Improve sha1 object file writing.

Make it much safer: we write to a temporary file, and then link that
temporary file to the final destination. This avoids all the nasty
races if several people write the same object at the same time.

It should also result in nicer on-disk layout, since it means that
objects all get created in the same subdirectory. That makes a lot
of block allocation algorithms happier, since the objects will now
be allocated from the same zone.

Author: Linus Torvalds

cache.h

@@ -128,9 +128,6 @@ extern int index_fd(unsigned char *sha1, int fd, struct stat *st);
 /* Return a statically allocated filename matching the sha1 signature */
 extern char *sha1_file_name(const unsigned char *sha1);
 
-/* Write a memory buffer out to the sha file */
-extern int write_sha1_buffer(const unsigned char *sha1, void *buf, unsigned int size);
-
 /* Read and unpack a sha1 file into memory, write memory to a sha1 file */
 extern void * map_sha1_file(const unsigned char *sha1, unsigned long *size);
 extern void * unpack_sha1_file(void *map, unsigned long mapsize, char *type, unsigned long *size);

sha1_file.c

@@ -284,8 +284,9 @@ int write_sha1_file(char *buf, unsigned long len, const char *type, unsigned cha
 	unsigned char sha1[20];
 	SHA_CTX c;
 	char *filename;
+	static char tmpfile[PATH_MAX];
 	char hdr[50];
-	int fd, hdrlen;
+	int fd, hdrlen, ret;
 
 	/* Generate the header */
 	hdrlen = sprintf(hdr, "%s %lu", type, len)+1;
@@ -300,18 +301,28 @@ int write_sha1_file(char *buf, unsigned long len, const char *type, unsigned cha
 		memcpy(returnsha1, sha1, 20);
 
 	filename = sha1_file_name(sha1);
-	fd = open(filename, O_WRONLY | O_CREAT | O_EXCL, 0666);
-	if (fd < 0) {
-		if (errno != EEXIST)
-			return -1;
-
+	fd = open(filename, O_RDONLY);
+	if (fd >= 0) {
 		/*
-		 * We might do collision checking here, but we'd need to
-		 * uncompress the old file and check it. Later.
+		 * FIXME!!! We might do collision checking here, but we'd
+		 * need to uncompress the old file and check it. Later.
 		 */
+		close(fd);
 		return 0;
 	}
 
+	if (errno != ENOENT) {
+		fprintf(stderr, "sha1 file %s: %s", filename, strerror(errno));
+		return -1;
+	}
+
+	snprintf(tmpfile, sizeof(tmpfile), "%s/obj_XXXXXX", get_object_directory());
+	fd = mkstemp(tmpfile);
+	if (fd < 0) {
+		fprintf(stderr, "unable to create temporary sha1 filename %s: %s", tmpfile, strerror(errno));
+		return -1;
+	}
+
 	/* Set it up */
 	memset(&stream, 0, sizeof(stream));
 	deflateInit(&stream, Z_BEST_COMPRESSION);
@@ -338,54 +349,21 @@ int write_sha1_file(char *buf, unsigned long len, const char *type, unsigned cha
 
 	if (write(fd, compressed, size) != size)
 		die("unable to write file");
+	fchmod(fd, 0444);
 	close(fd);
-		
-	return 0;
-}
 
-static inline int collision_check(char *filename, void *buf, unsigned int size)
-{
-#ifdef COLLISION_CHECK
-	void *map;
-	int fd = open(filename, O_RDONLY);
-	struct stat st;
-	int cmp;
-
-	/* Unreadable object, or object went away? Strange. */
-	if (fd < 0)
-		return -1;
-
-	if (fstat(fd, &st) < 0 || size != st.st_size)
-		return -1;
-
-	map = mmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
-	close(fd);
-	if (map == MAP_FAILED)
-		return -1;
-	cmp = memcmp(buf, map, size);
-	munmap(map, size);
-	if (cmp)
-		return -1;
-#endif
-	return 0;
-}
-
-int write_sha1_buffer(const unsigned char *sha1, void *buf, unsigned int size)
-{
-	char *filename = sha1_file_name(sha1);
-	int fd;
-
-	fd = open(filename, O_WRONLY | O_CREAT | O_EXCL, 0666);
-	if (fd < 0) {
-		if (errno != EEXIST)
+	ret = link(tmpfile, filename);
+	if (ret < 0)
+		ret = errno;
+	unlink(tmpfile);
+	if (ret) {
+		if (ret != EEXIST) {
+			fprintf(stderr, "unable to write sha1 filename %s: %s", filename, strerror(ret));
 			return -1;
-		if (collision_check(filename, buf, size))
-			return error("SHA1 collision detected!"
-					" This is bad, bad, BAD!\a\n");
-		return 0;
+		}
+		/* FIXME!!! Collision check here ? */
 	}
-	write(fd, buf, size);
-	close(fd);
+
 	return 0;
 }
 
@@ -463,51 +441,19 @@ int has_sha1_file(const unsigned char *sha1)
 
 int index_fd(unsigned char *sha1, int fd, struct stat *st)
 {
-	z_stream stream;
 	unsigned long size = st->st_size;
-	int max_out_bytes = size + 200;
-	void *out = xmalloc(max_out_bytes);
-	void *metadata = xmalloc(200);
-	int metadata_size;
-	void *in;
-	SHA_CTX c;
+	void *buf;
+	int ret;
 
-	in = "";
+	buf = "";
 	if (size)
-		in = mmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
+		buf = mmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
 	close(fd);
-	if (!out || (int)(long)in == -1)
+	if ((int)(long)buf == -1)
 		return -1;
 
-	metadata_size = 1+sprintf(metadata, "blob %lu", size);
-
-	SHA1_Init(&c);
-	SHA1_Update(&c, metadata, metadata_size);
-	SHA1_Update(&c, in, size);
-	SHA1_Final(sha1, &c);
-
-	memset(&stream, 0, sizeof(stream));
-	deflateInit(&stream, Z_BEST_COMPRESSION);
-
-	/*
-	 * ASCII size + nul byte
-	 */	
-	stream.next_in = metadata;
-	stream.avail_in = metadata_size;
-	stream.next_out = out;
-	stream.avail_out = max_out_bytes;
-	while (deflate(&stream, 0) == Z_OK)
-		/* nothing */;
-
-	/*
-	 * File content
-	 */
-	stream.next_in = in;
-	stream.avail_in = size;
-	while (deflate(&stream, Z_FINISH) == Z_OK)
-		/*nothing */;
-
-	deflateEnd(&stream);
-	
-	return write_sha1_buffer(sha1, out, stream.total_out);
+	ret = write_sha1_file(buf, size, "blob", sha1);
+	if (size)
+		munmap(buf, size);
+	return ret;
 }