add receive.denyNonFastforwards config variable

If receive.denyNonFastforwards is set to true, git-receive-pack will deny
non fast-forwards, i.e. forced updates. Most notably, a push to a repository
which has that flag set will fail.

As a first user, 'git-init-db --shared' sets this flag, since in a shared
setup, you are most unlikely to want forced pushes to succeed.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-init-db.c

@@ -311,6 +311,7 @@ int cmd_init_db(int argc, const char **argv, const char *prefix)
 		 */
 		sprintf(buf, "%d", shared_repository);
 		git_config_set("core.sharedrepository", buf);
+		git_config_set("receive.denyNonFastforwards", "true");
 	}
 
 	return 0;

cache.h

@@ -188,6 +188,7 @@ extern int prefer_symlink_refs;
 extern int log_all_ref_updates;
 extern int warn_ambiguous_refs;
 extern int shared_repository;
+extern int deny_non_fast_forwards;
 extern const char *apply_default_whitespace;
 extern int zlib_compression_level;
 

environment.c

@@ -20,6 +20,7 @@ int warn_ambiguous_refs = 1;
 int repository_format_version;
 char git_commit_encoding[MAX_ENCODING_LENGTH] = "utf-8";
 int shared_repository = PERM_UMASK;
+int deny_non_fast_forwards = 0;
 const char *apply_default_whitespace;
 int zlib_compression_level = Z_DEFAULT_COMPRESSION;
 int pager_in_use;

receive-pack.c

@@ -2,6 +2,8 @@
 #include "refs.h"
 #include "pkt-line.h"
 #include "run-command.h"
+#include "commit.h"
+#include "object.h"
 
 static const char receive_pack_usage[] = "git-receive-pack <git-dir>";
 
@@ -127,6 +129,20 @@ static int update(struct command *cmd)
 		return error("unpack should have generated %s, "
 			     "but I can't find it!", new_hex);
 	}
+	if (deny_non_fast_forwards && !is_null_sha1(old_sha1)) {
+		struct commit *old_commit, *new_commit;
+		struct commit_list *bases;
+
+		old_commit = (struct commit *)parse_object(old_sha1);
+		new_commit = (struct commit *)parse_object(new_sha1);
+		for (bases = get_merge_bases(old_commit, new_commit, 1);
+				bases; bases = bases->next)
+			if (!hashcmp(old_sha1, bases->item->object.sha1))
+				break;
+		if (!bases)
+			return error("denying non-fast forward;"
+					" you should pull first");
+	}
 	safe_create_leading_directories(lock_name);
 
 	newfd = open(lock_name, O_CREAT | O_EXCL | O_WRONLY, 0666);

setup.c

@@ -244,6 +244,8 @@ int check_repository_format_version(const char *var, const char *value)
                repository_format_version = git_config_int(var, value);
 	else if (strcmp(var, "core.sharedrepository") == 0)
 		shared_repository = git_config_perm(var, value);
+	else if (strcmp(var, "receive.denynonfastforwards") == 0)
+		deny_non_fast_forwards = git_config_bool(var, value);
        return 0;
 }