mergetools/p4merge: create a base if none available

kjbracey2 · gitster · commit 4549162e8d62 · 2013-03-13T10:46:07.000-07:00
Originally, with no base, Git gave P4Merge $LOCAL as a dummy base: p4merge "$LOCAL" "$LOCAL" "$REMOTE" "$MERGED" Commit 0a0ec7b changed this to: p4merge "empty file" "$LOCAL" "$REMOTE" "$MERGED" to avoid the problem of being unable to save in some circumstances with similar inputs. Unfortunately this approach produces much worse results on differing inputs. P4Merge really regards the blank file as the base, and once you have just a couple of differences between the two branches you end up with one a massive full-file conflict. The 3-way diff is not readable, and you have to invoke "difftool MERGE_HEAD HEAD" manually to get a useful view. The original approach appears to have invoked special 2-way merge behaviour in P4Merge that occurs only if the base filename is "" or equal to the left input. You get a good visual comparison, and it does not auto-resolve differences. (Normally if one branch matched the base, it would autoresolve to the other branch). But there appears to be no way of getting this 2-way behaviour and being able to reliably save. Having base==left appears to be triggering other assumptions. There are tricks the user can use to force the save icon on, but it's not intuitive. So we now follow a suggestion given in the original patch's discussion: generate a virtual base, consisting of the lines common to the two branches. This is the same as the technique used in resolve and octopus merges, so we relocate that code to a shared function. Note that if there are no differences at the same location, this technique can lead to automatic resolution without conflict, combining everything from the 2 files. As with the other merges using this technique, we assume the user will inspect the result before saving. Signed-off-by: Kevin Bracey <kevin@bracey.fi> Reviewed-by: David Aguilar <davvid@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-sh-setup.txt b/Documentation/git-sh-setup.txt
@@ -82,6 +82,12 @@ get_author_ident_from_commit::
 	outputs code for use with eval to set the GIT_AUTHOR_NAME,
 	GIT_AUTHOR_EMAIL and GIT_AUTHOR_DATE variables for a given commit.
 
+create_virtual_base::
+	modifies the first file so only lines in common with the
+	second file remain. If there is insufficient common material,
+	then the first file is left empty. The result is suitable
+	as a virtual base input for a 3-way merge.
+
 GIT
 ---
 Part of the linkgit:git[1] suite
diff --git a/git-merge-one-file.sh b/git-merge-one-file.sh
@@ -104,30 +104,22 @@ case "${1:-.}${2:-.}${3:-.}" in
 		;;
 	esac
 
-	src2=`git-unpack-file $3`
+	src1=$(git-unpack-file $2)
+	src2=$(git-unpack-file $3)
 	case "$1" in
 	'')
 		echo "Added $4 in both, but differently."
-		# This extracts OUR file in $orig, and uses git apply to
-		# remove lines that are unique to ours.
-		orig=`git-unpack-file $2`
-		sz0=`wc -c <"$orig"`
-		@@DIFF@@ -u -La/$orig -Lb/$orig $orig $src2 | git apply --no-add
-		sz1=`wc -c <"$orig"`
-
-		# If we do not have enough common material, it is not
-		# worth trying two-file merge using common subsections.
-		expr $sz0 \< $sz1 \* 2 >/dev/null || : >$orig
+		orig=$(git-unpack-file $2)
+		create_virtual_base "$orig" "$src2"
 		;;
 	*)
 		echo "Auto-merging $4"
-		orig=`git-unpack-file $1`
+		orig=$(git-unpack-file $1)
 		;;
 	esac
 
 	# Be careful for funny filename such as "-L" in "$4", which
 	# would confuse "merge" greatly.
-	src1=`git-unpack-file $2`
 	git merge-file "$src1" "$orig" "$src2"
 	ret=$?
 	msg=
diff --git a/git-sh-setup.sh b/git-sh-setup.sh
@@ -249,6 +249,18 @@ clear_local_git_env() {
 	unset $(git rev-parse --local-env-vars)
 }
 
+# Generate a virtual base file for a two-file merge. Uses git apply to
+# remove lines from $1 that are not in $2, leaving only common lines.
+create_virtual_base() {
+	sz0=$(wc -c <"$1")
+	@@DIFF@@ -u -La/"$1" -Lb/"$1" "$1" "$2" | git apply --no-add
+	sz1=$(wc -c <"$1")
+
+	# If we do not have enough common material, it is not
+	# worth trying two-file merge using common subsections.
+	expr $sz0 \< $sz1 \* 2 >/dev/null || : >"$1"
+}
+
 
 # Platform specific tweaks to work around some commands
 case $(uname -s) in
diff --git a/mergetools/p4merge b/mergetools/p4merge
@@ -21,7 +21,11 @@ diff_cmd () {
 
 merge_cmd () {
 	touch "$BACKUP"
-	$base_present || >"$BASE"
+	if ! $base_present
+	then
+		cp -- "$LOCAL" "$BASE"
+		create_virtual_base "$BASE" "$REMOTE"
+	fi
 	"$merge_tool_path" "$BASE" "$REMOTE" "$LOCAL" "$MERGED"
 	check_unchanged
 }
