git/mkdelta.c

/*
 * Deltafication of a GIT database.
 *
 * (C) 2005 Nicolas Pitre <nico@cam.org>
 *
 * This code is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 */

#include "cache.h"
#include "delta.h"

static int replace_object(char *buf, unsigned long size, unsigned char *sha1)
{
	char tmpfile[PATH_MAX];
	int fd;

	snprintf(tmpfile, sizeof(tmpfile), "%s/obj_XXXXXX", get_object_directory());
	fd = mkstemp(tmpfile);
	if (fd < 0)
		return error("%s: %s\n", tmpfile, strerror(errno));
	if (write(fd, buf, size) != size) {
		perror("unable to write file");
		close(fd);
		unlink(tmpfile);
		return -1;
	}
	fchmod(fd, 0444);
	close(fd);
	if (rename(tmpfile, sha1_file_name(sha1))) {
		perror("unable to replace original object");
		unlink(tmpfile);
		return -1;
	}
	return 0;
}

static void *create_object(char *buf, unsigned long len, char *hdr, int hdrlen,
			   unsigned long *retsize)
{
	char *compressed;
	unsigned long size;
	z_stream stream;

	/* Set it up */
	memset(&stream, 0, sizeof(stream));
	deflateInit(&stream, Z_BEST_COMPRESSION);
	size = deflateBound(&stream, len+hdrlen);
	compressed = xmalloc(size);

	/* Compress it */
	stream.next_out = compressed;
	stream.avail_out = size;

	/* First header.. */
	stream.next_in = hdr;
	stream.avail_in = hdrlen;
	while (deflate(&stream, 0) == Z_OK)
		/* nothing */;

	/* Then the data itself.. */
	stream.next_in = buf;
	stream.avail_in = len;
	while (deflate(&stream, Z_FINISH) == Z_OK)
		/* nothing */;
	deflateEnd(&stream);
	*retsize = stream.total_out;
	return compressed;
}

static int restore_original_object(char *buf, unsigned long len,
				   char *type, unsigned char *sha1)
{
	char hdr[50];
	int hdrlen, ret;
	void *compressed;
	unsigned long size;

	hdrlen = sprintf(hdr, "%s %lu", type, len)+1;
	compressed = create_object(buf, len, hdr, hdrlen, &size);
	ret = replace_object(compressed, size, sha1);
	free(compressed);
	return ret;
}

static void *create_delta_object(char *buf, unsigned long len,
				 unsigned char *sha1_ref, unsigned long *size)
{
	char hdr[50];
	int hdrlen;

	/* Generate the header + sha1 of reference for delta */
	hdrlen = sprintf(hdr, "delta %lu", len+20)+1;
	memcpy(hdr + hdrlen, sha1_ref, 20);
	hdrlen += 20;

	return create_object(buf, len, hdr, hdrlen, size);
}

static unsigned long get_object_size(unsigned char *sha1)
{
	struct stat st;
	if (stat(sha1_file_name(sha1), &st))
		die("%s: %s", sha1_to_hex(sha1), strerror(errno));
	return st.st_size;
}

static void *get_buffer(unsigned char *sha1, char *type, unsigned long *size)
{
	unsigned long mapsize;
	void *map = map_sha1_file(sha1, &mapsize);
	if (map) {
		void *buffer = unpack_sha1_file(map, mapsize, type, size);
		munmap(map, mapsize);
		if (buffer)
			return buffer;
	}
	error("unable to get object %s", sha1_to_hex(sha1));
	return NULL;
}

static void *expand_delta(void *delta, unsigned long delta_size, char *type,
			  unsigned long *size, unsigned int *depth, char *head)
{
	void *buf = NULL;
	*depth++;
	if (delta_size < 20) {
		error("delta object is bad");
		free(delta);
	} else {
		unsigned long ref_size;
		void *ref = get_buffer(delta, type, &ref_size);
		if (ref && !strcmp(type, "delta"))
			ref = expand_delta(ref, ref_size, type, &ref_size,
					   depth, head);
		else
			memcpy(head, delta, 20);
		if (ref)
			buf = patch_delta(ref, ref_size, delta+20,
					  delta_size-20, size);
		free(ref);
		free(delta);
	}
	return buf;
}

static char *mkdelta_usage =
"mkdelta [ --max-depth=N ] <reference_sha1> <target_sha1> [ <next_sha1> ... ]";

int main(int argc, char **argv)
{
	unsigned char sha1_ref[20], sha1_trg[20], head_ref[20], head_trg[20];
	char type_ref[20], type_trg[20];
	void *buf_ref, *buf_trg, *buf_delta;
	unsigned long size_ref, size_trg, size_orig, size_delta;
	unsigned int depth_ref, depth_trg, depth_max = -1;
	int i, verbose = 0;

	for (i = 1; i < argc; i++) {
		if (!strcmp(argv[i], "-v")) {
			verbose = 1;
		} else if (!strcmp(argv[i], "-d") && i+1 < argc) {
			depth_max = atoi(argv[++i]);
		} else if (!strncmp(argv[i], "--max-depth=", 12)) {
			depth_max = atoi(argv[i]+12);
		} else
			break;
	}

