[PATCH] git-apply: silence errors for success cases

[PATCH] git-apply: silence errors for success cases

[Jerry Zhang]

Certain invocations of "git apply --3way"
will print error messages even though git
is able to fall back on apply_fragments and
apply the patch successfully with a return
value of 0. To fix, return early from
try_threeway() in the following cases:

When the patch is a rename and no lines have
changed. In this case, "git diff" doesn't
record the blob info, so 3way is neither
possible nor necessary.

When the patch is an addition and there is
no add/add conflict, i.e. direct_to_threeway
is false. In this case, threeway will fail
since the preimage is not in cache, but isn't
necessary anyway since there is no conflict.

Only error messaging is affected and other
behavior does not change.

Signed-off-by: Jerry Zhang <jerry@skydio.com>
---
 apply.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/apply.c b/apply.c
index 8c5b29809b..a36d4002ca 100644
--- a/apply.c
+++ b/apply.c
@@ -3560,7 +3560,9 @@ static int try_threeway(struct apply_state *state,
 
 	/* No point falling back to 3-way merge in these cases */
 	if (patch->is_delete ||
-	    S_ISGITLINK(patch->old_mode) || S_ISGITLINK(patch->new_mode))
+	    S_ISGITLINK(patch->old_mode) || S_ISGITLINK(patch->new_mode) ||
+	    (patch->is_new && !patch->direct_to_threeway) ||
+	    (patch->is_rename && !patch->lines_added && !patch->lines_deleted))
 		return -1;
 
 	/* Preimage the patch was prepared for */
-- 
2.29.0

Re: [PATCH] git-apply: silence errors for success cases

[Junio C Hamano]

Jerry Zhang <jerry@skydio.com> writes:

> Certain invocations of "git apply --3way"
> will print error messages even though git
> is able to fall back on apply_fragments and
> apply the patch successfully with a return
> value of 0. To fix, return early from
> try_threeway() in the following cases:

I suspect that this is a recent breakage after we swapped the order
of 3way fallback.  It used to be that we tried the straight
application first (while suppressing the error messages) and then
fell back on 3way (while fully exposing the error messages).

It is understandable if we just swapped the order without changing
when errors are squelched, we may leak unnecessary error messages
while trying 3way and failing.

After having written all of the above, I just realized that the
swapping of the order was _your_ topic and you didn't have to be
explained any of the above ;-)  Just consider it as my thinking
aloud for the benefit of those who are reading from the sideline
(i.e. those not on the To/Cc list but are reading because this
message is sent to git@ mailing list).

Re: [PATCH] git-apply: silence errors for success cases

[Junio C Hamano]

Junio C Hamano <gitster@pobox.com> writes:

> Jerry Zhang <jerry@skydio.com> writes:
>
>> Certain invocations of "git apply --3way"
>> will print error messages even though git
>> is able to fall back on apply_fragments and
>> apply the patch successfully with a return
>> value of 0. To fix, return early from
>> try_threeway() in the following cases:
>
> I suspect that this is a recent breakage after we swapped the order
> of 3way fallback.  It used to be that we tried the straight
> application first (while suppressing the error messages) and then
> fell back on 3way (while fully exposing the error messages).

The other side of the coin is that we should say, before falling
back to non-3way codepath, that we saw 3way application failed, and
falling back to the straight patch, if we don't say so already with
the recent change, just like we used to say the opposite after seeing
the patch application to fail and then fell back to 3way.

In other words, we may have said, under the verbose mode, that we
are falling back to threeway only after seeing apply-fragments
failed, but if we are doing 3-way first, telling that we are doing
three-way application in try_threeway() function is actively wrong.
The message was there only because try_threeway() was a fallback and
sometimes did not happen even when the user asked to do --3way (hence
deserved a message).  Now we always try 3way first when the user asked,
the extra message should be given only after try_threeway() failed
and should talk about falling back to the straight application.

IOW, a fix something along this line, perhaps, is needed to complete
the "swap the order between 3way and direct application" series we
earlier worked on.


 apply.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git i/apply.c w/apply.c
index 8c5b29809b..6634305e9e 100644
--- i/apply.c
+++ w/apply.c
@@ -3570,7 +3570,7 @@ static int try_threeway(struct apply_state *state,
 		 read_blob_object(&buf, &pre_oid, patch->old_mode))
 		return error(_("repository lacks the necessary blob to perform 3-way merge."));
 
