Subject: [RFC PATCH 2/5] merge-recursive: new function for better colliding conflict resolutions

From: Elijah Newren <[email-masked]>
Date: Mon, 5 Mar 2018 09:11:22 -0800

There are three conflict types that represent two (possibly entirely
unrelated) files colliding at the same location:
  * add/add
  * rename/add
  * rename/rename(2to1)

These three conflict types already share more similarity than might be
immediately apparent from their description: (1) the handling of the
rename variants already involves removing any entries from the index
corresponding to the original file names[*], thus only leaving entries
in the index for the colliding path; (2) likewise, any trace of the
original file name in the working tree is also removed.  So, in all
three cases we're left with how to represent two colliding files in both
the index and the working copy.

[*] Technically, this isn't quite true because rename/rename(2to1)
conflicts in the recursive (o->call_depth > 0) case do an "unrename"
since about seven years ago.  But even in that case, Junio felt
compelled to explain that my decision to "unrename" wasn't necessarily
the only or right answer -- search for "Comment from Junio" in t6036 for
details.

My initial motivation for looking at these three conflict types was that
if the handling of these three conflict types is the same, at least in
the limited set of cases where a renamed file is unmodified on the side
of history where the file is not renamed, then a significant performance
improvement for rename detection during merges is possible.  However,
while that served as motivation to look at these three types of
conflicts, the actual goal of this new function is to try to improve the
handling for all three cases, not to merely make them the same as each
other in that special circumstance.

=== Handling the working tree ===

The previous behavior for these conflict types in regards to the
working tree (assuming the file collision occurs at 'foo') was:
  * add/add does a two-way merge of the two files and records it as 'foo'.
  * rename/rename(2to1) records the two different files into two new
    uniquely named files (foo~HEAD and foo~$MERGE), while removing 'foo'
    from the working tree.
  * rename/add records the two different files into two different
    locations, recording the add at foo~$SIDE and, oddly, recording
    the rename at foo (why is the rename more important than the add?)

So, the question for what to write to the working tree boils down to
whether the two colliding files should be two-way merged and recorded in
place, or recorded into separate files.  If the files are similar enough,
the two-way merge is probably preferable, but if they're not similar,
recording as separate files is probably preferable.  (The same logic that
applies for the working directory here would also apply to the recursive
case, i.e. the o->call_depth > 0 case, as well.)  The code handling the
different types of conflicts appear to have been written with different
assumptions about whether the colliding files would be similar.

But, rather than make an assumption about whether the two files will be
similar, why not just check?  If we simply call estimate_similarity(),
we can two-way merge the files if they are similar, and otherwise record
the two files at different locations.

=== Handling of the index ===

For a typical rename, unpack_trees() would set up the index in the
following fashion:
           old_path  new_path
   stage1: 5ca1ab1e  00000000
   stage2: f005ba11  00000000
   stage3: 00000000  b0a710ad
And merge-recursive would rewrite this to
           new_path
   stage1: 5ca1ab1e
   stage2: f005ba11
   stage3: b0a710ad
Removing old_path from the index means the user won't have to `git rm
old_path` manually every time a renamed path has a content conflict.
It also means they can use `git checkout [--ours|--theirs|--conflict|-m]
new_path`, `git diff [--ours|--theirs]` and various other commands that
would be difficult otherwise.

This strategy becomes a problem when we have a rename/add or
rename/rename(2to1) conflict, however, because then we have only three
slots to store blob sha1s and we need either four or six.  Previously,
this was handled by continuing to delete old_path from the index, and
just outright ignoring any blob shas from old_path.  That had the
downside of deleting any trace of changes made to old_path on the other
side of history.  This function instead does a three-way content merge of
the renamed file, and stores the blob sha1 for that at either stage2 or
stage3 for new_path (depending on which side the rename came from).  That
has the advantage of bringing information about changes on both sides and
still allows for easy resolution (no need to git rm old_path, etc.), but
does have the downside that if the content merge had conflict markers,
then what we store in the index is the sha1 of a blob with conflict
markers.  While that is a downside, it seems less problematic than the
downsides of any obvious alternatives, and certainly makes more sense
than the previous handling.

Signed-off-by: Elijah Newren <[email-masked]>
---
 diff.h            |   4 ++
 diffcore-rename.c |   6 +--
 merge-recursive.c | 124 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 131 insertions(+), 3 deletions(-)

diff --git a/diff.h b/diff.h
index 7cf276f077..810944247c 100644
--- a/diff.h
+++ b/diff.h
@@ -442,4 +442,8 @@ extern void print_stat_summary(FILE *fp, int files,
 			       int insertions, int deletions);
 extern void setup_diff_pager(struct diff_options *);
 
+extern int estimate_similarity(struct diff_filespec *src,
+			       struct diff_filespec *dst,
+			       int minimum_score);
+
 #endif /* DIFF_H */
diff --git a/diffcore-rename.c b/diffcore-rename.c
index 245e999fe5..0f4388a9e0 100644
--- a/diffcore-rename.c
+++ b/diffcore-rename.c
@@ -127,9 +127,9 @@ struct diff_score {
 	short name_score;
 };
 
