config: retry acquiring config.lock, configurable via core.configLockTimeout Concurrent config writers race for the ".lock" file, which is taken with open(O_EXCL) and no retry, so the losers fail right away with "could not lock config file". This shows up with parallel "git worktree add -b" against the same repository: each one writes a couple of branch.* keys and the losers fail at random. Worse, "git worktree add" doesn't propagate that failure to its exit code, so the tracking config is silently dropped. (The swallowed error is a separate bug.) Retry instead of giving up on the first EEXIST. The lock is only held while rewriting a small file, so the loser only has to wait out the other writers. Same approach as 4ff0f01cb7 (refs: retry acquiring reference locks for 100ms, 2017-08-21). On the semantics: the on-disk config is read only after the lock is taken, so writers touching different keys can't lose each other's change. Writers touching the same key still get last-writer-wins, but that is already the case today and would need a compare-and-swap config API to fix. The retry only turns hard failures into successes. Default to 1000ms, like core.packedRefsTimeout: same shape of problem, one shared file everyone serializes through. A larger timeout only costs anything when a stale lock is left behind by a crash, which is rare; a smaller one fails spuriously on slow filesystems (NTFS has been seen needing more than 100ms). Make it configurable as core.configLockTimeout. There is no chicken-and-egg problem: we read the config before we lock it. microsoft/git carries a similar patch (core.configWriteLockTimeoutMS, default off) for Scalar's tests. Defaulting to non-zero here because the worktree case fails silently. Helped-by: Patrick Steinhardt <ps@pks.im> Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de> Signed-off-by: Jörg Thalheim <joerg@thalheim.io> Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/config/core.adoc b/Documentation/config/core.adoc index a0ebf03..340329e 100644 --- a/Documentation/config/core.adoc +++ b/Documentation/config/core.adoc
@@ -589,6 +589,14 @@ all; -1 means to try indefinitely. Default is 1000 (i.e., retry for 1 second). +core.configLockTimeout:: + The length of time, in milliseconds, to retry when trying to + lock a configuration file for writing. Value 0 means not to + retry at all; -1 means to try indefinitely. Default is 1000 + (i.e., retry for 1 second). This is read from the configuration + that is already on disk before the lock is taken, so it can be + set persistently like any other option. + core.pager:: Text viewer for use by Git commands (e.g., 'less'). The value is meant to be interpreted by the shell. The order of preference
diff --git a/config.c b/config.c index 156f2a2..ac95637 100644 --- a/config.c +++ b/config.c
@@ -2903,6 +2903,24 @@ char *git_config_prepare_comment_string(const char *comment) return prepared; } +/* + * How long to retry acquiring config.lock when another process holds + * it. Default matches core.packedRefsTimeout; override via + * core.configLockTimeout. + */ +static long config_lock_timeout_ms(struct repository *r) +{ + static int configured; + static int timeout_ms = 1000; + + if (!configured) { + repo_config_get_int(r, "core.configlocktimeout", &timeout_ms); + configured = 1; + } + + return timeout_ms; +} + static void validate_comment_string(const char *comment) { size_t leading_blanks; @@ -2986,7 +3004,8 @@ int repo_config_set_multivar_in_file_gently(struct repository *r, * The lock serves a purpose in addition to locking: the new * contents of .git/config will be written into it. */ - fd = hold_lock_file_for_update(&lock, config_filename, 0); + fd = hold_lock_file_for_update_timeout(&lock, config_filename, 0, + config_lock_timeout_ms(r)); if (fd < 0) { error_errno(_("could not lock config file %s"), config_filename); ret = CONFIG_NO_LOCK; @@ -3331,7 +3350,8 @@ static int repo_config_copy_or_rename_section_in_file( if (!config_filename) config_filename = filename_buf = repo_git_path(r, "config"); - out_fd = hold_lock_file_for_update(&lock, config_filename, 0); + out_fd = hold_lock_file_for_update_timeout(&lock, config_filename, 0, + config_lock_timeout_ms(r)); if (out_fd < 0) { ret = error(_("could not lock config file %s"), config_filename); goto out;
diff --git a/t/t1300-config.sh b/t/t1300-config.sh index 128971e..12a1cb8 100755 --- a/t/t1300-config.sh +++ b/t/t1300-config.sh
@@ -2939,4 +2939,21 @@ test_cmp expect actual ' +test_expect_success 'writing config fails immediately with core.configLockTimeout=0' ' + test_when_finished "rm -f .git/config.lock" && + >.git/config.lock && + test_must_fail git -c core.configLockTimeout=0 config foo.bar baz 2>err && + test_grep "could not lock config file" err +' + +test_expect_success 'writing config retries until lock is released' ' + test_when_finished "rm -f .git/config.lock" && + >.git/config.lock && + { + ( sleep 1 && rm -f .git/config.lock ) & + } && + git -c core.configLockTimeout=5000 config retried.key value && + test "$(git config retried.key)" = value +' + test_done
diff --git a/t/t3200-branch.sh b/t/t3200-branch.sh index e7829c2..5f8a31c 100755 --- a/t/t3200-branch.sh +++ b/t/t3200-branch.sh
@@ -1037,7 +1037,8 @@ test_when_finished "rm -f .git/config.lock" && >.git/config.lock && git branch locked && - test_must_fail git branch --set-upstream-to locked 2>err && + test_must_fail git -c core.configLockTimeout=0 \ + branch --set-upstream-to locked 2>err && test_grep "could not lock config file .git/config" err ' @@ -1068,7 +1069,8 @@ test_when_finished "rm -f .git/config.lock" && git branch --set-upstream-to locked && >.git/config.lock && - test_must_fail git branch --unset-upstream 2>err && + test_must_fail git -c core.configLockTimeout=0 \ + branch --unset-upstream 2>err && test_grep "could not lock config file .git/config" err '
diff --git a/t/t5505-remote.sh b/t/t5505-remote.sh index e592c0b..aea9222 100755 --- a/t/t5505-remote.sh +++ b/t/t5505-remote.sh
@@ -1327,7 +1327,8 @@ test_when_finished "rm -f .git/config.lock" && git config --get-all remote.someremote.url >expect && >.git/config.lock && - test_must_fail git remote set-url someremote baz && + test_must_fail git -c core.configLockTimeout=0 \ + remote set-url someremote baz && git config --get-all remote.someremote.url >actual && cmp expect actual '