[PATCH] Add -i option to cat-file to allow operation on displaced git object files

[Jon Seymour <jon.seymour@gmail.com>]

For the purposes of merging the contents of remote git object
repositories at the filesystem level, it would be helpful to be able
to verify the integrity of a remote git object file before it is
actually copied into the local repository.

To enable this, the option syntax for the usage of the cat-file tool
is extended with a -i option, per the modified usage string quoted
below:

    usage: cat-file [-t | tagname] <sha1> [ -i displaced-git-object-file ]

If the -i option is specified, cat-file uses the filename specified on
the command line rather than the derived filename to locate and
process the git object file implied by the sha1 argument.

In addition, the -i option forces cat-file to check the SHA1 signature
of the specified input file against the SHA1 signature specified on
the command line and report an error if there is a mismatch.

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
--
Index: cache.h
===================================================================
--- e6d3a81c30ad237102e2ca82f3dfc57d0b9f0ddf/cache.h  (mode:100644
sha1:bf30ac4741d2eeeb483079f566182505898082f3)
+++ 1b503f0571727a865b413e28641d2be09c8c3304/cache.h  (mode:100644
sha1:56eacfa0ab43c93a838efedca429e865c95f16bc)
@@ -121,8 +121,9 @@
  extern void * map_sha1_file(const unsigned char *sha1, unsigned long *size);
  extern void * unpack_sha1_file(void *map, unsigned long mapsize,
char *type, unsigned long *size);
  extern void * read_sha1_file(const unsigned char *sha1, char *type,
unsigned long *size);
+extern void * read_displaced_sha1_file(char *type, unsigned long
*size, const char *filename);
  extern int write_sha1_file(char *buf, unsigned len, unsigned char
*return_sha1);
-extern int check_sha1_signature(unsigned char *sha1, void *buf,
unsigned long size, const char *type);
+extern int check_sha1_signature(const unsigned char *sha1, void *buf,
unsigned long size, const char *type);
 
 /* Read a tree into the cache */
 extern int read_tree(void *buffer, unsigned long size, int stage);
Index: cat-file.c
===================================================================
--- e6d3a81c30ad237102e2ca82f3dfc57d0b9f0ddf/cat-file.c  (mode:100644
sha1:3c47d79a16305d326a65768fe9f37ee25928510b)
+++ 1b503f0571727a865b413e28641d2be09c8c3304/cat-file.c  (mode:100644
sha1:c88df7287b6ce7d6d33d016ffb524cc5a793cad2)
@@ -5,16 +5,32 @@
  */
 #include "cache.h"
 
+static char _usage[]="cat-file [-t | tagname] <sha1> [ -i
displaced-git-object-file ]";
+
  int main(int argc, char **argv)
 {
  	unsigned char sha1[20];
 	char type[20];
-	void *buf;
+	void *buf = NULL;
 	unsigned long size;
 
-	if (argc != 3 || get_sha1_hex(argv[2], sha1))
-		usage("cat-file [-t | tagname] <sha1>");
-	buf = read_sha1_file(sha1, type, &size);
+	if (argc < 3 || get_sha1_hex(argv[2], sha1))
+		usage(_usage);
+
+        if (argc == 3) {
+	    buf = read_sha1_file(sha1, type, &size);
+        } else if (argc == 5 && !strcmp(argv[3], "-i")) {
+            buf = read_displaced_sha1_file(type, &size, argv[4]);
+            if (!buf) 
+                    die("bad file: %s", argv[4]);
+	    
+            if (check_sha1_signature(sha1, buf, size, type) < 0)
+                    die("signature mismatch for %s", argv[4]);
+
+        } else {
+	    usage(_usage);
+        }
+
  	if (!buf)
  		die("cat-file %s: bad file", argv[2]);
  	if (!strcmp("-t", argv[1])) {
Index: sha1_file.c
===================================================================
--- e6d3a81c30ad237102e2ca82f3dfc57d0b9f0ddf/sha1_file.c  (mode:100644
sha1:6f7228e106d4e24b18f8416cc6adc2a6fd303eb7)
+++ 1b503f0571727a865b413e28641d2be09c8c3304/sha1_file.c  (mode:100644
sha1:25e2e8b22cf9590a6df05f234ce88db8c2cd1030)
@@ -80,7 +80,7 @@
 	return base;
 }
 
-int check_sha1_signature(unsigned char *sha1, void *map, unsigned
long size, const char *type)
+int check_sha1_signature(const unsigned char *sha1, void *map,
unsigned long size, const char *type)
 {
 	char header[100];
 	unsigned char real_sha1[20];
@@ -93,13 +93,12 @@
  	return memcmp(sha1, real_sha1, 20) ? -1 : 0;
 }
 
-void *map_sha1_file(const unsigned char *sha1, unsigned long *size)
+static void *map_file(const char *filename, unsigned long *size)
 {
-	char *filename = sha1_file_name(sha1);
-	int fd = open(filename, O_RDONLY);
 	struct stat st;
 	void *map;
 
+	int fd = open(filename, O_RDONLY);
 	if (fd < 0) {
 		perror(filename);
 		return NULL;
@@ -116,6 +115,12 @@
 	return map;
 }
 
+void *map_sha1_file(const unsigned char *sha1, unsigned long *size)
+{
+        char *filename = sha1_file_name(sha1);
+        return map_file(filename, size);
+}
+
  void * unpack_sha1_file(void *map, unsigned long mapsize, char
*type, unsigned long *size)
 {
 	int ret, bytes;
@@ -166,6 +171,20 @@
 	return NULL;
 }
 
+void * read_displaced_sha1_file(char *type, unsigned long *size,
const char *filename)
+{
+	unsigned long mapsize;
+	void *map, *buf;
+
+	map = map_file(filename, &mapsize);
+	if (map) {
+		buf = unpack_sha1_file(map, mapsize, type, size);
+		munmap(map, mapsize);
+		return buf;
+	}
+	return NULL;
+}
+
  void *read_tree_with_tree_or_commit_sha1(const unsigned char *sha1,
 					 unsigned long *size,
 					 unsigned char *tree_sha1_return)