Update diff engine for symlinks stored in the cache.

This patch updates the external diff interface engine for the change
to store the symbolic links in the cache, recently done by Kay
Sievers.

The main thing it does is when comparing with the work tree, it
prepares the counterpart to the blob being compared by doing a
readlink followed by sending that result to a temporary file to
be diffed.

Signed-off-by: Junio C Hamano <junkio@cox.net>

diff.c

@@ -125,9 +125,16 @@ static void builtin_diff(const char *name,
 		printf("Created: %s (mode:%s)\n", name, temp[1].mode);
 	else if (!path1[1][0])
 		printf("Deleted: %s\n", name);
-	else if (strcmp(temp[0].mode, temp[1].mode))
+	else if (strcmp(temp[0].mode, temp[1].mode)) {
 		printf("Mode changed: %s (%s->%s)\n", name,
 		       temp[0].mode, temp[1].mode);
+		/* Be careful.  We do not want to diff between
+		 * symlink and a file.
+		 */
+		if (strncmp(temp[0].mode, "120", 3) !=
+		    strncmp(temp[1].mode, "120", 3))
+			exit(0);
+	}
 	fflush(NULL);
 	execlp("/bin/sh","sh", "-c", cmd, NULL);
 }
@@ -163,13 +170,35 @@ static int work_tree_matches(const char *name, const unsigned char *sha1)
 	if (pos < 0)
 		return 0;
 	ce = active_cache[pos];
-	if ((stat(name, &st) < 0) ||
+	if ((lstat(name, &st) < 0) ||
+	    !S_ISREG(st.st_mode) ||
 	    cache_match_stat(ce, &st) ||
 	    memcmp(sha1, ce->sha1, 20))
 		return 0;
 	return 1;
 }
 
+static void prep_temp_blob(struct diff_tempfile *temp,
+			   void *blob,
+			   unsigned long size,
+			   unsigned char *sha1,
+			   int mode)
+{
+	int fd;
+
+	strcpy(temp->tmp_path, ".diff_XXXXXX");
+	fd = mkstemp(temp->tmp_path);
+	if (fd < 0)
+		die("unable to create temp-file");
+	if (write(fd, blob, size) != size)
+		die("unable to write temp-file");
+	close(fd);
+	temp->name = temp->tmp_path;
+	strcpy(temp->hex, sha1_to_hex(sha1));
+	temp->hex[40] = 0;
+	sprintf(temp->mode, "%06o", mode);
+}
+
 static void prepare_temp_file(const char *name,
 			      struct diff_tempfile *temp,
 			      struct diff_spec *one)
@@ -196,11 +225,26 @@ static void prepare_temp_file(const char *name,
 	if (!one->sha1_valid || use_work_tree) {
 		struct stat st;
 		temp->name = name;
-		if (stat(temp->name, &st) < 0) {
+		if (lstat(temp->name, &st) < 0) {
 			if (errno == ENOENT)
 				goto not_a_valid_file;
 			die("stat(%s): %s", temp->name, strerror(errno));
 		}
+		if (S_ISLNK(st.st_mode)) {
+			int ret;
+			char *buf, buf_[1024];
+			buf = ((sizeof(buf_) < st.st_size) ?
+			       xmalloc(st.st_size) : buf_);
+			ret = readlink(name, buf, st.st_size);
+			if (ret < 0)
+				die("readlink(%s)", name);
+			prep_temp_blob(temp, buf, st.st_size,
+				       (one->sha1_valid ?
+					one->blob_sha1 : null_sha1),
+				       (one->sha1_valid ?
+					one->mode : S_IFLNK));
+		}
+		else {
 			if (!one->sha1_valid)
 				strcpy(temp->hex, sha1_to_hex(null_sha1));
 			else
@@ -208,8 +252,9 @@ static void prepare_temp_file(const char *name,
 			sprintf(temp->mode, "%06o",
 				S_IFREG |ce_permissions(st.st_mode));
 		}
+		return;
+	}
 	else {
-		int fd;
 		void *blob;
 		char type[20];
 		unsigned long size;
@@ -218,19 +263,8 @@ static void prepare_temp_file(const char *name,
 		if (!blob || strcmp(type, "blob"))
 			die("unable to read blob object for %s (%s)",
 			    name, sha1_to_hex(one->blob_sha1));
-
+		prep_temp_blob(temp, blob, size, one->blob_sha1, one->mode);
-		strcpy(temp->tmp_path, ".diff_XXXXXX");
-		fd = mkstemp(temp->tmp_path);
-		if (fd < 0)
-			die("unable to create temp-file");
-		if (write(fd, blob, size) != size)
-			die("unable to write temp-file");
-		close(fd);
 		free(blob);
-		temp->name = temp->tmp_path;
-		strcpy(temp->hex, sha1_to_hex(one->blob_sha1));
-		temp->hex[40] = 0;
-		sprintf(temp->mode, "%06o", one->mode);
 	}
 }