Add mutex around mapping and unmapping pack files

When I was writing threading tests for the new cache, the main
error I kept running into was a pack file having it's content
unmapped underneath the running thread.  This adds a lock around
the routines that map and unmap the pack data so that threads can
effectively reload the data when they need it.

This also required reworking the error handling paths in a couple
places in the code which I tried to make consistent.

src/pack.c

@@ -296,24 +296,34 @@ static int pack_index_check(const char *path, struct git_pack_file *p)
 static int pack_index_open(struct git_pack_file *p)
 {
 	char *idx_name;
-	int error;
-	size_t name_len, offset;
+	int error = 0;
+	size_t name_len, base_len;
+
+	if ((error = git_mutex_lock(&p->lock)) < 0)
+		return error;
 
 	if (p->index_map.data)
-		return 0;
+		goto done;
 
-	idx_name = git__strdup(p->pack_name);
-	GITERR_CHECK_ALLOC(idx_name);
+	name_len = strlen(p->pack_name);
+	assert(name_len > strlen(".pack")); /* checked by git_pack_file alloc */
 
-	name_len = strlen(idx_name);
-	offset = name_len - strlen(".pack");
-	assert(offset < name_len); /* make sure no underflow */
+	if ((idx_name = git__malloc(name_len)) == NULL) {
+		error = -1;
+		goto done;
+	}
 
-	strncpy(idx_name + offset, ".idx", name_len - offset);
+	base_len = name_len - strlen(".pack");
+	memcpy(idx_name, p->pack_name, base_len);
+	memcpy(idx_name + base_len, ".idx", sizeof(".idx"));
 
 	error = pack_index_check(idx_name, p);
+
 	git__free(idx_name);
 
+done:
+	git_mutex_unlock(&p->lock);
+
 	return error;
 }
 
@@ -389,7 +399,7 @@ int git_packfile_unpack_header(
 	 * the maximum deflated object size is 2^137, which is just
 	 * insane, so we know won't exceed what we have been given.
 	 */
-//	base = pack_window_open(p, w_curs, *curpos, &left);
+/*	base = pack_window_open(p, w_curs, *curpos, &left); */
 	base = git_mwindow_open(mwf, w_curs, *curpos, 20, &left);
 	if (base == NULL)
 		return GIT_EBUFS;
@@ -789,15 +799,23 @@ git_off_t get_delta_base(
 static struct git_pack_file *packfile_alloc(size_t extra)
 {
 	struct git_pack_file *p = git__calloc(1, sizeof(*p) + extra);
-	if (p != NULL)
+	if (!p)
+		return NULL;
+
 	p->mwf.fd = -1;
+	git_mutex_init(&p->lock);
+
 	return p;
 }
 
 
 void git_packfile_free(struct git_pack_file *p)
 {
-	assert(p);
+	if (!p)
+		return;
+
+	if (git_mutex_lock(&p->lock) < 0)
+		return;
 
 	cache_free(&p->bases);
 
@@ -810,6 +828,10 @@ void git_packfile_free(struct git_pack_file *p)
 	pack_index_free(p);
 
 	git__free(p->bad_object_sha1);
+
+	git_mutex_unlock(&p->lock);
+
+	git_mutex_free(&p->lock);
 	git__free(p);
 }
 
@@ -820,8 +842,6 @@ static int packfile_open(struct git_pack_file *p)
 	git_oid sha1;
 	unsigned char *idx_sha1;
 
-	assert(p->index_map.data);
-
 	if (!p->index_map.data && pack_index_open(p) < 0)
 		return git_odb__error_notfound("failed to open packfile", NULL);
 
@@ -888,7 +908,10 @@ int git_packfile_check(struct git_pack_file **pack_out, const char *path)
 	size_t path_len;
 
 	*pack_out = NULL;
-	path_len = strlen(path);
+
+	if (!path || (path_len = strlen(path)) <= strlen(".idx"))
+		return git_odb__error_notfound("invalid packfile path", NULL);
+
 	p = packfile_alloc(path_len + 2);
 	GITERR_CHECK_ALLOC(p);
 
@@ -897,18 +920,13 @@ int git_packfile_check(struct git_pack_file **pack_out, const char *path)
 	 * the index looks sane.
 	 */
 	path_len -= strlen(".idx");
-	if (path_len < 1) {
-		git__free(p);
-		return git_odb__error_notfound("invalid packfile path", NULL);
-	}
-
 	memcpy(p->pack_name, path, path_len);
 
-	strcpy(p->pack_name + path_len, ".keep");
+	memcpy(p->pack_name + path_len, ".keep", sizeof(".keep"));
 	if (git_path_exists(p->pack_name) == true)
 		p->pack_keep = 1;
 
-	strcpy(p->pack_name + path_len, ".pack");
+	memcpy(p->pack_name + path_len, ".pack", sizeof(".pack"));
 	if (p_stat(p->pack_name, &st) < 0 || !S_ISREG(st.st_mode)) {
 		git__free(p);
 		return git_odb__error_notfound("packfile not found", NULL);
@@ -1039,7 +1057,6 @@ static int pack_entry_find_offset(
 
 		if ((error = pack_index_open(p)) < 0)
 			return error;
-
 		assert(p->index_map.data);
 
 		index = p->index_map.data;
@@ -1099,6 +1116,7 @@ static int pack_entry_find_offset(
 		return git_odb__error_notfound("failed to find offset for pack entry", short_oid);
 	if (found > 1)
 		return git_odb__error_ambiguous("found multiple offsets for pack entry");
+
 	*offset_out = nth_packed_object_offset(p, pos);
 	git_oid_fromraw(found_oid, current);
 
@@ -1110,6 +1128,7 @@ static int pack_entry_find_offset(
 		printf("found lo=%d %s\n", lo, hex_sha1);
 	}
 #endif
+
 	return 0;
 }
 

src/pack.h

@@ -79,6 +79,7 @@ typedef struct {
 struct git_pack_file {
 	git_mwindow_file mwf;
 	git_map index_map;
+	git_mutex lock;
 
 	uint32_t num_objects;
 	uint32_t num_bad_objects;