-	if (state->apply_verbosity > verbosity_silent)
+	if (state->apply_verbosity > verbosity_silent && patch->direct_to_threeway)
 		fprintf(stderr, _("Performing three-way merge...\n"));
 
 	img = strbuf_detach(&buf, &len);
@@ -3637,6 +3637,9 @@ static int apply_data(struct apply_state *state, struct patch *patch,
 		return -1;
 
 	if (!state->threeway || try_threeway(state, &image, patch, st, ce) < 0) {
+		if (state->threeway && !patch->direct_to_threeway)
+			fprintf(stderr, _("Falling back to direct application...\n"));
+
 		/* Note: with --reject, apply_fragments() returns 0 */
 		if (patch->direct_to_threeway || apply_fragments(state, &image, patch) < 0)
 			return -1;

[PATCH V2] git-apply: silence errors for success cases

[Jerry Zhang]

Certain invocations of "git apply --3way"
will print error messages even though git
is able to fall back on apply_fragments and
apply the patch successfully with a return
value of 0. To fix, return early from
try_threeway() in the following cases:

When the patch is a rename and no lines have
changed. In this case, "git diff" doesn't
record the blob info, so 3way is neither
possible nor necessary.

When the patch is an addition and there is
no add/add conflict, i.e. direct_to_threeway
is false. In this case, threeway will fail
since the preimage is not in cache, but isn't
necessary anyway since there is no conflict.

Only error messaging is affected and other
behavior does not change.

Signed-off-by: Jerry Zhang <jerry@skydio.com>
---
V1->V2: rebase onto master and rerun tests

I think I addressed previous comments. What
are the next steps for this patch?

 apply.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/apply.c b/apply.c
index 44bc31d6eb..fb321c707b 100644
--- a/apply.c
+++ b/apply.c
@@ -3560,7 +3560,9 @@ static int try_threeway(struct apply_state *state,
 
 	/* No point falling back to 3-way merge in these cases */
 	if (patch->is_delete ||
-	    S_ISGITLINK(patch->old_mode) || S_ISGITLINK(patch->new_mode))
+	    S_ISGITLINK(patch->old_mode) || S_ISGITLINK(patch->new_mode) ||
+	    (patch->is_new && !patch->direct_to_threeway) ||
+	    (patch->is_rename && !patch->lines_added && !patch->lines_deleted))
 		return -1;
 
 	/* Preimage the patch was prepared for */
-- 
2.32.0.1314.g6ed4fcc4cc

Re: [PATCH V2] git-apply: silence errors for success cases

[Jerry Zhang]

On Tue, Jul 27, 2021 at 8:14 PM Jerry Zhang <jerry@skydio.com> wrote:
>
> Certain invocations of "git apply --3way"
> will print error messages even though git
> is able to fall back on apply_fragments and
> apply the patch successfully with a return
> value of 0. To fix, return early from
> try_threeway() in the following cases:
>
> When the patch is a rename and no lines have
> changed. In this case, "git diff" doesn't
> record the blob info, so 3way is neither
> possible nor necessary.
>
> When the patch is an addition and there is
> no add/add conflict, i.e. direct_to_threeway
> is false. In this case, threeway will fail
> since the preimage is not in cache, but isn't
> necessary anyway since there is no conflict.
>
> Only error messaging is affected and other
> behavior does not change.
>
> Signed-off-by: Jerry Zhang <jerry@skydio.com>
> ---
> V1->V2: rebase onto master and rerun tests
>
> I think I addressed previous comments. What
> are the next steps for this patch?
>
>  apply.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/apply.c b/apply.c
> index 44bc31d6eb..fb321c707b 100644
> --- a/apply.c
> +++ b/apply.c
> @@ -3560,7 +3560,9 @@ static int try_threeway(struct apply_state *state,
>
>         /* No point falling back to 3-way merge in these cases */
>         if (patch->is_delete ||
> -           S_ISGITLINK(patch->old_mode) || S_ISGITLINK(patch->new_mode))
> +           S_ISGITLINK(patch->old_mode) || S_ISGITLINK(patch->new_mode) ||
> +           (patch->is_new && !patch->direct_to_threeway) ||
> +           (patch->is_rename && !patch->lines_added && !patch->lines_deleted))
>                 return -1;
>
>         /* Preimage the patch was prepared for */
> --
> 2.32.0.1314.g6ed4fcc4cc
>
Any updates on this patch? I had addressed the previous comment in this
thread in a different patch "apply: adjust messages to account for
--3way changes"
that was already merged in a while ago.

I originally intended this to mainly fix print message behavior, but
while testing
I found that it also fixed a separately reported issue from this thread:
https://www.spinics.net/lists/git/msg421481.html.

I probably owe an update / refresher on why I originally submitted these
features in the first place. I've written a tool for branchless / patch stack
based code review and development w/github. The core of this is a mechanism
that can cherry-pick commits without touching the cache or working tree
by using "git apply --cached --3way" with a custom index file. We've been
using this internally with a custom built git for a while now and gotten pretty
good feedback. We'd like to open source this tool soon, and it'd be nice
if our open-source users (eventually) don't need to build git themselves :).