Don't truncate valid cachefiles

An embarrassing thinko in cgit_check_cache() would truncate valid cachefiles
in the following situation:
  1) process A notices a missing/expired cachefile
  2) process B gets scheduled, locks, fills and unlocks the cachefile
  3) process A gets scheduled, locks the cachefile, notices that the cachefile
     now exist/is not expired anymore, and continues to overwrite it with an
     empty lockfile.

Thanks to Linus for noticing (again).

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

3 files changed, 14 insertions, 2 deletions

diff --git a/cache.c b/cache.c
index 2e1ef8c..b947a34 100644
--- a/cache.c
+++ b/cache.c
@@ -54,54 +54,59 @@ int cache_create_dirs()
 
 	if (cgit_query_page) {
 		path = fmt("%s/%s/%s", cgit_cache_root, cgit_query_repo, 
 			   cgit_query_page);
 		if (mkdir(path, S_IRWXU) && errno!=EEXIST)
 			return 0;
 	}
 	return 1;
 }
 
 int cache_refill_overdue(const char *lockfile)
 {
 	struct stat st;
 
 	if (stat(lockfile, &st))
 		return 0;
 	else
 		return (time(NULL) - st.st_mtime > cgit_cache_max_create_time);
 }
 
 int cache_lock(struct cacheitem *item)
 {
 	int i = 0;
 	char *lockfile = fmt("%s.lock", item->name);
 
  top:
 	if (++i > cgit_max_lock_attempts)
 		die("cache_lock: unable to lock %s: %s",
 		    item->name, strerror(errno));
 
        	item->fd = open(lockfile, O_WRONLY|O_CREAT|O_EXCL, S_IRUSR|S_IWUSR);
 
 	if (item->fd == NOLOCK && errno == ENOENT && cache_create_dirs())
 		goto top;
 
 	if (item->fd == NOLOCK && errno == EEXIST &&
 	    cache_refill_overdue(lockfile) && !unlink(lockfile))
 			goto top;
 
 	return (item->fd > 0);
 }
 
 int cache_unlock(struct cacheitem *item)
 {
 	close(item->fd);
 	return (rename(fmt("%s.lock", item->name), item->name) == 0);
 }
 
+int cache_cancel_lock(struct cacheitem *item)
+{
+	return (unlink(fmt("%s.lock", item->name)) == 0);
+}
+
 int cache_expired(struct cacheitem *item)
 {
 	if (item->ttl < 0)
 		return 0;
 	return item->st.st_mtime + item->ttl * 60 < time(NULL);
 }
diff --git a/cgit.c b/cgit.c
index d1abaa0..0f72f2d 100644
--- a/cgit.c
+++ b/cgit.c
@@ -16,85 +16,91 @@ static void cgit_print_repo_page(struct cacheitem *item)
 	    cgit_read_config("info/cgit", cgit_repo_config_cb)) {
 		char *title = fmt("%s - %s", cgit_root_title, "Bad request");
 		cgit_print_docstart(title, item);
 		cgit_print_pageheader(title);
 		cgit_print_error(fmt("Unable to scan repository: %s",
 				     strerror(errno)));
 		cgit_print_docend();
 		return;
 	}
 	setenv("GIT_DIR", fmt("%s/%s", cgit_root, cgit_query_repo), 1);
 	char *title = fmt("%s - %s", cgit_repo_name, cgit_repo_desc);
 	cgit_print_docstart(title, item);
 	cgit_print_pageheader(title);
 	if (!cgit_query_page)
 		cgit_print_summary();
 	else if (!strcmp(cgit_query_page, "log")) {
 		cgit_print_log(cgit_query_head, 0, 100);
 	} else if (!strcmp(cgit_query_page, "view")) {
 		cgit_print_view(cgit_query_sha1);
 	}
 	cgit_print_docend();
 }
 
 static void cgit_fill_cache(struct cacheitem *item)
 {
 	htmlfd = item->fd;
 	item->st.st_mtime = time(NULL);
 	if (cgit_query_repo)
 		cgit_print_repo_page(item);
 	else
 		cgit_print_repolist(item);
 }
 
 static void cgit_check_cache(struct cacheitem *item)
 {
 	int i = 0;
 
