Re: [PATCH 6/6] Add git-rewrite-commits

> +Filters
> +~~~~~~~
> +
> +The filters are applied in the order as listed below.  The <command>
> +argument is run as "sh -c '<command'>", with the $GIT_COMMIT
> +environment variable set to the commit that is being rewritten.
> +If any call to a filter fails, then git-rewrite-commits will abort.

> +Examples
> +--------
> +
> +Suppose you want to remove a file (containing confidential information
> +or copyright violation) from all commits:
> +
> +----------------------------------------------------------------------------
> +git rewrite-commits --index-filter 'git update-index --remove filename || :'

> +To move the whole tree into a subdirectory, or remove it from there:
> +
> +---------------------------------------------------------------
> +git rewrite-commits --index-filter \
> +	'git ls-files -s | sed "s-\t-&newsubdir/-" |
> +		GIT_INDEX_FILE=$GIT_INDEX_FILE.new \
> +			git update-index --index-info &&
> +	 mv $GIT_INDEX_FILE.new $GIT_INDEX_FILE'
> +---------------------------------------------------------------

> +struct decoration rewrite_decoration = { "rewritten as" };
> [...]
> +
> +struct rewrite_decoration {
> +	struct rewrite_decoration *next;
> +	unsigned char sha1[20];
> +};

> +/* Replace any (short) sha1 of a rewritten commit by the new (short) sha1 */
> +static char *rewrite_body(char *dest, unsigned long len, char *buf)
> +{
> +	unsigned char sha1[20];
> +
> +	while (len) {
> +		size_t ll = non_hex_len(buf, len);
> +		memcpy(dest, buf, ll);
> +		dest += ll;
> +		buf += ll;
> +		len -= ll;

> +		    !get_short_sha1(buf, ll, sha1, 1) &&
> +		    !get_rewritten_sha1(sha1))
> +			memcpy(dest, sha1_to_hex(sha1), ll);
> +		else
> +			memcpy(dest, buf, ll);

> +static int is_ref_to_be_rewritten(const char *ref)
> +{
> +	unsigned char sha1[20];
> +	int flag;
> +
> +	if (prefixcmp(ref, "refs/"))
> +		return 0;
> +	if (!prefixcmp(ref, "refs/remotes/"))
> +		return 0;
> +	if (!prefixcmp(ref+5, original_prefix))

> +static void rewrite_sha1(struct object *obj, unsigned char *new_sha1)
> +{
> +	if (!hashcmp(obj->sha1, new_sha1))
> +		return;
> +
> +	add_rewrite_decoration(obj, new_sha1);

> +/*
> + * Replace any parent that has been removed by its parents
> + * and return the number of new parents.
> + * We directly modify the parent list, so any libification
> + * should probably adapt this function.

> +static int rewrite_ref(const char *refname, const unsigned char *sha1,
> +			int flags, void *cb_data)
> +{
> +	int prefix_len;
> +	int len;
> +	char buffer[256], *p;
> +	struct object *obj = parse_object(sha1);
> +	unsigned char new_sha1[20];
> +	struct commit *commit;
> +	int pruned;
> +
> +	if (!obj)
> +		return 0;
> +	if (obj->type == OBJ_TAG)

> +	commit = lookup_commit_reference(sha1);
> +	pruned = is_pruned(commit, !!cb_data);
> +
> +	hashcpy(new_sha1, sha1);
> +	if (!pruned && get_rewritten_sha1(new_sha1))
> +		return 0;

> +static void filter_and_write_commit(char *commit_body, size_t len,
> +				    struct commit *commit, unsigned char *sha1)
> +{
> +	char commit_path[PATH_MAX];
> +	struct child_process cmd;
> +	int argc;
> +	const char *argv[10];
> +	int fd;
> +	const char *env[] = { commit_env, NULL };
> +	char hex[41];
> +	struct commit_list *list = NULL, **end = &list;
> +
> +	memcpy(commit_env+sizeof(commit_env)-41,
> +		sha1_to_hex(commit->object.sha1), 40);
> +
> +	fd = git_mkstemp(commit_path, sizeof(commit_path), ".commit_XXXXXX");;
> +	write_or_die(fd, commit_body, len);
> +
> +	argc = 0;
> +	argv[argc++] = "sh";
> +	argv[argc++] = "-c";
> +	argv[argc++] = commit_filter;
> +	argv[argc] = NULL;
> +	memset(&cmd, 0, sizeof(cmd));
> +	cmd.in = open(commit_path, O_RDONLY);
> +	if (cmd.in < 0)
> +		die("Unable to read commit from file '%s'", commit_path);
> +	unlink(commit_path);

> +		hashclr(sha1);
> +		commit->object.flags |= PRUNED;
> +		/*
> +		 * If the filter returns two or more commits,
> +		 * we consider the original commit to have been
> +		 * removed and put the list in the old commit's
> +		 * parent list so that all the old commit's children
> +		 * will copy them.
> +		 */
> +		if (list) {
> +			free_commit_list(commit->parents);
> +			commit->parents = list;
> +		} else
> +		    add_sha1_map(commit->object.sha1, NULL);

> +	p = rewrite_header(p, &orig_len, &orig_buf, commit);
> +	p = rewrite_body(p, orig_len, orig_buf);
> +	if (!commit_filter) {
> +		if (write_sha1_file(buf, p-buf, commit_type, sha1))
> +			die("Unable to write new commit");
> +		add_sha1_map(commit->object.sha1, sha1);
> +	} else
> +		filter_and_write_commit(buf, p-buf, commit, sha1);
> +	free(buf);

> +"commit()\n"
> +"{\n"
> +"	git-hash-object -w -t commit --stdin\n"
> +"}\n"
> +"map()\n"
> +"{\n"
> +"	# if it was not rewritten, take the original\n"
> +"	if test -r \"$GIT_REWRITE_DIR/map/$1\"\n"
> +"	then\n"
> +"		cat \"$GIT_REWRITE_DIR/map/$1\"\n"
> +"	else\n"
> +"		echo \"$1\"\n"
> +"	fi\n"
> +"}\n"
> +"cd t\n";

> +static char *create_filter(const char *command)
> +{
> +	int prefix_len = sizeof(filter_prefix)-1;
> +	int command_len = strlen(command);
> +	char *filter = xmalloc(prefix_len + command_len + 1);
> +
> +	memcpy(filter, filter_prefix, prefix_len);
> +	memcpy(filter+prefix_len, command, command_len+1);
> +	return filter;
> +}

> +static void setup_temp_dir()
> +{
> +	int aux;
> +
> +	absolute_git_dir = create_absolute_path(get_git_dir());
> +	setenv(GIT_DIR_ENVIRONMENT, absolute_git_dir, 1);
> +
> +	if (!rewrite_dir) {
> +		rewrite_dir = xstrdup(git_path("rewrite"));

> +	if (index_filter || commit_filter)
> +		setup_temp_dir();
> +	else
> +		/* They'll never know.  BWUHAHA */
> +		write_sha1_mapping = 0;