rebase: fast-forward --fork-point in more cases

Before, when we rebased with a --fork-point invocation where the
fork-point wasn't empty, we would be setting options.restrict_revision.
The fast-forward logic would automatically declare that the rebase was
not fast-forwardable if it was set. However, this was painting with a
very broad brush.

Refine the logic so that we can fast-forward in the case where the
restricted revision is equal to the merge base, since we stop rebasing
at the merge base anyway.

Helped-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Denton Liu <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Denton-L

builtin/rebase.c

@@ -1261,6 +1261,7 @@ static int is_linear_history(struct commit *from, struct commit *to)
 }
 
 static int can_fast_forward(struct commit *onto, struct commit *upstream,
+			    struct commit *restrict_revision,
 			    struct object_id *head_oid, struct object_id *merge_base)
 {
 	struct commit *head = lookup_commit(the_repository, head_oid);
@@ -1280,6 +1281,9 @@ static int can_fast_forward(struct commit *onto, struct commit *upstream,
 	if (!oideq(merge_base, &onto->object.oid))
 		goto done;
 
+	if (restrict_revision && !oideq(&restrict_revision->object.oid, merge_base))
+		goto done;
+
 	if (!upstream)
 		goto done;
 
@@ -2042,9 +2046,9 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	 * with new commits recreated by replaying their changes. This
 	 * optimization must not be done if this is an interactive rebase.
 	 */
-	if (can_fast_forward(options.onto, options.upstream, &options.orig_head,
-		    &merge_base) &&
-	    !is_interactive(&options) && !options.restrict_revision) {
+	if (can_fast_forward(options.onto, options.upstream, options.restrict_revision,
+		    &options.orig_head, &merge_base) &&
+	    !is_interactive(&options)) {
 		int flag;
 
 		if (!(options.flags & REBASE_FORCE)) {

t/t3432-rebase-fast-forward.sh

@@ -70,32 +70,32 @@ test_rebase_same_head_ () {
 }
 
 changes='no changes'
-test_rebase_same_head success work same success work same
+test_rebase_same_head success noop same success work same
 test_rebase_same_head success noop same success noop-force same master
 test_rebase_same_head success noop same success noop-force diff --onto B B
 test_rebase_same_head success noop same success noop-force diff --onto B... B
 test_rebase_same_head success noop same success noop-force same --onto master... master
 test_rebase_same_head success noop same success noop-force same --no-fork-point
-test_rebase_same_head success work same success work same --fork-point master
-test_rebase_same_head failure noop same success work diff --fork-point --onto B B
-test_rebase_same_head failure work same success work diff --fork-point --onto B... B
-test_rebase_same_head success work same success work same --fork-point --onto master... master
+test_rebase_same_head success noop same success work same --fork-point master
+test_rebase_same_head success noop same success work diff --fork-point --onto B B
+test_rebase_same_head success noop same success work diff --fork-point --onto B... B
+test_rebase_same_head success noop same success work same --fork-point --onto master... master
 
 test_expect_success 'add work same to side' '
 	test_commit E
 '
 
 changes='our changes'
-test_rebase_same_head success work same success work same
+test_rebase_same_head success noop same success work same
 test_rebase_same_head success noop same success noop-force same master
 test_rebase_same_head success noop same success noop-force diff --onto B B
 test_rebase_same_head success noop same success noop-force diff --onto B... B
 test_rebase_same_head success noop same success noop-force same --onto master... master
 test_rebase_same_head success noop same success noop-force same --no-fork-point
-test_rebase_same_head success work same success work same --fork-point master
-test_rebase_same_head failure work same success work diff --fork-point --onto B B
-test_rebase_same_head failure work same success work diff --fork-point --onto B... B
-test_rebase_same_head success work same success work same --fork-point --onto master... master
+test_rebase_same_head success noop same success work same --fork-point master
+test_rebase_same_head success noop same success work diff --fork-point --onto B B
+test_rebase_same_head success noop same success work diff --fork-point --onto B... B
+test_rebase_same_head success noop same success work same --fork-point --onto master... master
 
 test_expect_success 'add work same to upstream' '
 	git checkout master &&