[PATCH v2 12/12] read_cache: roll back lock in `update_index_if_able()`

["Martin Ågren" <martin.agren@gmail.com>]

`update_index_if_able()` used to always commit the lock or roll it back.
Commit 03b866477 (read-cache: new API write_locked_index instead of
write_index/write_cache, 2014-06-13) stopped rolling it back in case a
write was not even attempted. This change in behavior is not motivated
in the commit message and appears to be accidental: the `else`-path was
removed, although that changed the behavior in case the `if` shortcuts.

Reintroduce the rollback and document this behavior. While at it, move
the documentation on this function from the function definition to the
function declaration in cache.h.

If `write_locked_index(..., COMMIT_LOCK)` fails, it will roll back the
lock for us (see the previous commit).

Noticed-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
---
v2: New patch.

 cache.h      | 4 ++++
 read-cache.c | 5 ++---
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/cache.h b/cache.h
index 0e26224b9..8c2493766 100644
--- a/cache.h
+++ b/cache.h
@@ -734,6 +734,10 @@ extern void fill_stat_cache_info(struct cache_entry *ce, struct stat *st);
 extern int refresh_index(struct index_state *, unsigned int flags, const struct pathspec *pathspec, char *seen, const char *header_msg);
 extern struct cache_entry *refresh_cache_entry(struct cache_entry *, unsigned int);
 
+/*
+ * Opportunistically update the index but do not complain if we can't.
+ * The lockfile is always committed or rolled back.
+ */
 extern void update_index_if_able(struct index_state *, struct lock_file *);
 
 extern int hold_locked_index(struct lock_file *, int);
diff --git a/read-cache.c b/read-cache.c
index 0090ab886..92ea5df96 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -2176,14 +2176,13 @@ static int has_racy_timestamp(struct index_state *istate)
 	return 0;
 }
 
-/*
- * Opportunistically update the index but do not complain if we can't
- */
 void update_index_if_able(struct index_state *istate, struct lock_file *lockfile)
 {
 	if ((istate->cache_changed || has_racy_timestamp(istate)) &&
 	    verify_index(istate))
 		write_locked_index(istate, lockfile, COMMIT_LOCK);
+	else
+		rollback_lock_file(lockfile);
 }
 
 static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
-- 
2.14.2.666.gea220ee40