journal: fix already offline check and thread leak (#2810)

Early in journal_file_set_offline() f->header->state is tested to see if
it's != STATE_ONLINE, and since there's no need to do anything if the
journal isn't online, the function simply returned here.

Since moving part of the offlining process to a separate thread, there
are two problems here:

1. We can't simply check f->header->state, because if there is an
offline thread active it may modify f->header->state.

2. Even if the journal is deemed offline, the thread responsible may
still need joining, so a bare return may leak the thread's resources
like its stack.

To address #1, the helper journal_file_is_offlining() is called prior to
accessing f->header->state.

If journal_file_is_offlining() returns true, f->header->state isn't even
checked, because an offlining journal is obviously online, and we'll
just continue with the normal set offline code path.

If journal_file_is_offlining() returns false, then it's safe to check
f->header->state, because the offline_state is beyond the point of
modifying f->header->state, and there's a memory barrier in the helper.

If we find f->header->state is != STATE_ONLINE, then we call the
idempotent journal_file_set_offline_thread_join() on the way out of the
function, to join a potential lingering offline thread.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/journal/journal-file.c b/src/journal/journal-file.c
index bed825cdc3..12902d9f91 100644
--- a/src/journal/journal-file.c
+++ b/src/journal/journal-file.c
@@ -217,8 +217,10 @@ int journal_file_set_offline(JournalFile *f, bool wait) {
         if (!(f->fd >= 0 && f->header))
                 return -EINVAL;
 
-        if (f->header->state != STATE_ONLINE)
-                return 0;
+        /* An offlining journal is implicitly online and may modify f->header->state,
+         * we must also join any potentially lingering offline thread when not online. */
+        if (!journal_file_is_offlining(f) && f->header->state != STATE_ONLINE)
+                return journal_file_set_offline_thread_join(f);
 
         /* Restart an in-flight offline thread and wait if needed, or join a lingering done one. */
         restarted = journal_file_set_offline_try_restart(f);