Fix fork done handler wrongly update fsync metrics and enhance AOF_ FSYNC_ALWAYS (#11973)

This PR fix several unrelated bugs that were discovered by the same set of tests
(WAITAOF tests in #11713), could make the `WAITAOF` test hang. 

The change in `backgroundRewriteDoneHandler` is about MP-AOF.
That leftover / old code assumes that we started a new AOF file just now
(when we have a new base into which we're gonna incrementally write), but
the fact is that with MP-AOF, the fork done handler doesn't really affect the
incremental file being maintained by the parent process, there's no reason to
re-issue `SELECT`, and no reason to update any of the fsync variables in that flow.
This should have been deleted with MP-AOF (introduced in #9788, 7.0).
The damage is that the update to `aof_fsync_offset` will cause us to miss an fsync
in `flushAppendOnlyFile`, that happens if we stop write commands in `AOF_FSYNC_EVERYSEC`
while an AOFRW is in progress. This caused a new `WAITAOF` test to sometime hang forever.

Also because of MP-AOF, we needed to change `aof_fsync_offset` to `aof_last_incr_fsync_offset`
and match it to `aof_last_incr_size` in `flushAppendOnlyFile`. This is because in the past we compared
`aof_fsync_offset` and `aof_current_size`, but with MP-AOF it could be the total AOF file will be
smaller after AOFRW, and the (already existing) incr file still has data that needs to be fsynced.

The change in `flushAppendOnlyFile`, about the `AOF_FSYNC_ALWAYS`, it is follow #6053
(the details is in #5985), we also check `AOF_FSYNC_ALWAYS` to handle a case where
appendfsync is changed from everysec to always while there is data that's written but not yet fsynced.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/server.h b/src/server.h
index 1a68071f1..43d85008a 100644
--- a/src/server.h
+++ b/src/server.h
@@ -1742,7 +1742,8 @@ struct redisServer {
     off_t aof_rewrite_base_size;    /* AOF size on latest startup or rewrite. */
     off_t aof_current_size;         /* AOF current size (Including BASE + INCRs). */
     off_t aof_last_incr_size;       /* The size of the latest incr AOF. */
-    off_t aof_fsync_offset;         /* AOF offset which is already synced to disk. */
+    off_t aof_last_incr_fsync_offset; /* AOF offset which is already requested to be synced to disk.
+                                       * Compare with the aof_last_incr_size. */
     int aof_flush_sleep;            /* Micros to sleep before flush. (used by tests) */
     int aof_rewrite_scheduled;      /* Rewrite once BGSAVE terminates. */
     sds aof_buf;      /* AOF buffer, written before entering the event loop */