diff.c: output correct index lines for a split diff

A patch that changes the filetype (e.g. regular file to symlink) of a path
must be split into a deletion event followed by a creation event, which
means that we need to have two independent metainfo lines for each.
However, the code reused the single set of metainfo lines.

As the blob object names recorded on the index lines are usually not used
nor validated on the receiving end, this is not an issue with normal use
of the resulting patch.  However, when accepting a binary patch to delete
a blob, git-apply verified that the postimage blob object name on the
index line is 0{40}, hence a patch that deletes a regular file blob that
records binary contents to create a blob with different filetype (e.g. a
symbolic link) failed to apply.  "git am -3" also uses the blob object
names recorded on the index line, so it would also misbehave when
synthesizing a preimage tree.

This moves the code to generate metainfo lines around, so that two
independent sets of metainfo lines are used for the split halves.

Additional tests by Jeff King.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -2096,16 +2096,86 @@ static const char *external_diff_attr(const char *name)
 	return NULL;
 }
 
+static int similarity_index(struct diff_filepair *p)
+{
+	return p->score * 100 / MAX_SCORE;
+}
+
+static void fill_metainfo(struct strbuf *msg,
+			  const char *name,
+			  const char *other,
+			  struct diff_filespec *one,
+			  struct diff_filespec *two,
+			  struct diff_options *o,
+			  struct diff_filepair *p)
+{
+	strbuf_init(msg, PATH_MAX * 2 + 300);
+	switch (p->status) {
+	case DIFF_STATUS_COPIED:
+		strbuf_addf(msg, "similarity index %d%%", similarity_index(p));
+		strbuf_addstr(msg, "\ncopy from ");
+		quote_c_style(name, msg, NULL, 0);
+		strbuf_addstr(msg, "\ncopy to ");
+		quote_c_style(other, msg, NULL, 0);
+		strbuf_addch(msg, '\n');
+		break;
+	case DIFF_STATUS_RENAMED:
+		strbuf_addf(msg, "similarity index %d%%", similarity_index(p));
+		strbuf_addstr(msg, "\nrename from ");
+		quote_c_style(name, msg, NULL, 0);
+		strbuf_addstr(msg, "\nrename to ");
+		quote_c_style(other, msg, NULL, 0);
+		strbuf_addch(msg, '\n');
+		break;
+	case DIFF_STATUS_MODIFIED:
+		if (p->score) {
+			strbuf_addf(msg, "dissimilarity index %d%%\n",
+				    similarity_index(p));
+			break;
+		}
+		/* fallthru */
+	default:
+		/* nothing */
+		;
+	}
+	if (one && two && hashcmp(one->sha1, two->sha1)) {
+		int abbrev = DIFF_OPT_TST(o, FULL_INDEX) ? 40 : DEFAULT_ABBREV;
+
+		if (DIFF_OPT_TST(o, BINARY)) {
+			mmfile_t mf;
+			if ((!fill_mmfile(&mf, one) && diff_filespec_is_binary(one)) ||
+			    (!fill_mmfile(&mf, two) && diff_filespec_is_binary(two)))
+				abbrev = 40;
+		}
+		strbuf_addf(msg, "index %.*s..%.*s",
+			    abbrev, sha1_to_hex(one->sha1),
+			    abbrev, sha1_to_hex(two->sha1));
+		if (one->mode == two->mode)
+			strbuf_addf(msg, " %06o", one->mode);
+		strbuf_addch(msg, '\n');
+	}
+	if (msg->len)
+		strbuf_setlen(msg, msg->len - 1);
+}
+
 static void run_diff_cmd(const char *pgm,
 			 const char *name,
 			 const char *other,
 			 const char *attr_path,
 			 struct diff_filespec *one,
 			 struct diff_filespec *two,
-			 const char *xfrm_msg,
+			 struct strbuf *msg,
 			 struct diff_options *o,
-			 int complete_rewrite)
+			 struct diff_filepair *p)
 {
+	const char *xfrm_msg = NULL;
+	int complete_rewrite = (p->status == DIFF_STATUS_MODIFIED) && p->score;
+
+	if (msg) {
+		fill_metainfo(msg, name, other, one, two, o, p);
+		xfrm_msg = msg->len ? msg->buf : NULL;
+	}
+
 	if (!DIFF_OPT_TST(o, ALLOW_EXTERNAL))
 		pgm = NULL;
 	else {
@@ -2145,11 +2215,6 @@ static void diff_fill_sha1_info(struct diff_filespec *one)
 		hashclr(one->sha1);
 }
 
-static int similarity_index(struct diff_filepair *p)
-{
-	return p->score * 100 / MAX_SCORE;
-}
-
 static void strip_prefix(int prefix_length, const char **namep, const char **otherp)
 {
 	/* Strip the prefix but do not molest /dev/null and absolute paths */
@@ -2163,13 +2228,11 @@ static void run_diff(struct diff_filepair *p, struct diff_options *o)
 {
 	const char *pgm = external_diff();
 	struct strbuf msg;
-	char *xfrm_msg;
 	struct diff_filespec *one = p->one;
 	struct diff_filespec *two = p->two;
 	const char *name;
 	const char *other;
 	const char *attr_path;
-	int complete_rewrite = 0;
 
 	name  = p->one->path;
 	other = (strcmp(name, p->two->path) ? p->two->path : NULL);
@@ -2179,83 +2242,34 @@ static void run_diff(struct diff_filepair *p, struct diff_options *o)
 
 	if (DIFF_PAIR_UNMERGED(p)) {
 		run_diff_cmd(pgm, name, NULL, attr_path,
-			     NULL, NULL, NULL, o, 0);
+			     NULL, NULL, NULL, o, p);
 		return;
 	}
 
 	diff_fill_sha1_info(one);
 	diff_fill_sha1_info(two);
 