 	cache_prepare(item);
  top:
 	if (++i > cgit_max_lock_attempts) {
 		die("cgit_refresh_cache: unable to lock %s: %s",
 		    item->name, strerror(errno));
 	}
 	if (!cache_exist(item)) {
 		if (!cache_lock(item)) {
 			sleep(1);
 			goto top;
 		}
-		if (!cache_exist(item))
+		if (!cache_exist(item)) {
 			cgit_fill_cache(item);
 		cache_unlock(item);
+		} else {
+			cache_cancel_lock(item);
+		}
 	} else if (cache_expired(item) && cache_lock(item)) {
-		if (cache_expired(item))
+		if (cache_expired(item)) {
 			cgit_fill_cache(item);
 		cache_unlock(item);
+		} else {
+			cache_cancel_lock(item);
+		}
 	}
 }
 
 static void cgit_print_cache(struct cacheitem *item)
 {
 	static char buf[4096];
 	ssize_t i;
 
 	int fd = open(item->name, O_RDONLY);
 	if (fd<0)
 		die("Unable to open cached file %s", item->name);
 
 	while((i=read(fd, buf, sizeof(buf))) > 0)
 		write(STDOUT_FILENO, buf, i);
 
 	close(fd);
 }
 
 int main(int argc, const char **argv)
 {
 	struct cacheitem item;
 
 	cgit_read_config("/etc/cgitrc", cgit_global_config_cb);
 	cgit_querystring = xstrdup(getenv("QUERY_STRING"));
 	cgit_parse_query(cgit_querystring, cgit_querystring_cb);
 
 	cgit_check_cache(&item);
 	cgit_print_cache(&item);
 	return 0;
 }
diff --git a/cgit.h b/cgit.h
index c1dcc97..c9554a7 100644
--- a/cgit.h
+++ b/cgit.h
@@ -21,66 +21,67 @@ extern char *cgit_root;
 extern char *cgit_root_title;
 extern char *cgit_css;
 extern char *cgit_logo;
 extern char *cgit_logo_link;
 extern char *cgit_virtual_root;
 extern char *cgit_cache_root;
 
 extern int cgit_max_lock_attempts;
 extern int cgit_cache_root_ttl;
 extern int cgit_cache_repo_ttl;
 extern int cgit_cache_dynamic_ttl;
 extern int cgit_cache_static_ttl;
 extern int cgit_cache_max_create_time;
 
 extern char *cgit_repo_name;
 extern char *cgit_repo_desc;
 extern char *cgit_repo_owner;
 
 extern int cgit_query_has_symref;
 extern int cgit_query_has_sha1;
 
 extern char *cgit_querystring;
 extern char *cgit_query_repo;
 extern char *cgit_query_page;
 extern char *cgit_query_head;
 extern char *cgit_query_sha1;
 
 extern int htmlfd;
 
 extern void cgit_global_config_cb(const char *name, const char *value);
 extern void cgit_repo_config_cb(const char *name, const char *value);
 extern void cgit_querystring_cb(const char *name, const char *value);
 
 extern char *fmt(const char *format,...);
 
 extern void html(const char *txt);
 extern void htmlf(const char *format,...);
 extern void html_txt(char *txt);
 extern void html_attr(char *txt);
 extern void html_link_open(char *url, char *title, char *class);
 extern void html_link_close(void);
 
 extern int cgit_read_config(const char *filename, configfn fn);
 extern int cgit_parse_query(char *txt, configfn fn);
 
 extern void cache_prepare(struct cacheitem *item);
 extern int cache_lock(struct cacheitem *item);
 extern int cache_unlock(struct cacheitem *item);
+extern int cache_cancel_lock(struct cacheitem *item);
 extern int cache_exist(struct cacheitem *item);
 extern int cache_expired(struct cacheitem *item);
 
 extern char *cgit_repourl(const char *reponame);
 extern char *cgit_pageurl(const char *reponame, const char *pagename, 
 			  const char *query);
 
 extern void cgit_print_error(char *msg);
 extern void cgit_print_docstart(char *title, struct cacheitem *item);
 extern void cgit_print_docend();
 extern void cgit_print_pageheader(char *title);
 
 extern void cgit_print_repolist(struct cacheitem *item);
 extern void cgit_print_summary();
 extern void cgit_print_log(const char *tip, int ofs, int cnt);
 extern void cgit_print_view(char *hex);
 
 #endif /* CGIT_H */