Refactor how we open pack files to prepare for multiple windows.

To efficiently support mmaping of multiple regions of the same pack
file we want to keep the pack's file descriptor open while we are
actively working with that pack.  So we are now keeping that file
descriptor in packed_git.pack_fd and closing it only after we unmap
the last window.

This is going to increase the number of file descriptors that are
in use at once, however that will be bounded by the total number of
pack files present and therefore should not be very high.  It is
a small tradeoff which we may need to revisit after some testing
can be done on various repositories and systems.

For code clarity we also want to seperate out the implementation
of how we open a pack file from the implementation which locates
a suitable window (or makes a new one) from the given pack file.
Since this is a rather large delta I'm taking advantage of doing
it now, in a fairly isolated change.

When we open a pack file we need to examine the header and trailer
without having a mmap in place, as we may only need to mmap
the middle section of this particular pack.  Consequently the
verification code has been refactored to make use of the new
read_or_die function.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

cache.h

@@ -351,6 +351,7 @@ extern struct packed_git {
 	unsigned int *index_base;
 	off_t index_size;
 	off_t pack_size;
+	int pack_fd;
 	int pack_local;
 	unsigned char sha1[20];
 	/* something like ".git/objects/pack/xxxxx.pack" */

sha1_file.c

@@ -465,6 +465,8 @@ static int unuse_one_packed_git(void)
 	munmap(lru->windows->base, lru->windows->len);
 	free(lru->windows);
 	lru->windows = NULL;
+	close(p->pack_fd);
+	p->pack_fd = -1;
 	return 1;
 }
 
@@ -474,62 +476,64 @@ void unuse_packed_git(struct packed_git *p)
 		p->windows->inuse_cnt--;
 }
 
-int use_packed_git(struct packed_git *p)
+static void open_packed_git(struct packed_git *p)
 {
+	struct stat st;
+	struct pack_header hdr;
+	unsigned char sha1[20];
+	unsigned char *idx_sha1;
+
+	p->pack_fd = open(p->pack_name, O_RDONLY);
+	if (p->pack_fd < 0 || fstat(p->pack_fd, &st))
+		die("packfile %s cannot be opened", p->pack_name);
+
+	/* If we created the struct before we had the pack we lack size. */
 	if (!p->pack_size) {
-		struct stat st;
-		/* We created the struct before we had the pack */
-		stat(p->pack_name, &st);
 		if (!S_ISREG(st.st_mode))
 			die("packfile %s not a regular file", p->pack_name);
 		p->pack_size = st.st_size;
-	}
+	} else if (p->pack_size != st.st_size)
+		die("packfile %s size changed", p->pack_name);
+
+	/* Verify we recognize this pack file format. */
+	read_or_die(p->pack_fd, &hdr, sizeof(hdr));
+	if (hdr.hdr_signature != htonl(PACK_SIGNATURE))
+		die("file %s is not a GIT packfile", p->pack_name);
+	if (!pack_version_ok(hdr.hdr_version))
+		die("packfile %s is version %u and not supported"
+			" (try upgrading GIT to a newer version)",
+			p->pack_name, ntohl(hdr.hdr_version));
+
+	/* Verify the pack matches its index. */
+	if (num_packed_objects(p) != ntohl(hdr.hdr_entries))
+		die("packfile %s claims to have %u objects"
+			" while index size indicates %u objects",
+			p->pack_name, ntohl(hdr.hdr_entries),
+			num_packed_objects(p));
+	if (lseek(p->pack_fd, p->pack_size - sizeof(sha1), SEEK_SET) == -1)
+		die("end of packfile %s is unavailable", p->pack_name);
+	read_or_die(p->pack_fd, sha1, sizeof(sha1));
+	idx_sha1 = ((unsigned char *)p->index_base) + p->index_size - 40;
+	if (hashcmp(sha1, idx_sha1))
+		die("packfile %s does not match index", p->pack_name);
+}
+
+int use_packed_git(struct packed_git *p)
+{
+	if (p->pack_fd == -1)
+		open_packed_git(p);
 	if (!p->windows) {
-		int fd;
-		struct stat st;
 		struct pack_window *win;
-		struct pack_header *hdr;
 
 		pack_mapped += p->pack_size;
 		while (packed_git_limit < pack_mapped && unuse_one_packed_git())
 			; /* nothing */
-		fd = open(p->pack_name, O_RDONLY);
-		if (fd < 0)
-			die("packfile %s cannot be opened", p->pack_name);
-		if (fstat(fd, &st)) {
-			close(fd);
-			die("packfile %s cannot be opened", p->pack_name);
-		}
-		if (st.st_size != p->pack_size)
-			die("packfile %s size mismatch.", p->pack_name);
 		win = xcalloc(1, sizeof(*win));
 		win->len = p->pack_size;
-		win->base = mmap(NULL, p->pack_size, PROT_READ, MAP_PRIVATE, fd, 0);
-		close(fd);
+		win->base = mmap(NULL, p->pack_size, PROT_READ, MAP_PRIVATE, p->pack_fd, 0);
 		if (win->base == MAP_FAILED)
 			die("packfile %s cannot be mapped.", p->pack_name);
 		p->windows = win;
-
-		/* Check if we understand this pack file.  If we don't we're
-		 * likely too old to handle it.
-		 */
-		hdr = (struct pack_header*)win->base;
-		if (hdr->hdr_signature != htonl(PACK_SIGNATURE))
-			die("packfile %s isn't actually a pack.", p->pack_name);
-		if (!pack_version_ok(hdr->hdr_version))
-			die("packfile %s is version %i and not supported"
-				" (try upgrading GIT to a newer version)",
-				p->pack_name, ntohl(hdr->hdr_version));
-
-		/* Check if the pack file matches with the index file.
-		 * this is cheap.
-		 */
-		if (hashcmp((unsigned char *)(p->index_base) +
-			    p->index_size - 40,
-			    p->windows->base +
-			    p->pack_size - 20)) {
-			die("packfile %s does not match index.", p->pack_name);
-		}
 	}
 	p->windows->last_used = pack_used_ctr++;
 	p->windows->inuse_cnt++;
@@ -563,6 +567,7 @@ struct packed_git *add_packed_git(char *path, int path_len, int local)
 	p->index_base = idx_map;
 	p->next = NULL;
 	p->windows = NULL;
+	p->pack_fd = -1;
 	p->pack_local = local;
 	if ((path_len > 44) && !get_sha1_hex(path + path_len - 44, sha1))
 		hashcpy(p->sha1, sha1);
@@ -594,6 +599,7 @@ struct packed_git *parse_pack_index_file(const unsigned char *sha1, char *idx_pa
 	p->index_base = idx_map;
 	p->next = NULL;
 	p->windows = NULL;
+	p->pack_fd = -1;
 	hashcpy(p->sha1, sha1);
 	return p;
 }