[PATCH v3 0/4] 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 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/d3dd805c-7c1d-30a9-6574-a7bfcb7fc013@syntevo.com/

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              | 28 ++++++++++++
 6 files changed, 130 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-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1105/mstrap/feature/update-index-refresh-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/1105

Range-diff vs v2:

 -:  ----------- > 1:  e6301e9d770 test-lib: introduce API for verifying file mtime
 -:  ----------- > 2:  d15a23cc804 t7508: fix bogus mtime verification
 1:  7d58f806111 ! 3:  3567ef91e7a t7508: add tests capturing racy timestamp handling
     @@ Commit message
      
       ## t/t7508-status.sh ##
      @@ t/t7508-status.sh: test_expect_success '--no-optional-locks prevents index update' '
     - 	! grep ^1234567890 out
     + 	! test_is_magic_mtime .git/index
       '
       
      +test_expect_success 'racy timestamps will be fixed for clean worktree' '
     @@ t/t7508-status.sh: test_expect_success '--no-optional-locks prevents index updat
      +	# no more index writes unless caused by racy timestamps; note that
      +	# timestamps may already be racy now (depending on previous tests)
      +	git status &&
     -+	test-tool chmtime =1234567890 .git/index &&
     -+	test-tool chmtime --get .git/index >out &&
     -+	grep ^1234567890 out &&
     ++	test_set_magic_mtime .git/index &&
      +	git status &&
     -+	test-tool chmtime --get .git/index >out &&
     -+	! grep ^1234567890 out
     ++	! test_is_magic_mtime .git/index
      +'
      +
      +test_expect_success 'racy timestamps will be fixed for dirty worktree' '
      +	echo content2 >racy-dirty &&
      +	git status &&
     -+	test-tool chmtime =1234567890 .git/index &&
     -+	test-tool chmtime --get .git/index >out &&
     -+	grep ^1234567890 out &&
     ++	test_set_magic_mtime .git/index &&
      +	git status &&
     -+	test-tool chmtime --get .git/index >out &&
     -+	! grep ^1234567890 out
     ++	! test_is_magic_mtime .git/index
      +'
      +
       test_done
 2:  dfeabf6af15 ! 4:  4a6b18fb304 update-index: refresh should rewrite index in case of racy timestamps
     @@ t/t2108-update-index-refresh-racy.sh (new)
      +reset_files () {
      +	echo content >file &&
      +	echo content >other &&
     -+	test-tool chmtime =1234567890 file &&
     -+	test-tool chmtime =1234567890 other
     ++	test_set_magic_mtime file &&
     ++	test_set_magic_mtime other
      +}
      +
      +update_assert_changed () {
     -+	test-tool chmtime =1234567890 .git/index &&
     ++	test_set_magic_mtime .git/index &&
      +	test_might_fail git update-index "$1" &&
     -+	test-tool chmtime --get .git/index >.git/out &&
     -+	! grep ^1234567890 .git/out
     ++	! test_is_magic_mtime .git/index
      +}
      +
      +test_expect_success 'setup' '
     @@ t/t2108-update-index-refresh-racy.sh (new)
      +
      +test_expect_success '--refresh has no racy timestamps to fix' '
      +	reset_files &&
     -+	test-tool chmtime =1234567891 .git/index &&
     ++	# set the index time far enough to the future;
     ++	# it must be at least 3 seconds for VFAT
     ++	test_set_magic_mtime .git/index +60 &&
      +	git update-index --refresh &&
     -+	test-tool chmtime --get .git/index >.git/out &&
     -+	grep ^1234567891 .git/out
     ++	test_is_magic_mtime .git/index +60
      +'
      +
      +test_expect_success '--refresh should fix racy timestamp' '
     @@ t/t2108-update-index-refresh-racy.sh (new)
      +test_expect_success '--refresh should fix racy timestamp if other file needs update' '
      +	reset_files &&
      +	echo content2 >other &&
     -+	test-tool chmtime =1234567890 other &&
     ++	test_set_magic_mtime other &&
      +	update_assert_changed --refresh
      +'
      +
      +test_expect_success '--refresh should fix racy timestamp if racy file needs update' '
      +	reset_files &&
      +	echo content2 >file &&
     -+	test-tool chmtime =1234567890 file &&
     ++	test_set_magic_mtime file &&
      +	update_assert_changed --refresh
      +'
      +