builtin/apply: make write_out_results() return -1 on error

To libify `git apply` functionality we have to signal errors to the
caller instead of exit()ing.

To do that in a compatible manner with the rest of the error handling
in "builtin/apply.c", write_out_results() should return -1 instead of
calling exit().

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/apply.c

@@ -4383,6 +4383,12 @@ static int write_out_one_reject(struct apply_state *state, struct patch *patch)
 	return -1;
 }
 
+/*
+ * Returns:
+ *  -1 if an error happened
+ *   0 if the patch applied cleanly
+ *   1 if the patch did not apply cleanly
+ */
 static int write_out_results(struct apply_state *state, struct patch *list)
 {
 	int phase;
@@ -4396,8 +4402,10 @@ static int write_out_results(struct apply_state *state, struct patch *list)
 			if (l->rejected)
 				errs = 1;
 			else {
-				if (write_out_one_result(state, l, phase))
-					exit(128);
+				if (write_out_one_result(state, l, phase)) {
+					string_list_clear(&cpath, 0);
+					return -1;
+				}
 				if (phase == 1) {
 					if (write_out_one_reject(state, l))
 						errs = 1;
@@ -4517,11 +4525,18 @@ static int apply_patch(struct apply_state *state,
 		}
 	}
 
-	if (state->apply && write_out_results(state, list)) {
+	if (state->apply) {
+		int write_res = write_out_results(state, list);
+		if (write_res < 0) {
+			res = -128;
+			goto end;
+		}
+		if (write_res > 0) {
 			/* with --3way, we still need to write the index out */
 			res = state->apply_with_reject ? -1 : 1;
 			goto end;
 		}
+	}
 
 	if (state->fake_ancestor &&
 	    build_fake_ancestor(list, state->fake_ancestor)) {