git-apply --binary: clean up and prepare for --reverse

This cleans up the implementation of "git-apply --binary", and
implements reverse application of binary patches (when git-diff
is converted to emit reversible binary patches).

Earlier, the types of encoding (either deflated literal or
deflated delta) were stored in is_binary field in struct patch,
which meant that we cannot store more than one fragment that
differ in the encoding for a patch.  This moves the information
to a field in struct fragment that is otherwise unused for
binary patches, and makes it possible to hang two (or more, but
two is enough) hunks for a binary patch.

The original "binary patch" output from git-diff is internally
parsed into an "is_binary" patch with one fragment.  Upcoming
reversible binary patch output will have two fragments, the
first one being the forward patch and the second one the reverse
patch.

Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-apply.c

@@ -109,6 +109,13 @@ static int max_change, max_len;
  */
 static int linenr = 1;
 
+/*
+ * This represents one "hunk" from a patch, starting with
+ * "@@ -oldpos,oldlines +newpos,newlines @@" marker.  The
+ * patch text is pointed at by patch, and its byte length
+ * is stored in size.  leading and trailing are the number
+ * of context lines.
+ */
 struct fragment {
 	unsigned long leading, trailing;
 	unsigned long oldpos, oldlines;
@@ -118,12 +125,19 @@ struct fragment {
 	struct fragment *next;
 };
 
+/*
+ * When dealing with a binary patch, we reuse "leading" field
+ * to store the type of the binary hunk, either deflated "delta"
+ * or deflated "literal".
+ */
+#define binary_patch_method leading
+#define BINARY_DELTA_DEFLATED	1
+#define BINARY_LITERAL_DEFLATED 2
+
 struct patch {
 	char *new_name, *old_name, *def_name;
 	unsigned int old_mode, new_mode;
 	int is_rename, is_copy, is_new, is_delete, is_binary;
-#define BINARY_DELTA_DEFLATED 1
-#define BINARY_LITERAL_DEFLATED 2
 	unsigned long deflate_origlen;
 	int lines_added, lines_deleted;
 	int score;
@@ -979,43 +993,70 @@ static inline int metadata_changes(struct patch *patch)
 		 patch->old_mode != patch->new_mode);
 }
 
-static int parse_binary(char *buffer, unsigned long size, struct patch *patch)
+static char *inflate_it(const void *data, unsigned long size,
+			unsigned long inflated_size)
 {
-	/* We have read "GIT binary patch\n"; what follows is a line
-	 * that says the patch method (currently, either "deflated
-	 * literal" or "deflated delta") and the length of data before
-	 * deflating; a sequence of 'length-byte' followed by base-85
-	 * encoded data follows.
+	z_stream stream;
+	void *out;
+	int st;
+
+	memset(&stream, 0, sizeof(stream));
+
+	stream.next_in = (unsigned char *)data;
+	stream.avail_in = size;
+	stream.next_out = out = xmalloc(inflated_size);
+	stream.avail_out = inflated_size;
+	inflateInit(&stream);
+	st = inflate(&stream, Z_FINISH);
+	if ((st != Z_STREAM_END) || stream.total_out != inflated_size) {
+		free(out);
+		return NULL;
+	}
+	return out;
+}
+
+static struct fragment *parse_binary_hunk(char **buf_p,
+					  unsigned long *sz_p,
+					  int *status_p,
+					  int *used_p)
+{
+	/* Expect a line that begins with binary patch method ("literal"
+	 * or "delta"), followed by the length of data before deflating.
+	 * a sequence of 'length-byte' followed by base-85 encoded data
+	 * should follow, terminated by a newline.
 	 *
 	 * Each 5-byte sequence of base-85 encodes up to 4 bytes,
 	 * and we would limit the patch line to 66 characters,
 	 * so one line can fit up to 13 groups that would decode
 	 * to 52 bytes max.  The length byte 'A'-'Z' corresponds
 	 * to 1-26 bytes, and 'a'-'z' corresponds to 27-52 bytes.
-	 * The end of binary is signaled with an empty line.
 	 */
 	int llen, used;
-	struct fragment *fragment;
+	unsigned long size = *sz_p;
+	char *buffer = *buf_p;
+	int patch_method;
+	unsigned long origlen;
 	char *data = NULL;
+	int hunk_size = 0;
+	struct fragment *frag;
 
-	patch->fragments = fragment = xcalloc(1, sizeof(*fragment));
-
-	/* Grab the type of patch */
 	llen = linelen(buffer, size);
 	used = llen;
-	linenr++;
+
+	*status_p = 0;
 
