Implemented tree reloading in fast-import.

Tree reloading allows fast-import to swap out the least-recently used
branch by simply deallocating the data structures from memory that
were associated with that branch.  Later if the branch becomes active
again it can lazily recreate those structures on demand by reloading
the necessary trees from the pack file it originally wrote them to.

The reloading process is implemented by mmap'ing the pack into
memory and using a much tighter variant of the pack reading code
contained in sha1_file.c.  This was a blatent copy from sha1_file.c
but the unpacking functions were significantly simplified and are
actually now in a form that should make it easier to map only the
necessary regions of a pack rather than the entire file.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

fast-import.c

@@ -198,6 +198,7 @@ static unsigned long max_depth = 10;
 static unsigned long alloc_count;
 static unsigned long branch_count;
 static unsigned long branch_load_count;
+static unsigned long remap_count;
 static unsigned long object_count;
 static unsigned long duplicate_count;
 static unsigned long marks_set_count;
@@ -216,8 +217,10 @@ static struct atom_str **atom_table;
 
 /* The .pack file being generated */
 static int pack_fd;
-static unsigned long pack_offset;
+static unsigned long pack_size;
 static unsigned char pack_sha1[20];
+static void* pack_base;
+static size_t pack_mlen;
 
 /* Table of objects we've written. */
 static unsigned int object_entry_alloc = 1000;
@@ -616,7 +619,7 @@ static int store_object(
 		return 1;
 	}
 	e->type = type;
-	e->offset = pack_offset;
+	e->offset = pack_size;
 	object_count++;
 	object_count_by_type[type]++;
 
@@ -637,7 +640,7 @@ static int store_object(
 		hdrlen = encode_header(OBJ_DELTA, deltalen, hdr);
 		ywrite(pack_fd, hdr, hdrlen);
 		ywrite(pack_fd, last->sha1, sizeof(sha1));
-		pack_offset += hdrlen + sizeof(sha1);
+		pack_size += hdrlen + sizeof(sha1);
 	} else {
 		if (last)
 			last->depth = 0;
@@ -645,7 +648,7 @@ static int store_object(
 		s.avail_in = datlen;
 		hdrlen = encode_header(type, datlen, hdr);
 		ywrite(pack_fd, hdr, hdrlen);
-		pack_offset += hdrlen;
+		pack_size += hdrlen;
 	}
 
 	s.avail_out = deflateBound(&s, s.avail_in);
@@ -655,7 +658,7 @@ static int store_object(
 	deflateEnd(&s);
 
 	ywrite(pack_fd, out, s.total_out);
-	pack_offset += s.total_out;
+	pack_size += s.total_out;
 
 	free(out);
 	if (delta)
@@ -670,6 +673,127 @@ static int store_object(
 	return 0;
 }
 
+static void* map_pack(unsigned long offset)
+{
+	if (offset >= pack_size)
+		die("object offset outside of pack file");
+	if (offset >= pack_mlen) {
+		if (pack_base)
+			munmap(pack_base, pack_mlen);
+		/* round out how much we map to 16 MB units */
+		pack_mlen = pack_size;
+		if (pack_mlen & ((1 << 24) - 1))
+			pack_mlen = ((pack_mlen >> 24) + 1) << 24;
+		pack_base = mmap(NULL,pack_mlen,PROT_READ,MAP_SHARED,pack_fd,0);
+		if (pack_base == MAP_FAILED)
+			die("Failed to map generated pack: %s", strerror(errno));
+		remap_count++;
+	}
+	return (char*)pack_base + offset;
+}
+
+static unsigned long unpack_object_header(unsigned long offset,
+	enum object_type *type,
+	unsigned long *sizep)
+{
+	unsigned shift;
+	unsigned char c;
+	unsigned long size;
+
+	c = *(unsigned char*)map_pack(offset++);
+	*type = (c >> 4) & 7;
+	size = c & 15;
+	shift = 4;
+	while (c & 0x80) {
+		c = *(unsigned char*)map_pack(offset++);
+		size += (c & 0x7f) << shift;
+		shift += 7;
+	}
+	*sizep = size;
+	return offset;
+}
+
+static void *unpack_non_delta_entry(unsigned long o, unsigned long sz)
+{
+	z_stream stream;
+	unsigned char *result;
+
+	result = xmalloc(sz + 1);
+	result[sz] = 0;
+
+	memset(&stream, 0, sizeof(stream));
+	stream.next_in = map_pack(o);
+	stream.avail_in = pack_mlen - o;
+	stream.next_out = result;
+	stream.avail_out = sz;
+
+	inflateInit(&stream);
+	for (;;) {
+		int st = inflate(&stream, Z_FINISH);
+		if (st == Z_STREAM_END)
+			break;
+		if (st == Z_OK) {
+			o = stream.next_in - (unsigned char*)pack_base;
+			stream.next_in = map_pack(o);
+			stream.avail_in = pack_mlen - o;
+			continue;
+		}
+		die("Error from zlib during inflate.");
+	}
+	inflateEnd(&stream);
+	if (stream.total_out != sz)
+		die("Error after inflate: sizes mismatch");
+	return result;
+}
+
+static void *unpack_entry(unsigned long offset, unsigned long *sizep);
+
+static void *unpack_delta_entry(unsigned long offset,
+	unsigned long delta_size,
+	unsigned long *sizep)
+{
+	struct object_entry *base_oe;
+	unsigned char *base_sha1;
+	void *delta_data, *base, *result;
+	unsigned long base_size, result_size;
+
+	base_sha1 = (unsigned char*)map_pack(offset + 20) - 20;
+	base_oe = find_object(base_sha1);
+	if (!base_oe)
+		die("I'm broken; I can't find a base I know must be here.");
+	base = unpack_entry(base_oe->offset, &base_size);
+	delta_data = unpack_non_delta_entry(offset + 20, delta_size);
+	result = patch_delta(base, base_size,
+			     delta_data, delta_size,
+			     &result_size);
+	if (!result)
+		die("failed to apply delta");
+	free(delta_data);
+	free(base);
+	*sizep = result_size;
+	return result;
+}
+
+static void *unpack_entry(unsigned long offset, unsigned long *sizep)
+{
+	unsigned long size;
+	enum object_type kind;
+
+	offset = unpack_object_header(offset, &kind, &size);
+	switch (kind) {
+	case OBJ_DELTA:
+		return unpack_delta_entry(offset, size, sizep);
+	case OBJ_COMMIT:
+	case OBJ_TREE:
+	case OBJ_BLOB:
+	case OBJ_TAG:
+		*sizep = size;
+		return unpack_non_delta_entry(offset, size);
+	default:
+		die("I created an object I can't read!");
+	}
+}
+
 static const char *get_mode(const char *str, unsigned int *modep)
 {
 	unsigned char c;
@@ -691,7 +815,6 @@ static void load_tree(struct tree_entry *root)
 	unsigned long size;
 	char *buf;
 	const char *c;
-	char type[20];
 
