bisect: libify `check_good_are_ancestors_of_bad` and its dependents

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.

Code that turns BISECT_INTERNAL_SUCCESS_MERGE_BASE (-11)
 to BISECT_OK (0) from `check_good_are_ancestors_of_bad()` has been moved to
`cmd_bisect__helper()`.

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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bisect.c

@@ -872,20 +872,23 @@ static int check_ancestors(struct repository *r, int rev_nr,
  *
  * If that's not the case, we need to check the merge bases.
  * If a merge base must be tested by the user, its source code will be
- * checked out to be tested by the user and we will exit.
+ * checked out to be tested by the user and we will return.
  */
-static void check_good_are_ancestors_of_bad(struct repository *r,
+
+static enum bisect_error check_good_are_ancestors_of_bad(struct repository *r,
 					    const char *prefix,
 					    int no_checkout)
 {
-	char *filename = git_pathdup("BISECT_ANCESTORS_OK");
+	char *filename;
 	struct stat st;
 	int fd, rev_nr;
 	enum bisect_error res = BISECT_OK;
 	struct commit **rev;
 
 	if (!current_bad_oid)
-		die(_("a %s revision is needed"), term_bad);
+		return error(_("a %s revision is needed"), term_bad);
+
+	filename = git_pathdup("BISECT_ANCESTORS_OK");
 
 	/* Check if file BISECT_ANCESTORS_OK exists. */
 	if (!stat(filename, &st) && S_ISREG(st.st_mode))
@@ -901,18 +904,26 @@ static void check_good_are_ancestors_of_bad(struct repository *r,
 	if (check_ancestors(r, rev_nr, rev, prefix))
 		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);
-	if (fd < 0)
-		warning_errno(_("could not create file '%s'"),
-			      filename);
-	else
-		close(fd);
+	if (!res) {
+		/* Create file BISECT_ANCESTORS_OK. */
+		fd = open(filename, O_CREAT | O_TRUNC | O_WRONLY, 0600);
+		if (fd < 0)
+			/*
+			 * BISECT_ANCESTORS_OK file is not absolutely necessary,
+			 * the bisection process will continue at the next
+			 * bisection step.
+			 * So, just signal with a warning that something
+			 * might be wrong.
+			 */
+			warning_errno(_("could not create file '%s'"),
+				filename);
+		else
+			close(fd);
+	}
  done:
 	free(filename);
+	return res;
 }
 
 /*
@@ -984,7 +995,9 @@ enum bisect_error bisect_next_all(struct repository *r, const char *prefix, int
 	if (read_bisect_refs())
 		die(_("reading bisect refs failed"));
 
-	check_good_are_ancestors_of_bad(r, prefix, no_checkout);
+	res = check_good_are_ancestors_of_bad(r, prefix, no_checkout);
+	if (res)
+		return res;
 
 	bisect_rev_setup(r, &revs, prefix, "%s", "^%s", 1);
 	revs.limited = 1;

builtin/bisect--helper.c

@@ -666,7 +666,8 @@ int cmd_bisect__helper(int argc, const char **argv, const char *prefix)
 
 	switch (cmdmode) {
 	case NEXT_ALL:
-		return bisect_next_all(the_repository, prefix, no_checkout);
+		res = bisect_next_all(the_repository, prefix, no_checkout);
+		break;
 	case WRITE_TERMS:
 		if (argc != 2)
 			return error(_("--write-terms requires two arguments"));
@@ -713,5 +714,13 @@ int cmd_bisect__helper(int argc, const char **argv, const char *prefix)
 		return error("BUG: unknown subcommand '%d'", cmdmode);
 	}
 	free_terms(&terms);
+
+	/*
+	 * Handle early success
+	 * From check_merge_bases > check_good_are_ancestors_of_bad > bisect_next_all
+	 */
+	if (res == BISECT_INTERNAL_SUCCESS_MERGE_BASE)
+		res = BISECT_OK;
+
 	return abs(res);
 }