[PATCH v3 10/13] bisect: libify `check_merge_bases` and its dependents

[Miriam Rubio <mirucam@gmail.com>]

From: Pranit Bauva <pranit.bauva@gmail.com>

Since we want to get rid of git-bisect.sh, it would be necessary to
convert those exit() calls to return statements so that errors can be
reported.

Emulate try catch in C by converting `exit(<positive-value>)` to
`return <negative-value>`. Follow POSIX conventions to return
<negative-value> to indicate error.

In `check_merge_bases()` there is an early success special case,
so we have introduced special error code
BISECT_INTERNAL_SUCCESS_MERGE_BASE (-11) which indicates early
success. This BISECT_INTERNAL_SUCCESS_MERGE_BASE is converted back
to BISECT_OK (0) in `check_good_are_ancestors_of_bad()`.

Update all callers to handle the error returns.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Pranit Bauva <pranit.bauva@gmail.com>
Signed-off-by: Tanushree Tumane <tanushreetumane@gmail.com>
Signed-off-by: Miriam Rubio <mirucam@gmail.com>
---
 bisect.c | 22 ++++++++++++++++++----
 bisect.h |  7 ++++++-
 2 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/bisect.c b/bisect.c
index f6582ddfed..382e0b471f 100644
--- a/bisect.c
+++ b/bisect.c
@@ -811,13 +811,18 @@ static void handle_skipped_merge_base(const struct object_id *mb)
  * "check_merge_bases" checks that merge bases are not "bad" (or "new").
  *
  * - If one is "bad" (or "new"), it means the user assumed something wrong
- * and we must exit with a non 0 error code.
+ * and we must return error with a non 0 error code.
  * - If one is "good" (or "old"), that's good, we have nothing to do.
  * - If one is "skipped", we can't know but we should warn.
  * - If we don't know, we should check it out and ask the user to test.
+ * - If a merge base must be tested, on success return
+ * BISECT_INTERNAL_SUCCESS_MERGE_BASE (-11) a special condition
+ * for early success, this will be converted back to 0 in
+ * check_good_are_ancestors_of_bad().
  */
-static void check_merge_bases(int rev_nr, struct commit **rev, int no_checkout)
+static enum bisect_error check_merge_bases(int rev_nr, struct commit **rev, int no_checkout)
 {
+	enum bisect_error res = BISECT_OK;
 	struct commit_list *result;
 
 	result = get_merge_bases_many(rev[0], rev_nr - 1, rev + 1);
@@ -832,11 +837,16 @@ static void check_merge_bases(int rev_nr, struct commit **rev, int no_checkout)
 			handle_skipped_merge_base(mb);
 		} else {
 			printf(_("Bisecting: a merge base must be tested\n"));
-			exit(bisect_checkout(mb, no_checkout));
+			res = bisect_checkout(mb, no_checkout);
+			if (!res)
+				/* indicate early success */
+				res = BISECT_INTERNAL_SUCCESS_MERGE_BASE;
+			break;
 		}
 	}
 
 	free_commit_list(result);
+	return res;
 }
 
 static int check_ancestors(struct repository *r, int rev_nr,
@@ -871,6 +881,7 @@ static void check_good_are_ancestors_of_bad(struct repository *r,
 	char *filename = git_pathdup("BISECT_ANCESTORS_OK");
 	struct stat st;
 	int fd, rev_nr;
+	enum bisect_error res = BISECT_OK;
 	struct commit **rev;
 
 	if (!current_bad_oid)
@@ -885,10 +896,13 @@ static void check_good_are_ancestors_of_bad(struct repository *r,
 		goto done;
 
 	/* Check if all good revs are ancestor of the bad rev. */
+
 	rev = get_bad_and_good_commits(r, &rev_nr);
 	if (check_ancestors(r, rev_nr, rev, prefix))
-		check_merge_bases(rev_nr, rev, no_checkout);
+		res = check_merge_bases(rev_nr, rev, no_checkout);
 	free(rev);
+	if (res)
+		exit(res == BISECT_INTERNAL_SUCCESS_MERGE_BASE ? BISECT_OK : -res);
 
 	/* Create file BISECT_ANCESTORS_OK. */
 	fd = open(filename, O_CREAT | O_TRUNC | O_WRONLY, 0600);
diff --git a/bisect.h b/bisect.h
index 19d90e4870..f68ae85376 100644
--- a/bisect.h
+++ b/bisect.h
@@ -39,11 +39,16 @@ struct rev_list_info {
  * BISECT_FAILED error code: default error code.
  * BISECT_ONLY_SKIPPED_LEFT error code: only skipped
  * commits left to be tested.
+ * BISECT_INTERNAL_SUCCESS_MERGE_BASE early success
+ * code: found merge base that should be tested.
+ * Early success code BISECT_INTERNAL_SUCCESS_MERGE_BASE
+ * should be only an internal code.
  */
 enum bisect_error {
 	BISECT_OK = 0,
 	BISECT_FAILED = -1,
-	BISECT_ONLY_SKIPPED_LEFT = -2
+	BISECT_ONLY_SKIPPED_LEFT = -2,
+	BISECT_INTERNAL_SUCCESS_MERGE_BASE = -11
 };
 
 enum bisect_error bisect_next_all(struct repository *r,
-- 
2.25.0