Re: [PATCH] git-apply: fix --3way with binary patch
Jerry Zhang <jerry@skydio.com> writes:
Show 30 quoted lines
> Binary patches applied with "--3way" will
> always return a conflict even if the patch
> should cleanly apply because the low level
> merge function considers all binary merges
> without a variant to be conflicting.
>
> Fix by falling back to normal patch application
> for all binary patches.
>
> Add tests for --3way and normal applications
> of binary patches.
>
> Fixes: 923cd87ac8 ("git-apply: try threeway first when "--3way" is used")
> Signed-off-by: Jerry Zhang <jerry@skydio.com>
> ---
> apply.c | 3 ++-
> t/t4108-apply-threeway.sh | 45 +++++++++++++++++++++++++++++++++++++++
> 2 files changed, 47 insertions(+), 1 deletion(-)
>
> diff --git a/apply.c b/apply.c
> index 1d2d7e124e..78e52f0dc1 100644
> --- a/apply.c
> +++ b/apply.c
> @@ -3638,7 +3638,8 @@ static int apply_data(struct apply_state *state, struct patch *patch,
> if (load_preimage(state, &image, patch, st, ce) < 0)
> return -1;
>
> - if (!state->threeway || try_threeway(state, &image, patch, st, ce) < 0) {
> + if (!state->threeway || patch->is_binary ||
> + try_threeway(state, &image, patch, st, ce) < 0) {Thanks for a quick turnaround. However.
Because apply.c::three_way_merge() calls into ll_merge() that lets the low-level custom merge drivers to take over the actual merge, I do not think your "if binary, bypass and never call try_threway() at all" is the right solution. The custom merge driver user uses for the path may successfully perform such a "trivial" three-way merge and return success.
Why does the current code that lets threeway tried first fails to fall back to direct application? The code before your change, if fed a binary patch that does not apply, would have failed the direct application first *and* then fell back to the threeway (if only to fail because we do not let binary files be merged), no?
Is it that try_threeway()'s way to express failure slightly different from how direct application reports failure, but your change used the same "only if it is negative, we fail and fallback" logic? IIRC, apply_fragments() which is the meat of the direct application logic reports failures by negative, but try_threeway() can return positive non-zero to signal a "recoverable" failure (aka "conflicted merge"). Which should lead us to explore a different approach, which is ...
Would it be possible for a patch to leave conflicts when
try_threeway() was attempted, but will cleanly apply if direct
application is done?If so, perhaps
- we first run try_threeway() and see if it cleanly resolves; if
so, we are done.
- then we try direct application and see if it cleanly applies; if
so, we are done.
- finally we run try_threeway() again and let it fail with
conflict.
might be the right sequence? We theoretically could omit the first of these three steps, but that would mean we'd write 923cd87a (git-apply: try threeway first when "--3way" is used, 2021-04-06) off as a failed experiment and revert it, which would not be ideal.
Also, independent from this "if we claim we try threeway first and fall back to direct application, we really should do so" fix we are discussing, I think our default binary merge can be a bit more lenient and resolve this particular case of applying the binary patch taken from itself (i.e. a patch that takes A to B gets applied using --3way option to A). I wonder if it can be as simple as the attached patch. FWIW, this change is sufficient (without the change to apply.c we are reviewing here) to make your new tests in t4108 pass.
---- >8 ------- >8 ------- >8 ------- >8 ------- >8 ------- >8 ----
Subject: ll-merge: teach ll_binary_merge() a trivial three-way merge
The low-level binary merge code assumed that the caller will not feed trivial merges that would have been resolved at the tree level; because of this, ll_binary_merge() assumes the ancestor is different from either side, always failing the merge in conflict unless -Xours or -Xtheirs is in effect.
But "git apply --3way" codepath could ask us to perform three-way merge between two binaries A and B using A as the ancestor version. The current code always fails such an application, but when given a binary patch that turns A into B and asked to apply it to A, there is no reason to fail such a request---we can trivially tell that the result must be B.
Arguably, this fix may belong to one level higher at ll_merge() function, which dispatches to lower-level merge drivers, possibly even before it renormalizes the three input buffers. But let's first see how this goes.
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
ll-merge.c | 56 +++++++++++++++++++++++++++++++++++++++-----------------
1 file changed, 39 insertions(+), 17 deletions(-)
Show changes to diff +39 −17
diff --git c/ll-merge.c w/ll-merge.c
index 261657578c..bc8038d404 100644
--- c/ll-merge.c
+++ w/ll-merge.c
@@ -46,6 +46,13 @@ void reset_merge_attributes(void)
merge_attributes = NULL;
}
+static int same_mmfile(mmfile_t *a, mmfile_t *b)
+{
+ if (a->size != b->size)
+ return 0;
+ return !memcmp(a->ptr, b->ptr, a->size);
+}
+
/*
* Built-in low-levels
*/
@@ -58,9 +65,18 @@ static int ll_binary_merge(const struct ll_merge_driver *drv_unused,
const struct ll_merge_options *opts,
int marker_size)
{
+ int status;
mmfile_t *stolen;
assert(opts);
+ /*
+ * With -Xtheirs or -Xours, we have cleanly merged;
+ * otherwise we got a conflict, unless 3way trivially
+ * resolves.
+ */
+ status = (opts->variant == XDL_MERGE_FAVOR_OURS ||
+ opts->variant == XDL_MERGE_FAVOR_THEIRS) ? 0 : 1;
+
/*
* The tentative merge result is the common ancestor for an
* internal merge. For the final merge, it is "ours" by
@@ -68,18 +84,30 @@ static int ll_binary_merge(const struct ll_merge_driver *drv_unused,
*/
if (opts->virtual_ancestor) {
stolen = orig;
+ status = 0;
} else {
- switch (opts->variant) {
- default:
- warning("Cannot merge binary files: %s (%s vs. %s)",
- path, name1, name2);
- /* fallthru */
- case XDL_MERGE_FAVOR_OURS:
- stolen = src1;
- break;
- case XDL_MERGE_FAVOR_THEIRS:
+ if (same_mmfile(orig, src1)) {
stolen = src2;
- break;
+ status = 0;
+ } else if (same_mmfile(orig, src2)) {
+ stolen = src1;
+ status = 0;
+ } else if (same_mmfile(src1, src2)) {
+ stolen = src1;
+ status = 0;
+ } else {
+ switch (opts->variant) {
+ default:
+ warning("Cannot merge binary files: %s (%s vs. %s)",
+ path, name1, name2);
+ /* fallthru */
+ case XDL_MERGE_FAVOR_OURS:
+ stolen = src1;
+ break;
+ case XDL_MERGE_FAVOR_THEIRS:
+ stolen = src2;
+ break;
+ }
}
}
@@ -87,13 +115,7 @@ static int ll_binary_merge(const struct ll_merge_driver *drv_unused,
result->size = stolen->size;
stolen->ptr = NULL;
- /*
- * With -Xtheirs or -Xours, we have cleanly merged;
- * otherwise we got a conflict.
- */
- return opts->variant == XDL_MERGE_FAVOR_OURS ||
- opts->variant == XDL_MERGE_FAVOR_THEIRS ?
- 0 : 1;
+ return status;
}
static int ll_xdl_merge(const struct ll_merge_driver *drv_unused,
Jerry Zhang
