[PATCH] Allow reading "symbolic refs" that point to other refs

This extends the ref reading to understand a "symbolic ref": a ref file
that starts with "ref: " and points to another ref file, and thus
introduces the notion of ref aliases.

This is in preparation of allowing HEAD to eventually not be a symlink,
but one of these symbolic refs instead.

[jc: Linus originally required the prefix to be "ref: " five bytes
 and nothing else, but I changed it to allow and strip any number of
 leading whitespaces to match what update-ref.c does.]

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

cache.h

@@ -229,6 +229,7 @@ extern int has_pack_index(const unsigned char *sha1);
 extern int get_sha1(const char *str, unsigned char *sha1);
 extern int get_sha1_hex(const char *hex, unsigned char *sha1);
 extern char *sha1_to_hex(const unsigned char *sha1);	/* static buffer result! */
+extern int read_ref(const char *filename, unsigned char *sha1);
 
 /* General helper functions */
 extern void usage(const char *err) NORETURN;

refs.c

@@ -2,17 +2,43 @@
 #include "cache.h"
 
 #include <errno.h>
+#include <ctype.h>
 
-static int read_ref(const char *refname, unsigned char *sha1)
+/* We allow "recursive" symbolic refs. Only within reason, though */
+#define MAXDEPTH 5
+
+int read_ref(const char *filename, unsigned char *sha1)
 {
-	int ret = -1;
-	int fd = open(git_path("%s", refname), O_RDONLY);
+	int depth = 0;
+	int ret = -1, fd;
+
+	while ((fd = open(filename, O_RDONLY)) >= 0) {
+		char buffer[256];
+		int len = read(fd, buffer, sizeof(buffer)-1);
 
-	if (fd >= 0) {
-		char buffer[60];
-		if (read(fd, buffer, sizeof(buffer)) >= 40)
-			ret = get_sha1_hex(buffer, sha1);
 		close(fd);
+		if (len < 0)
+			break;
+
+		buffer[len] = 0;
+		while (len && isspace(buffer[len-1]))
+			buffer[--len] = 0;
+
+		if (!strncmp(buffer, "ref:", 4)) {
+			char *buf;
+			if (depth > MAXDEPTH)
+				break;
+			depth++;
+			buf = buffer + 4;
+			len -= 4;
+			while (len && isspace(*buf))
+				buf++, len--;
+			filename = git_path("%.*s", len, buf);
+			continue;
+		}
+		if (len >= 40)
+			ret = get_sha1_hex(buffer, sha1);
+		break;
 	}
 	return ret;
 }
@@ -54,7 +80,7 @@ static int do_for_each_ref(const char *base, int (*fn)(const char *path, const u
 					break;
 				continue;
 			}
-			if (read_ref(path, sha1) < 0)
+			if (read_ref(git_path("%s", path), sha1) < 0)
 				continue;
 			if (!has_sha1_file(sha1))
 				continue;
@@ -71,7 +97,7 @@ static int do_for_each_ref(const char *base, int (*fn)(const char *path, const u
 int head_ref(int (*fn)(const char *path, const unsigned char *sha1))
 {
 	unsigned char sha1[20];
-	if (!read_ref("HEAD", sha1))
+	if (!read_ref(git_path("HEAD"), sha1))
 		return fn("HEAD", sha1);
 	return 0;
 }
@@ -101,33 +127,14 @@ static char *ref_lock_file_name(const char *ref)
 	return ret;
 }
 
-static int read_ref_file(const char *filename, unsigned char *sha1) {
-	int fd = open(filename, O_RDONLY);
-	char hex[41];
-	if (fd < 0) {
-		return error("Couldn't open %s\n", filename);
-	}
-	if ((read(fd, hex, 41) < 41) ||
-	    (hex[40] != '\n') ||
-	    get_sha1_hex(hex, sha1)) {
-		error("Couldn't read a hash from %s\n", filename);
-		close(fd);
-		return -1;
-	}
-	close(fd);
-	return 0;
-}
-
 int get_ref_sha1(const char *ref, unsigned char *sha1)
 {
-	char *filename;
-	int retval;
+	const char *filename;
+
 	if (check_ref_format(ref))
 		return -1;
-	filename = ref_file_name(ref);
-	retval = read_ref_file(filename, sha1);
-	free(filename);
-	return retval;
+	filename = git_path("refs/%s", ref);
+	return read_ref(filename, sha1);
 }
 
 static int lock_ref_file(const char *filename, const char *lock_filename,
@@ -140,7 +147,7 @@ static int lock_ref_file(const char *filename, const char *lock_filename,
 		return error("Couldn't open lock file for %s: %s",
 			     filename, strerror(errno));
 	}
-	retval = read_ref_file(filename, current_sha1);
+	retval = read_ref(filename, current_sha1);
 	if (old_sha1) {
 		if (retval) {
 			close(fd);

sha1_name.c

@@ -119,21 +119,6 @@ static int get_short_sha1(const char *name, int len, unsigned char *sha1)
 	return -1;
 }
 
-static int get_sha1_file(const char *path, unsigned char *result)
-{
-	char buffer[60];
-	int fd = open(path, O_RDONLY);
-	int len;
-
-	if (fd < 0)
-		return -1;
-	len = read(fd, buffer, sizeof(buffer));
-	close(fd);
-	if (len < 40)
-		return -1;
-	return get_sha1_hex(buffer, result);
-}
-
 static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 {
 	static const char *prefix[] = {
@@ -150,7 +135,7 @@ static int get_sha1_basic(const char *str, int len, unsigned char *sha1)
 
 	for (p = prefix; *p; p++) {
 		char *pathname = git_path("%s/%.*s", *p, len, str);
-		if (!get_sha1_file(pathname, sha1))
+		if (!read_ref(pathname, sha1))
 			return 0;
 	}