replace_object: add mechanism to replace objects found in "refs/replace/"

The code implementing this mechanism has been copied more-or-less
from the commit graft code.

This mechanism is used in "read_sha1_file". sha1 passed to this
function that match a ref name in "refs/replace/" are replaced by
the sha1 that has been read in the ref.

We "die" if the replacement recursion depth is too high or if we
can't read the replacement object.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -512,6 +512,7 @@ LIB_OBJS += read-cache.o
 LIB_OBJS += reflog-walk.o
 LIB_OBJS += refs.o
 LIB_OBJS += remote.o
+LIB_OBJS += replace_object.o
 LIB_OBJS += rerere.o
 LIB_OBJS += revision.o
 LIB_OBJS += run-command.o

commit.h

@@ -124,6 +124,8 @@ struct commit_graft *read_graft_line(char *buf, int len);
 int register_commit_graft(struct commit_graft *, int);
 struct commit_graft *lookup_commit_graft(const unsigned char *sha1);
 
+const unsigned char *lookup_replace_object(const unsigned char *sha1);
+
 extern struct commit_list *get_merge_bases(struct commit *rev1, struct commit *rev2, int cleanup);
 extern struct commit_list *get_merge_bases_many(struct commit *one, int n, struct commit **twos, int cleanup);
 extern struct commit_list *get_octopus_merge_bases(struct commit_list *in);

replace_object.c

@@ -0,0 +1,111 @@
+#include "cache.h"
+#include "sha1-lookup.h"
+#include "refs.h"
+
+static struct replace_object {
+	unsigned char sha1[2][20];
+} **replace_object;
+
+static int replace_object_alloc, replace_object_nr;
+
+static const unsigned char *replace_sha1_access(size_t index, void *table)
+{
+	struct replace_object **replace = table;
+	return replace[index]->sha1[0];
+}
+
+static int replace_object_pos(const unsigned char *sha1)
+{
+	return sha1_pos(sha1, replace_object, replace_object_nr,
+			replace_sha1_access);
+}
+
+static int register_replace_object(struct replace_object *replace,
+				   int ignore_dups)
+{
+	int pos = replace_object_pos(replace->sha1[0]);
+
+	if (0 <= pos) {
+		if (ignore_dups)
+			free(replace);
+		else {
+			free(replace_object[pos]);
+			replace_object[pos] = replace;
+		}
+		return 1;
+	}
+	pos = -pos - 1;
+	if (replace_object_alloc <= ++replace_object_nr) {
+		replace_object_alloc = alloc_nr(replace_object_alloc);
+		replace_object = xrealloc(replace_object,
+					  sizeof(*replace_object) *
+					  replace_object_alloc);
+	}
+	if (pos < replace_object_nr)
+		memmove(replace_object + pos + 1,
+			replace_object + pos,
+			(replace_object_nr - pos - 1) *
+			sizeof(*replace_object));
+	replace_object[pos] = replace;
+	return 0;
+}
+
+static int register_replace_ref(const char *refname,
+				const unsigned char *sha1,
+				int flag, void *cb_data)
+{
+	/* Get sha1 from refname */
+	const char *slash = strrchr(refname, '/');
+	const char *hash = slash ? slash + 1 : refname;
+	struct replace_object *repl_obj = xmalloc(sizeof(*repl_obj));
+
+	if (strlen(hash) != 40 || get_sha1_hex(hash, repl_obj->sha1[0])) {
+		free(repl_obj);
+		warning("bad replace ref name: %s", refname);
+		return 0;
+	}
+
+	/* Copy sha1 from the read ref */
+	hashcpy(repl_obj->sha1[1], sha1);
+
+	/* Register new object */
+	if (register_replace_object(repl_obj, 1))
+		die("duplicate replace ref: %s", refname);
+
+	return 0;
+}
+
+static void prepare_replace_object(void)
+{
+	static int replace_object_prepared;
+
+	if (replace_object_prepared)
+		return;
+
+	for_each_replace_ref(register_replace_ref, NULL);
+	replace_object_prepared = 1;
+}
+
+/* We allow "recursive" replacement. Only within reason, though */
+#define MAXREPLACEDEPTH 5
+
+const unsigned char *lookup_replace_object(const unsigned char *sha1)
+{
+	int pos, depth = MAXREPLACEDEPTH;
+	const unsigned char *cur = sha1;
+
+	prepare_replace_object();
+
+	/* Try to recursively replace the object */
+	do {
+		if (--depth < 0)
+			die("replace depth too high for object %s",
+			    sha1_to_hex(sha1));
+
+		pos = replace_object_pos(cur);
+		if (0 <= pos)
+			cur = replace_object[pos]->sha1[1];
+	} while (0 <= pos);
+
+	return cur;
+}

sha1_file.c

@@ -2148,10 +2148,18 @@ static void *read_object(const unsigned char *sha1, enum object_type *type,
 void *read_sha1_file(const unsigned char *sha1, enum object_type *type,
 		     unsigned long *size)
 {
-	void *data = read_object(sha1, type, size);
+	const unsigned char *repl = lookup_replace_object(sha1);
+	void *data = read_object(repl, type, size);
+
+	/* die if we replaced an object with one that does not exist */
+	if (!data && repl != sha1)
+		die("replacement %s not found for %s",
+		    sha1_to_hex(repl), sha1_to_hex(sha1));
+
 	/* legacy behavior is to die on corrupted objects */
-	if (!data && (has_loose_object(sha1) || has_packed_and_bad(sha1)))
-		die("object %s is corrupted", sha1_to_hex(sha1));
+	if (!data && (has_loose_object(repl) || has_packed_and_bad(repl)))
+		die("object %s is corrupted", sha1_to_hex(repl));
+
 	return data;
 }