read_sha1_file(): allow selective bypassing of replacement mechanism

The way "object replacement" mechanism was tucked to the read_sha1_file()
interface was suboptimal in a couple of ways:

 - Callers that want it to die with useful diagnosis upon seeing a corrupt
   object does not have a way to say that they do not want any object
   replacement.

 - Callers who do not want it to die but want to handle the errors
   themselves are told to arrange to call read_object(), but the function
   does not use the replacement mechanism, and also it is a file scope
   static function that not many callers can call to begin with.

This adds a read_sha1_file_extended() that takes a set of flags; the
callers of read_sha1_file() passes a flag READ_SHA1_FILE_REPLACE to ask
for object replacement mechanism to kick in.

Later, we could add another flag bit to tell the function to return an
error instead of dying and then remove the misguided "call read_object()
yourself".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -757,7 +757,12 @@ int daemon_avoid_alias(const char *path);
 int offset_1st_component(const char *path);
 
 /* object replacement */
-extern void *read_sha1_file(const unsigned char *sha1, enum object_type *type, unsigned long *size);
+#define READ_SHA1_FILE_REPLACE 1
+extern void *read_sha1_file_extended(const unsigned char *sha1, enum object_type *type, unsigned long *size, unsigned flag);
+static inline void *read_sha1_file(const unsigned char *sha1, enum object_type *type, unsigned long *size)
+{
+	return read_sha1_file_extended(sha1, type, size, READ_SHA1_FILE_REPLACE);
+}
 extern const unsigned char *do_lookup_replace_object(const unsigned char *sha1);
 static inline const unsigned char *lookup_replace_object(const unsigned char *sha1)
 {

sha1_file.c

@@ -2206,14 +2206,16 @@ static void *read_object(const unsigned char *sha1, enum object_type *type,
  * deal with them should arrange to call read_object() and give error
  * messages themselves.
  */
-void *read_sha1_file(const unsigned char *sha1,
-		     enum object_type *type,
-		     unsigned long *size)
+void *read_sha1_file_extended(const unsigned char *sha1,
+			      enum object_type *type,
+			      unsigned long *size,
+			      unsigned flag)
 {
-	const unsigned char *repl = lookup_replace_object(sha1);
 	void *data;
 	char *path;
 	const struct packed_git *p;
+	const unsigned char *repl = (flag & READ_SHA1_FILE_REPLACE)
+		? lookup_replace_object(sha1) : sha1;
 
 	errno = 0;
 	data = read_object(repl, type, size);