-static int estimate_similarity(struct diff_filespec *src,
-			       struct diff_filespec *dst,
-			       int minimum_score)
+int estimate_similarity(struct diff_filespec *src,
+			struct diff_filespec *dst,
+			int minimum_score)
 {
 	/* src points at a file that existed in the original tree (or
 	 * optionally a file in the destination tree) and dst points
diff --git a/merge-recursive.c b/merge-recursive.c
index 5f42c677d5..c54b918dc8 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -1290,6 +1290,130 @@ static struct diff_filespec *filespec_from_entry(struct diff_filespec *target,
 	return target;
 }
 
+#if 0 // #if-0-ing avoids unused function warning; will make live in next commit
+static int handle_file_collision(struct merge_options *o,
+				 const char *collide_path,
+				 const char *prev_path1,
+				 const char *prev_path2,
+				 const char *branch1, const char *branch2,
+				 const struct object_id *a_oid,
+				 unsigned int a_mode,
+				 const struct object_id *b_oid,
+				 unsigned int b_mode,
+				 unsigned int conflict_markers_already_present)
+{
+	struct merge_file_info mfi;
+	struct diff_filespec null, a, b;
+	int minimum_score;
+	char *new_path1, *new_path2;
+
+	/* Remove rename sources if rename/add or rename/rename(2to1) */
+	if (prev_path1)
+		remove_file(o, 1, prev_path1,
+			    o->call_depth || would_lose_untracked(prev_path1));
+	if (prev_path2)
+		remove_file(o, 1, prev_path2,
+			    o->call_depth || would_lose_untracked(prev_path2));
+
+	/*
+	 * Remove the collision path, if it wouldn't cause dirty contents
+	 * or an untracked file to get lost.  We'll either overwrite with
+	 * merged contents, or just write out to differently named files.
+	 */
+	if (was_dirty(o, collide_path))
+		output(o, 1, _("Refusing to lose dirty file at %s"),
+		       collide_path);
+	else if (would_lose_untracked(collide_path))
+		/*
+		 * Only way we get here is if both renames were from
+		 * a directory rename AND user had an untracked file
+		 * at the location where both files end up after the
+		 * two directory renames.  See testcase 10d of t6043.
+		 */
+		output(o, 1, _("Refusing to lose untracked file at "
+			       "%s, even though it's in the way."),
+		       collide_path);
+	else
+		/*
+		 * FIXME: It's possible that neither of the two files have
+		 * conflict markers already present, and that they're
+		 * identical, and that the current working copy happens to
+		 * match, in which case we are unnecessarily touching the
+		 * working tree file.  It's not a likely enough scenario
+		 * that I want to code up the checks for it and a better
+		 * fix is available if we restructure how unpack_trees()
+		 * and merge-recursive interoperate anyway, so punting for
+		 * now...
+		 */
+		remove_file(o, 0, collide_path, 0);
+
+	/* Store things in diff_filespecs for functions that need it */
+	memset(&a, 0, sizeof(struct diff_filespec));
+	memset(&b, 0, sizeof(struct diff_filespec));
+	null.path = a.path = b.path = (char *)collide_path;
+	oidcpy(&null.oid, &null_oid);
+	null.mode = 0;
+	oidcpy(&a.oid, a_oid);
+	a.mode = a_mode;
+	a.oid_valid = 1;
+	oidcpy(&b.oid, b_oid);
+	b.mode = b_mode;
+	b.oid_valid = 1;
+
+	/*
+	 * If the colliding files are similar enough, we can simply merge
+	 * them.  But we don't want to merge files that have conflict
+	 * markers in them already, because nested conflict markers are
+	 * too confusing.
+	 */
+	minimum_score = o->rename_score ? o->rename_score
+					: DEFAULT_RENAME_SCORE;
+	if (!conflict_markers_already_present && minimum_score <=
+	    estimate_similarity(&a, &b, o->rename_score)) {
+		if (merge_file_1(o, &null, &a, &b, branch1, branch2, &mfi))
+			return -1;
+		if (update_file(o, mfi.clean, &mfi.oid, mfi.mode, collide_path))
+			return -1;
+		if (!mfi.clean && !o->call_depth &&
+		    update_stages(o, collide_path, NULL, &a, &b))
+			return -1;
+		return mfi.clean;
+	}
+
+	/*
+	 * Put the colliding files into different paths, and record the
+	 * updated sha1sums in the index
+	 */
+	new_path1 = (o->call_depth && prev_path1) ? strdup(prev_path1) :
+		    unique_path(o, collide_path, branch1);
+	new_path2 = (o->call_depth && prev_path2) ? strdup(prev_path2) :
+		    unique_path(o, collide_path, branch2);
+	output(o, 1, _("Renaming collisions at %s to %s and %s instead"),
+	       collide_path, new_path1, new_path2);
+
+	if (update_file(o, 0, a_oid, a_mode, new_path1))
+		return -1;
+	if (update_file(o, 0, b_oid, b_mode, new_path2))
+		return -1;
+
+	/* Update index too, making sure to get stage order correct. */
+	if (!o->call_depth) {
+		if (o->branch1 == branch1) {
+			if (update_stages(o, collide_path, NULL, &a, &b))
+				return -1;
+		} else {
+			if (update_stages(o, collide_path, NULL, &b, &a))
+				return -1;
+		}
+	}
+
+	free(new_path2);
+	free(new_path1);
+
+	return 0; /* not clean */
+}
+#endif
+
 static int handle_file(struct merge_options *o,
 			struct diff_filespec *rename,
 			int stage,
-- 
2.16.0.41.g6a66043158