xdiff: add new XDL_MERGE_FAVOR_BASE mode

Similar to XDL_MERGE_FAVOR_{OURS,THEIRS} add a new mode for favoring the
base version in the event of a conflict, and add a --base flag to
merge-file to allow testing this out.

This will be used in a subsequent commit to reduce the prevalence of
nested conflict markers which traditionally arise when using the diff3
merge.conflictStyle due to having conflicts in the virtual merge base.
By eschewing using conflicted regions as the resolution for the virtual
merge base, and instead preferring to use the version of the text from
the merge base of the merge bases, we can avoid causing nested conflicts
in the outer merge.

Signed-off-by: Elijah Newren <[email protected]>

Author: newren

builtin/merge-file.c

@@ -74,6 +74,8 @@ int cmd_merge_file(int argc,
 		OPT_SET_INT(0, "diff3", &xmp.style, N_("use a diff3 based merge"), XDL_MERGE_DIFF3),
 		OPT_SET_INT(0, "zdiff3", &xmp.style, N_("use a zealous diff3 based merge"),
 				XDL_MERGE_ZEALOUS_DIFF3),
+		OPT_SET_INT(0, "base", &xmp.favor, N_("for conflicts, use base version"),
+			    XDL_MERGE_FAVOR_BASE),
 		OPT_SET_INT(0, "ours", &xmp.favor, N_("for conflicts, use our version"),
 			    XDL_MERGE_FAVOR_OURS),
 		OPT_SET_INT(0, "theirs", &xmp.favor, N_("for conflicts, use their version"),

t/t6403-merge-file.sh

@@ -301,6 +301,26 @@ test_expect_success "merge conflicting with --union" '
 	test_cmp expect.txt test.txt
 '
 
+test_expect_success "merge conflicting with --base" '
+	cp backup.txt test.txt &&
+
+	cat >expect.txt <<-\EOF &&
+	Dominus regit me,
+	et nihil mihi deerit.
+	In loco pascuae ibi me collocavit,
+	super aquam refectionis educavit me;
+	animam meam convertit,
+	deduxit me super semitas jusitiae,
+	propter nomen suum.
+	Nam et si ambulavero in medio umbrae mortis,
+	non timebo mala, quoniam tu mecum es:
+	virga tua et baculus tuus ipsa me consolata sunt.
+	EOF
+
+	git merge-file --base test.txt orig.txt new3.txt &&
+	test_cmp expect.txt test.txt
+'
+
 test_expect_success "merge with conflicts, using -L" '
 	cp backup.txt test.txt &&
 

xdiff/xdiff.h

@@ -63,6 +63,7 @@ extern "C" {
 #define XDL_MERGE_FAVOR_OURS 1
 #define XDL_MERGE_FAVOR_THEIRS 2
 #define XDL_MERGE_FAVOR_UNION 3
+#define XDL_MERGE_FAVOR_BASE 4
 
 /* merge output styles */
 #define XDL_MERGE_DIFF3 1

xdiff/xmerge.c

@@ -26,9 +26,10 @@ typedef struct s_xdmerge {
 	struct s_xdmerge *next;
 	/*
 	 * 0 = conflict,
-	 * 1 = no conflict, take first,
+	 * 1 = no conflict, take first.
 	 * 2 = no conflict, take second.
-	 * 3 = no conflict, take both.
+	 * 3 = no conflict, take both first & second.
+	 * 4 = no conflict, take base.
 	 */
 	int mode;
 	/*
@@ -313,6 +314,13 @@ static int xdl_fill_merge_buffer(xdfenv_t *xe1, const char *name1,
 			if (m->mode & 2)
 				size += xdl_recs_copy(xe2, m->i2, m->chg2, 0, 0,
 						      dest ? dest + size : NULL);
+		} else if (m->mode == XDL_MERGE_FAVOR_BASE) {
+			/* Before conflicting part */
+			size += xdl_recs_copy(xe1, i, m->i1 - i, 0, 0,
+					      dest ? dest + size : NULL);
+			/* Image from merge base */
+			size += xdl_orig_copy(xe1, m->i0, m->chg0, 0, 0,
+					      dest ? dest + size : NULL);
 		} else
 			continue;
 		i = m->i1 + m->chg1;