	if (i + (depth_max != 0) >= argc)
		usage(mkdelta_usage);

	if (get_sha1(argv[i], sha1_ref))
		die("bad sha1 %s", argv[i]);
	depth_ref = 0;
	buf_ref = get_buffer(sha1_ref, type_ref, &size_ref);
	if (buf_ref && !strcmp(type_ref, "delta"))
		buf_ref = expand_delta(buf_ref, size_ref, type_ref,
				       &size_ref, &depth_ref, head_ref);
	else
		memcpy(head_ref, sha1_ref, 20);
	if (!buf_ref)
		die("unable to obtain initial object %s", argv[i]);

	if (depth_ref > depth_max) {
		if (restore_original_object(buf_ref, size_ref, type_ref, sha1_ref))
			die("unable to restore %s", argv[i]);
		if (verbose)
			printf("undelta %s (depth was %d)\n", argv[i], depth_ref);
		depth_ref = 0;
	}

	/*
	 * TODO: deltafication should be tried against any early object
	 * in the object list and not only the previous object.
	 */

	while (++i < argc) {
		if (get_sha1(argv[i], sha1_trg))
			die("bad sha1 %s", argv[i]);
		depth_trg = 0;
		buf_trg = get_buffer(sha1_trg, type_trg, &size_trg);
		if (buf_trg && !size_trg) {
			if (verbose)
				printf("skip    %s (object is empty)\n", argv[i]);
			continue;
		}
		size_orig = size_trg;
		if (buf_trg && !strcmp(type_trg, "delta")) {
			if (!memcmp(buf_trg, sha1_ref, 20)) {
				/* delta already in place */
				depth_ref++;
				memcpy(sha1_ref, sha1_trg, 20);
				buf_ref = patch_delta(buf_ref, size_ref,
						      buf_trg+20, size_trg-20,
						      &size_ref);
				if (!buf_ref)
					die("unable to apply delta %s", argv[i]);
				if (depth_ref > depth_max) {
					if (restore_original_object(buf_ref, size_ref,
								    type_ref, sha1_ref))
						die("unable to restore %s", argv[i]);
					if (verbose)
						printf("undelta %s (depth was %d)\n", argv[i], depth_ref);
					depth_ref = 0;
					continue;
				}
				if (verbose)
					printf("skip    %s (delta already in place)\n", argv[i]);
				continue;
			}
			buf_trg = expand_delta(buf_trg, size_trg, type_trg,
					       &size_trg, &depth_trg, head_trg);
		} else
			memcpy(head_trg, sha1_trg, 20);
		if (!buf_trg)
			die("unable to read target object %s", argv[i]);

		if (depth_trg > depth_max) {
			if (restore_original_object(buf_trg, size_trg, type_trg, sha1_trg))
				die("unable to restore %s", argv[i]);
			if (verbose)
				printf("undelta %s (depth was %d)\n", argv[i], depth_trg);
			depth_trg = 0;
			size_orig = size_trg;
		}

		if (depth_max == 0)
			goto skip;

		if (strcmp(type_ref, type_trg))
			die("type mismatch for object %s", argv[i]);

		if (!size_ref) {
			if (verbose)
				printf("skip    %s (initial object is empty)\n", argv[i]);
			goto skip;
		}
		
		if (depth_ref + 1 > depth_max) {
			if (verbose)
				printf("skip    %s (exceeding max link depth)\n", argv[i]);
			goto skip;
		}

		if (!memcmp(head_ref, sha1_trg, 20)) {
			if (verbose)
				printf("skip    %s (would create a loop)\n", argv[i]);
			goto skip;
		}

		buf_delta = diff_delta(buf_ref, size_ref, buf_trg, size_trg, &size_delta);
		if (!buf_delta)
			die("out of memory");

		/* no need to even try to compress if original
		   uncompressed is already smaller */
		if (size_delta+20 < size_orig) {
			void *buf_obj;
			unsigned long size_obj;
			buf_obj = create_delta_object(buf_delta, size_delta,
						      sha1_ref, &size_obj);
			free(buf_delta);
			size_orig = get_object_size(sha1_trg);
			if (size_obj >= size_orig) {
				free(buf_obj);
				if (verbose)
					printf("skip    %s (original is smaller)\n", argv[i]);
				goto skip;
			}
			if (replace_object(buf_obj, size_obj, sha1_trg))
				die("unable to write delta for %s", argv[i]);
			free(buf_obj);
			depth_ref++;
			if (verbose)
				printf("delta   %s (size=%ld.%02ld%%, depth=%d)\n",
				       argv[i], size_obj*100 / size_orig,
				       (size_obj*10000 / size_orig)%100,
				       depth_ref);
		} else {
			free(buf_delta);
			if (verbose)
				printf("skip    %s (original is smaller)\n", argv[i]);
			skip:
			depth_ref = depth_trg;
			memcpy(head_ref, head_trg, 20);
		}

		free(buf_ref);
		buf_ref = buf_trg;
		size_ref = size_trg;
		memcpy(sha1_ref, sha1_trg, 20);
	}

	return 0;
}