulations simplified and aligned to t7508
* code style fixes, as discussed

[1]
https://lore.kernel.org/git/[email protected]/

Marc Strapetz (4):
test-lib: introduce API for verifying file mtime
t7508: fix bogus mtime verification
t7508: add tests capturing racy timestamp handling
update-index: refresh should rewrite index in case of racy timestamps

 builtin/update-index.c               | 11 +++++
 cache.h                              |  1 +
 read-cache.c                         |  2 +-
 t/t2108-update-index-refresh-racy.sh | 64 ++++++++++++++++++++++++++++
 t/t7508-status.sh                    | 30 ++++++++++---
 t/test-lib-functions.sh              | 33 ++++++++++++++
 6 files changed, 135 insertions(+), 6 deletions(-)
 create mode 100755 t/t2108-update-index-refresh-racy.sh

base-commit: dcc0cd074f0c639a0df20461a301af6d45bd582e
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1105%2Fmstrap%2Ffeature%2Fupdate-index-refresh-v4
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1105/mstrap/feature/update-index-refresh-v4
Pull-Request: https://github.com/gitgitgadget/git/pull/1105

Range-diff vs v3:

1: e6301e9d770 ! 1: 37c11bfafc4 test-lib: introduce API for verifying file mtime

     @@ t/test-lib-functions.sh: test_region () {
       	perl -le 'print readlink($_) for @ARGV' "$@"
       }
      +
     -+# Set a fixed "magic" mtime to the given file,
     -+# with an optional increment specified as second argument.
     -+# Use in combination with test_is_magic_mtime.
     ++# Set mtime to a fixed "magic" timestamp in mid February 2009, before we
     ++# run an operation that may or may not touch the file.  If the file was
     ++# touched, its timestamp will not accidentally have such an old timestamp,
     ++# as long as your filesystem clock is reasonably correct.  To verify the
     ++# timestamp, follow up with test_is_magic_mtime.
     ++#
     ++# An optional increment to the magic timestamp may be specified as second
     ++# argument.
      +test_set_magic_mtime () {
     -+	# We are using 1234567890 because it's a common timestamp used in
     -+	# various tests. It represents date 2009-02-13 which should be safe
     -+	# to use as long as the filetime clock is reasonably accurate.
      +	local inc=${2:-0} &&
      +	local mtime=$((1234567890 + $inc)) &&
     -+	test-tool chmtime =$mtime $1 &&
     -+	test_is_magic_mtime $1 $inc
     ++	test-tool chmtime =$mtime "$1" &&
     ++	test_is_magic_mtime "$1" $inc
      +}
      +
     -+# Test whether the given file has the "magic" mtime set,
     -+# with an optional increment specified as second argument.
     -+# Use in combination with test_set_magic_mtime.
     ++# Test whether the given file has the "magic" mtime set.  This is meant to
     ++# be used in combination with test_set_magic_mtime.
     ++#
     ++# An optional increment to the magic timestamp may be specified as second
     ++# argument.  Usually, this should be the same increment which was used for
     ++# the associated test_set_magic_mtime.
      +test_is_magic_mtime () {
      +	local inc=${2:-0} &&
      +	local mtime=$((1234567890 + $inc)) &&
      +	echo $mtime >.git/test-mtime-expect &&
     -+	test-tool chmtime --get $1 >.git/test-mtime-actual &&
     ++	test-tool chmtime --get "$1" >.git/test-mtime-actual &&
      +	test_cmp .git/test-mtime-expect .git/test-mtime-actual
      +	local ret=$?
     -+	rm .git/test-mtime-expect
     -+	rm .git/test-mtime-actual
     ++	rm -f .git/test-mtime-expect
     ++	rm -f .git/test-mtime-actual
      +	return $ret
      +}
 2:  d15a23cc804 = 2:  c97a41af389 t7508: fix bogus mtime verification
 3:  3567ef91e7a = 3:  82d0b6ab8d2 t7508: add tests capturing racy timestamp handling
 4:  4a6b18fb304 = 4:  e31edb74e24 update-index: refresh should rewrite index in case of racy timestamps

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