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builtin/maintenance: fix locking race when handling "gc" task 

The "gc" task has a similar locking race as the one that we have fixed
for the "pack-refs" and "reflog-expire" tasks in preceding commits. Fix
this by splitting up the logic of the "gc" task:

  - We execute `gc_before_repack()` in the foreground, which contains
    the logic that git-gc(1) itself would execute in the foreground, as
    well.

  - We spawn git-gc(1) after detaching, but with a new hidden flag that
    suppresses calling `gc_before_repack()`.

Like this we have roughly the same logic as git-gc(1) itself and know to
repack refs and reflogs before detaching, thus fixing the race.

Note that `gc_before_repack()` is renamed to `gc_foreground_tasks()` to
better reflect what this function does.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
maint
Patrick Steinhardt 2025-06-03 16:01:20 +02:00 committed by Junio C Hamano
parent d2b084c660
commit 1b5074e614
2 changed files with 33 additions and 20 deletions
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41
builtin/gc.c
View File

@ -816,8 +816,8 @@ done:
return ret;
}

static int gc_before_repack(struct maintenance_run_opts *opts,
struct gc_config *cfg)
static int gc_foreground_tasks(struct maintenance_run_opts *opts,
struct gc_config *cfg)
{
if (cfg->pack_refs && maintenance_task_pack_refs(opts, cfg))
return error(FAILED_RUN, "pack-refs");
@ -837,6 +837,7 @@ int cmd_gc(int argc,
pid_t pid;
int daemonized = 0;
int keep_largest_pack = -1;
int skip_foreground_tasks = 0;
timestamp_t dummy;
struct maintenance_run_opts opts = MAINTENANCE_RUN_OPTS_INIT;
struct gc_config cfg = GC_CONFIG_INIT;
@ -869,6 +870,8 @@ int cmd_gc(int argc,
N_("repack all other packs except the largest pack")),
OPT_STRING(0, "expire-to", &cfg.repack_expire_to, N_("dir"),
N_("pack prefix to store a pack containing pruned objects")),
OPT_HIDDEN_BOOL(0, "skip-foreground-tasks", &skip_foreground_tasks,
N_("skip maintenance tasks typically done in the foreground")),
OPT_END()
};

@ -952,14 +955,16 @@ int cmd_gc(int argc,
goto out;
}

if (lock_repo_for_gc(force, &pid)) {
ret = 0;
goto out;
}
if (!skip_foreground_tasks) {
if (lock_repo_for_gc(force, &pid)) {
ret = 0;
goto out;
}

if (gc_before_repack(&opts, &cfg) < 0)
die(NULL);
delete_tempfile(&pidfile);
if (gc_foreground_tasks(&opts, &cfg) < 0)
die(NULL);
delete_tempfile(&pidfile);
}

/*
* failure to daemonize is ok, we'll continue
@ -988,8 +993,8 @@ int cmd_gc(int argc,
free(path);
}

if (opts.detach <= 0)
gc_before_repack(&opts, &cfg);
if (opts.detach <= 0 && !skip_foreground_tasks)
gc_foreground_tasks(&opts, &cfg);

if (!repository_format_precious_objects) {
struct child_process repack_cmd = CHILD_PROCESS_INIT;
@ -1225,8 +1230,14 @@ static int maintenance_task_prefetch(struct maintenance_run_opts *opts,
return 0;
}

static int maintenance_task_gc(struct maintenance_run_opts *opts,
struct gc_config *cfg UNUSED)
static int maintenance_task_gc_foreground(struct maintenance_run_opts *opts,
struct gc_config *cfg)
{
return gc_foreground_tasks(opts, cfg);
}

static int maintenance_task_gc_background(struct maintenance_run_opts *opts,
struct gc_config *cfg UNUSED)
{
struct child_process child = CHILD_PROCESS_INIT;

@ -1240,6 +1251,7 @@ static int maintenance_task_gc(struct maintenance_run_opts *opts,
else
strvec_push(&child.args, "--no-quiet");
strvec_push(&child.args, "--no-detach");
strvec_push(&child.args, "--skip-foreground-tasks");

return run_command(&child);
}
@ -1571,7 +1583,8 @@ static const struct maintenance_task tasks[] = {
},
[TASK_GC] = {
.name = "gc",
.background = maintenance_task_gc,
.foreground = maintenance_task_gc_foreground,
.background = maintenance_task_gc_background,
.auto_condition = need_to_gc,
},
[TASK_COMMIT_GRAPH] = {

12
t/t7900-maintenance.sh
View File

@ -49,9 +49,9 @@ test_expect_success 'run [--auto|--quiet]' '
git maintenance run --auto 2>/dev/null &&
GIT_TRACE2_EVENT="$(pwd)/run-no-quiet.txt" \
git maintenance run --no-quiet 2>/dev/null &&
test_subcommand git gc --quiet --no-detach <run-no-auto.txt &&
test_subcommand ! git gc --auto --quiet --no-detach <run-auto.txt &&
test_subcommand git gc --no-quiet --no-detach <run-no-quiet.txt
test_subcommand git gc --quiet --no-detach --skip-foreground-tasks <run-no-auto.txt &&
test_subcommand ! git gc --auto --quiet --no-detach --skip-foreground-tasks <run-auto.txt &&
test_subcommand git gc --no-quiet --no-detach --skip-foreground-tasks <run-no-quiet.txt
'

test_expect_success 'maintenance.auto config option' '
@ -154,9 +154,9 @@ test_expect_success 'run --task=<task>' '
git maintenance run --task=commit-graph 2>/dev/null &&
GIT_TRACE2_EVENT="$(pwd)/run-both.txt" \
git maintenance run --task=commit-graph --task=gc 2>/dev/null &&
test_subcommand ! git gc --quiet --no-detach <run-commit-graph.txt &&
test_subcommand git gc --quiet --no-detach <run-gc.txt &&
test_subcommand git gc --quiet --no-detach <run-both.txt &&
test_subcommand ! git gc --quiet --no-detach --skip-foreground-tasks <run-commit-graph.txt &&
test_subcommand git gc --quiet --no-detach --skip-foreground-tasks <run-gc.txt &&
test_subcommand git gc --quiet --no-detach --skip-foreground-tasks <run-both.txt &&
test_subcommand git commit-graph write --split --reachable --no-progress <run-commit-graph.txt &&
test_subcommand ! git commit-graph write --split --reachable --no-progress <run-gc.txt &&
test_subcommand git commit-graph write --split --reachable --no-progress <run-both.txt