[PATCH] Prepare diff side for upcoming symlink work.

[Junio C Hamano <junkio@cox.net>]

This patch prepares the external diff interface engine for the
change to store the symbolic links in the cache, being worked on
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>
---

--- a/diff.c
+++ b/diff.c
@@ -163,13 +163,35 @@ static int work_tree_matches(const char 
 	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)
@@ -196,20 +218,35 @@ static void prepare_temp_file(const char
 	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_IFREG|ce_permissions(st.st_mode)));
+		}
 		if (!one->sha1_valid)
 			strcpy(temp->hex, sha1_to_hex(null_sha1));
 		else
 			strcpy(temp->hex, sha1_to_hex(one->blob_sha1));
 		sprintf(temp->mode, "%06o",
-			S_IFREG |ce_permissions(st.st_mode));
+			S_IFREG | ce_permissions(st.st_mode));
+		return;
 	}
 	else {
-		int fd;
 		void *blob;
 		char type[20];
 		unsigned long size;
@@ -218,19 +255,8 @@ static void prepare_temp_file(const char
 		if (!blob || strcmp(type, "blob"))
 			die("unable to read blob object for %s (%s)",
 			    name, sha1_to_hex(one->blob_sha1));
-
-		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);
+		prep_temp_blob(temp, blob, size, one->blob_sha1, one->mode);
 		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);
 	}
 }