 	root->tree = t = new_tree_content(8);
 	if (!memcmp(root->sha1, null_sha1, 20))
@@ -699,11 +822,14 @@ static void load_tree(struct tree_entry *root)
 
 	myoe = find_object(root->sha1);
 	if (myoe) {
-		die("FIXME");
+		if (myoe->type != OBJ_TREE)
+			die("Not a tree: %s", sha1_to_hex(root->sha1));
+		buf = unpack_entry(myoe->offset, &size);
 	} else {
+		char type[20];
 		buf = read_sha1_file(root->sha1, type, &size);
-		if (!buf || strcmp(type, tree_type))
-			die("Can't load existing tree %s", sha1_to_hex(root->sha1));
+		if (!buf || !strcmp(type, tree_type))
+			die("Can't load tree %s", sha1_to_hex(root->sha1));
 	}
 
 	c = buf;
@@ -880,7 +1006,7 @@ static void init_pack_header()
 	hdr.hdr_entries = 0;
 
 	ywrite(pack_fd, &hdr, sizeof(hdr));
-	pack_offset = sizeof(hdr);
+	pack_size = sizeof(hdr);
 }
 
 static void fixup_header_footer()
@@ -1052,7 +1178,8 @@ static void cmd_new_blob()
 
 static void unload_one_branch()
 {
-	while (cur_active_branches >= max_active_branches) {
+	while (cur_active_branches
+		&& cur_active_branches >= max_active_branches) {
 		unsigned long min_commit = ULONG_MAX;
 		struct branch *e, *l = NULL, *p = NULL;
 
@@ -1210,7 +1337,7 @@ static void cmd_new_commit()
 	msg = cmd_data(&msglen);
 
 	/* ensure the branch is active/loaded */
-	if (!b->branch_tree.tree) {
+	if (!b->branch_tree.tree || !max_active_branches) {
 		unload_one_branch();
 		load_branch(b);
 	}
@@ -1297,10 +1424,18 @@ static void cmd_new_branch()
 		} else if (*from == ':') {
 			unsigned long idnum = strtoul(from + 1, NULL, 10);
 			struct object_entry *oe = find_mark(idnum);
+			unsigned long size;
+			char *buf;
 			if (oe->type != OBJ_COMMIT)
 				die("Mark :%lu not a commit", idnum);
 			memcpy(b->sha1, oe->sha1, 20);
-			memcpy(b->branch_tree.sha1, null_sha1, 20);
+			buf = unpack_entry(oe->offset, &size);
+			if (!buf || size < 46)
+				die("Not a valid commit: %s", from);
+			if (memcmp("tree ", buf, 5)
+				|| get_sha1_hex(buf + 5, b->branch_tree.sha1))
+				die("The commit %s is corrupt", sha1_to_hex(b->sha1));
+			free(buf);
 		} else if (!get_sha1(from, b->sha1)) {
 			if (!memcmp(b->sha1, null_sha1, 20))
 				memcpy(b->branch_tree.sha1, null_sha1, 20);
@@ -1515,6 +1650,7 @@ int main(int argc, const char **argv)
 	fprintf(stderr, "Memory total:    %10lu KiB\n", (total_allocd + alloc_count*sizeof(struct object_entry))/1024);
 	fprintf(stderr, "       pools:    %10lu KiB\n", total_allocd/1024);
 	fprintf(stderr, "     objects:    %10lu KiB\n", (alloc_count*sizeof(struct object_entry))/1024);
+	fprintf(stderr, "Pack remaps:     %10lu\n", remap_count);
 	fprintf(stderr, "---------------------------------------------------\n");
 
 	stat(pack_name, &sb);