-	strbuf_init(&msg, PATH_MAX * 2 + 300);
-	switch (p->status) {
-	case DIFF_STATUS_COPIED:
-		strbuf_addf(&msg, "similarity index %d%%", similarity_index(p));
-		strbuf_addstr(&msg, "\ncopy from ");
-		quote_c_style(name, &msg, NULL, 0);
-		strbuf_addstr(&msg, "\ncopy to ");
-		quote_c_style(other, &msg, NULL, 0);
-		strbuf_addch(&msg, '\n');
-		break;
-	case DIFF_STATUS_RENAMED:
-		strbuf_addf(&msg, "similarity index %d%%", similarity_index(p));
-		strbuf_addstr(&msg, "\nrename from ");
-		quote_c_style(name, &msg, NULL, 0);
-		strbuf_addstr(&msg, "\nrename to ");
-		quote_c_style(other, &msg, NULL, 0);
-		strbuf_addch(&msg, '\n');
-		break;
-	case DIFF_STATUS_MODIFIED:
-		if (p->score) {
-			strbuf_addf(&msg, "dissimilarity index %d%%\n",
-					similarity_index(p));
-			complete_rewrite = 1;
-			break;
-		}
-		/* fallthru */
-	default:
-		/* nothing */
-		;
-	}
-
-	if (hashcmp(one->sha1, two->sha1)) {
-		int abbrev = DIFF_OPT_TST(o, FULL_INDEX) ? 40 : DEFAULT_ABBREV;
-
-		if (DIFF_OPT_TST(o, BINARY)) {
-			mmfile_t mf;
-			if ((!fill_mmfile(&mf, one) && diff_filespec_is_binary(one)) ||
-			    (!fill_mmfile(&mf, two) && diff_filespec_is_binary(two)))
-				abbrev = 40;
-		}
-		strbuf_addf(&msg, "index %.*s..%.*s",
-				abbrev, sha1_to_hex(one->sha1),
-				abbrev, sha1_to_hex(two->sha1));
-		if (one->mode == two->mode)
-			strbuf_addf(&msg, " %06o", one->mode);
-		strbuf_addch(&msg, '\n');
-	}
-
-	if (msg.len)
-		strbuf_setlen(&msg, msg.len - 1);
-	xfrm_msg = msg.len ? msg.buf : NULL;
-
 	if (!pgm &&
 	    DIFF_FILE_VALID(one) && DIFF_FILE_VALID(two) &&
 	    (S_IFMT & one->mode) != (S_IFMT & two->mode)) {
-		/* a filepair that changes between file and symlink
+		/*
+		 * a filepair that changes between file and symlink
 		 * needs to be split into deletion and creation.
 		 */
 		struct diff_filespec *null = alloc_filespec(two->path);
 		run_diff_cmd(NULL, name, other, attr_path,
-			     one, null, xfrm_msg, o, 0);
+			     one, null, &msg, o, p);
 		free(null);
+		strbuf_release(&msg);
+
 		null = alloc_filespec(one->path);
 		run_diff_cmd(NULL, name, other, attr_path,
-			     null, two, xfrm_msg, o, 0);
+			     null, two, &msg, o, p);
 		free(null);
 	}
 	else
 		run_diff_cmd(pgm, name, other, attr_path,
-			     one, two, xfrm_msg, o, complete_rewrite);
+			     one, two, &msg, o, p);
 
 	strbuf_release(&msg);
 }

t/t4114-apply-typechange.sh

@@ -25,6 +25,10 @@ test_expect_success 'setup repository and commits' '
 	git update-index foo &&
 	git commit -m "foo back to file" &&
 	git branch foo-back-to-file &&
+	printf "\0" > foo &&
+	git update-index foo &&
+	git commit -m "foo becomes binary" &&
+	git branch foo-becomes-binary &&
 	rm -f foo &&
 	git update-index --remove foo &&
 	mkdir foo &&
@@ -85,6 +89,20 @@ test_expect_success 'symlink becomes file' '
 	'
 test_debug 'cat patch'
 
+test_expect_success 'binary file becomes symlink' '
+	git checkout -f foo-becomes-binary &&
+	git diff-tree -p --binary HEAD foo-symlinked-to-bar > patch &&
+	git apply --index < patch
+	'
+test_debug 'cat patch'
+
+test_expect_success 'symlink becomes binary file' '
+	git checkout -f foo-symlinked-to-bar &&
+	git diff-tree -p --binary HEAD foo-becomes-binary > patch &&
+	git apply --index < patch
+	'
+test_debug 'cat patch'
+
 
 test_expect_success 'symlink becomes directory' '
 	git checkout -f foo-symlinked-to-bar &&