git-commit: exit non-zero if we fail to commit the index

In certain rare cases, the creation of the commit object
and update of HEAD can succeed, but then installing the
updated index will fail. This is most likely caused by a
full disk or exceeded disk quota. When this happens the
new index file will be removed, and the repository will
be left with the original now-out-of-sync index. The
user can recover with a "git reset HEAD" once the disk
space issue is resolved.

We should detect this failure and offer the user some
helpful guidance.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-commit.c

@@ -122,19 +122,23 @@ static void rollback_index_files(void)
 	}
 }
 
-static void commit_index_files(void)
+static int commit_index_files(void)
 {
+	int err = 0;
+
 	switch (commit_style) {
 	case COMMIT_AS_IS:
 		break; /* nothing to do */
 	case COMMIT_NORMAL:
-		commit_lock_file(&index_lock);
+		err = commit_lock_file(&index_lock);
 		break;
 	case COMMIT_PARTIAL:
-		commit_lock_file(&index_lock);
+		err = commit_lock_file(&index_lock);
 		rollback_lock_file(&false_lock);
 		break;
 	}
+
+	return err;
 }
 
 /*
@@ -926,7 +930,10 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	unlink(git_path("MERGE_HEAD"));
 	unlink(git_path("MERGE_MSG"));
 
-	commit_index_files();
+	if (commit_index_files())
+		die ("Repository has been updated, but unable to write\n"
+		     "new_index file. Check that disk is not full or quota is\n"
+		     "not exceeded, and then \"git reset HEAD\" to recover.");
 
 	rerere();
 	run_hook(get_index_file(), "post-commit", NULL);