 	if (!strncmp(buffer, "delta ", 6)) {
-		patch->is_binary = BINARY_DELTA_DEFLATED;
-		patch->deflate_origlen = strtoul(buffer + 6, NULL, 10);
+		patch_method = BINARY_DELTA_DEFLATED;
+		origlen = strtoul(buffer + 6, NULL, 10);
 	}
 	else if (!strncmp(buffer, "literal ", 8)) {
-		patch->is_binary = BINARY_LITERAL_DEFLATED;
-		patch->deflate_origlen = strtoul(buffer + 8, NULL, 10);
+		patch_method = BINARY_LITERAL_DEFLATED;
+		origlen = strtoul(buffer + 8, NULL, 10);
 	}
 	else
-		return error("unrecognized binary patch at line %d: %.*s",
-			     linenr-1, llen-1, buffer);
+		return NULL;
+
+	linenr++;
 	buffer += llen;
 	while (1) {
 		int byte_length, max_byte_length, newsize;
@@ -1044,21 +1085,79 @@ static int parse_binary(char *buffer, unsigned long size, struct patch *patch)
 		if (max_byte_length < byte_length ||
 		    byte_length <= max_byte_length - 4)
 			goto corrupt;
-		newsize = fragment->size + byte_length;
+		newsize = hunk_size + byte_length;
 		data = xrealloc(data, newsize);
-		if (decode_85(data + fragment->size,
-			      buffer + 1,
-			      byte_length))
+		if (decode_85(data + hunk_size, buffer + 1, byte_length))
 			goto corrupt;
-		fragment->size = newsize;
+		hunk_size = newsize;
 		buffer += llen;
 		size -= llen;
 	}
-	fragment->patch = data;
-	return used;
+
+	frag = xcalloc(1, sizeof(*frag));
+	frag->patch = inflate_it(data, hunk_size, origlen);
+	if (!frag->patch)
+		goto corrupt;
+	free(data);
+	frag->size = origlen;
+	*buf_p = buffer;
+	*sz_p = size;
+	*used_p = used;
+	frag->binary_patch_method = patch_method;
+	return frag;
+
  corrupt:
-	return error("corrupt binary patch at line %d: %.*s",
+	if (data)
+		free(data);
+	*status_p = -1;
+	error("corrupt binary patch at line %d: %.*s",
 	      linenr-1, llen-1, buffer);
+	return NULL;
+}
+
+static int parse_binary(char *buffer, unsigned long size, struct patch *patch)
+{
+	/* We have read "GIT binary patch\n"; what follows is a line
+	 * that says the patch method (currently, either "literal" or
+	 * "delta") and the length of data before deflating; a
+	 * sequence of 'length-byte' followed by base-85 encoded data
+	 * follows.
+	 *
+	 * When a binary patch is reversible, there is another binary
+	 * hunk in the same format, starting with patch method (either
+	 * "literal" or "delta") with the length of data, and a sequence
+	 * of length-byte + base-85 encoded data, terminated with another
+	 * empty line.  This data, when applied to the postimage, produces
+	 * the preimage.
+	 */
+	struct fragment *forward;
+	struct fragment *reverse;
+	int status;
+	int used, used_1;
+
+	forward = parse_binary_hunk(&buffer, &size, &status, &used);
+	if (!forward && !status)
+		/* there has to be one hunk (forward hunk) */
+		return error("unrecognized binary patch at line %d", linenr-1);
+	if (status)
+		/* otherwise we already gave an error message */
+		return status;
+
+	reverse = parse_binary_hunk(&buffer, &size, &status, &used_1);
+	if (reverse)
+		used += used_1;
+	else if (status) {
+		/* not having reverse hunk is not an error, but having
+		 * a corrupt reverse hunk is.
+		 */
+		free((void*) forward->patch);
+		free(forward);
+		return status;
+	}
+	forward->next = reverse;
+	patch->fragments = forward;
+	patch->is_binary = 1;
+	return used;
 }
 
 static int parse_chunk(char *buffer, unsigned long size, struct patch *patch)
@@ -1505,28 +1604,6 @@ static int apply_one_fragment(struct buffer_desc *desc, struct fragment *frag, i
 	return offset;
 }
 
-static char *inflate_it(const void *data, unsigned long size,
-			unsigned long inflated_size)
-{
-	z_stream stream;
-	void *out;
-	int st;
-
-	memset(&stream, 0, sizeof(stream));
-
-	stream.next_in = (unsigned char *)data;
-	stream.avail_in = size;
-	stream.next_out = out = xmalloc(inflated_size);
-	stream.avail_out = inflated_size;
-	inflateInit(&stream);
-	st = inflate(&stream, Z_FINISH);
-	if ((st != Z_STREAM_END) || stream.total_out != inflated_size) {
-		free(out);
-		return NULL;
-	}
-	return out;
-}
-
 static int apply_binary_fragment(struct buffer_desc *desc, struct patch *patch)
 {
 	unsigned long dst_size;
@@ -1534,30 +1611,29 @@ static int apply_binary_fragment(struct buffer_desc *desc, struct patch *patch)
 	void *data;
 	void *result;
 
-	/* Binary patch is irreversible */
-	if (apply_in_reverse)
-		return error("cannot reverse-apply a binary patch to '%s'",
+	/* Binary patch is irreversible without the optional second hunk */
+	if (apply_in_reverse) {
+		if (!fragment->next)
+			return error("cannot reverse-apply a binary patch "
+				     "without the reverse hunk to '%s'",
 				     patch->new_name
 				     ? patch->new_name : patch->old_name);
-
-	data = inflate_it(fragment->patch, fragment->size,
-			  patch->deflate_origlen);
-	if (!data)
-		return error("corrupt patch data");
-	switch (patch->is_binary) {
+		fragment = fragment;
+	}
+	data = (void*) fragment->patch;
+	switch (fragment->binary_patch_method) {
 	case BINARY_DELTA_DEFLATED:
 		result = patch_delta(desc->buffer, desc->size,
 				     data,
-				     patch->deflate_origlen,
+				     fragment->size,
 				     &dst_size);
 		free(desc->buffer);
 		desc->buffer = result;
-		free(data);
 		break;
 	case BINARY_LITERAL_DEFLATED:
 		free(desc->buffer);
 		desc->buffer = data;
-		dst_size = patch->deflate_origlen;
+		dst_size = fragment->size;
 		break;
 	}
 	if (!desc->buffer)