Avoid infinite loops in caching layer

Add a global variable, cgit_max_lock_attemps, to avoid the possibility of
infinite loops when failing to acquire a lockfile. This could happen on
broken setups or under crazy server load.

Incidentally, this also fixes a lurking bug in cache_lock() where an
uninitialized returnvalue was used.

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

3 files changed, 31 insertions, 14 deletions

diff --git a/cache.c b/cache.c
index 0ffa502..2e1ef8c 100644
--- a/cache.c
+++ b/cache.c
@@ -16,83 +16,92 @@ void cache_prepare(struct cacheitem *item)
                 item->name = xstrdup(fmt("%s/index.html", cgit_cache_root));
                 item->ttl = cgit_cache_root_ttl;
         } else if (!cgit_query_page) {
                 item->name = xstrdup(fmt("%s/%s/index.html", cgit_cache_root, 
                            cgit_query_repo));
                 item->ttl = cgit_cache_repo_ttl;
         } else {
                 item->name = xstrdup(fmt("%s/%s/%s/%s.html", cgit_cache_root, 
                            cgit_query_repo, cgit_query_page, 
                            cgit_querystring));
                 if (cgit_query_has_symref)
                         item->ttl = cgit_cache_dynamic_ttl;
                 else if (cgit_query_has_sha1)
                         item->ttl = cgit_cache_static_ttl;
                 else
                         item->ttl = cgit_cache_repo_ttl;
         }
 }
 
 int cache_exist(struct cacheitem *item)
 {
         if (stat(item->name, &item->st)) {
                 item->st.st_mtime = 0;
                 return 0;
         }
         return 1;
 }
 
 int cache_create_dirs()
 {
         char *path;
 
         if (!cgit_query_repo)
                 return 0;
 
         path = fmt("%s/%s", cgit_cache_root, cgit_query_repo);
         if (mkdir(path, S_IRWXU) && errno!=EEXIST)
                 return 0;
 
         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 ret;
+        int i = 0;
         char *lockfile = fmt("%s.lock", item->name);
 
- top:  
-               item->fd = open(lockfile, O_WRONLY | O_CREAT | O_EXCL, S_IRUSR|S_IWUSR);
+ 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) {
-                struct stat st;
-                time_t t;
-                if (stat(lockfile, &st))
-                        return ret;
-                t = time(NULL);
-                if (t-st.st_mtime > cgit_cache_max_create_time && 
-                    !unlink(lockfile))
+
+        if (item->fd == NOLOCK && errno == EEXIST &&
+            cache_refill_overdue(lockfile) && !unlink(lockfile))
                         goto top;
-                return 0;
-        }
+
         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_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 7f14016..dc91125 100644
--- a/cgit.c
+++ b/cgit.c
@@ -1,81 +1,82 @@
 /* cgit.c: cgi for the git scm
  *
  * Copyright (C) 2006 Lars Hjemli
  *
  * Licensed under GNU General Public License v2
  *   (see COPYING for full license text)
  */
 
 #include "cgit.h"
 
 const char cgit_version[] = CGIT_VERSION;
 
 const char cgit_doctype[] =
 "<!DOCTYPE html PUBLIC \"-//W3C//DTD XHTML 1.0 Transitional//EN\"\n"
 "  \"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd\">\n";
 
 const char cgit_error[] =
 "<div class='error'>%s</div>";
 
 const char cgit_lib_error[] =
 "<div class='error'>%s: %s</div>";
 
 int htmlfd = 0;
 
 char *cgit_root         = "/usr/src/git";
 char *cgit_root_title   = "Git repository browser";
 char *cgit_css          = "/cgit.css";
 char *cgit_logo         = "/git-logo.png";
 char *cgit_logo_link    = "http://www.kernel.org/pub/software/scm/git/docs/";
 char *cgit_virtual_root = NULL;
 
 char *cgit_cache_root   = "/var/cache/cgit";
 
+int cgit_max_lock_attempts     =  5;
 int cgit_cache_root_ttl        =  5;
 int cgit_cache_repo_ttl        =  5;
 int cgit_cache_dynamic_ttl     =  5;
 int cgit_cache_static_ttl      = -1;
 int cgit_cache_max_create_time =  5;
 
 char *cgit_repo_name    = NULL;
 char *cgit_repo_desc    = NULL;
 char *cgit_repo_owner   = NULL;
 
 int cgit_query_has_symref = 0;
 int cgit_query_has_sha1   = 0;
 
 char *cgit_querystring  = NULL;
 char *cgit_query_repo   = NULL;
 char *cgit_query_page   = NULL;
 char *cgit_query_head   = NULL;
 char *cgit_query_sha1   = NULL;
 
 struct cacheitem cacheitem;
 
 int cgit_parse_query(char *txt, configfn fn)
 {
         char *t, *value = NULL, c;
 
         if (!txt)
                 return 0;
 
         t = txt = xstrdup(txt);
  
         while((c=*t) != '\0') {
                 if (c=='=') {
                         *t = '\0';
                         value = t+1;
                 } else if (c=='&') {
                         *t = '\0';
                         (*fn)(txt, value);
                         txt = t+1;
                         value = NULL;
                 }
                 t++;
         }
         if (t!=txt)
                 (*fn)(txt, value);
         return 0;
 }
 
 void cgit_global_config_cb(const char *name, const char *value)
@@ -420,89 +421,95 @@ static void cgit_print_object(char *hex)
         }
 
         buf[size] = '\0';
         html("<h2>Object view</h2>");
         htmlf("sha1=%s<br/>type=%s<br/>size=%i<br/>", hex, type, size);
         html("<pre>");
         html_txt(buf);
         html("</pre>");
 }
 
 static void cgit_print_repo_page()
 {
         if (chdir(fmt("%s/%s", cgit_root, cgit_query_repo)) || 
             cgit_read_config("info/cgit", cgit_repo_config_cb)) {
                 char *title = fmt("%s - %s", cgit_root_title, "Bad request");
                 cgit_print_docstart(title);
                 cgit_print_pageheader(title);
                 htmlf(cgit_lib_error, "Unable to scan repository",
                       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);
         cgit_print_pageheader(title);
         if (!cgit_query_page)
                 cgit_print_repo_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_object(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();
         else
                 cgit_print_repolist();
 }
 
 static void cgit_refresh_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)) {
-                        sched_yield();
+                        sleep(1);
                         goto top;
                 }
                 if (!cache_exist(item))
                         cgit_fill_cache(item);
                 cache_unlock(item);
         } else if (cache_expired(item) && cache_lock(item)) {
                 if (cache_expired(item))
                         cgit_fill_cache(item);
                 cache_unlock(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)
 {
         cgit_read_config("/etc/cgitrc", cgit_global_config_cb);
         cgit_querystring = xstrdup(getenv("QUERY_STRING"));
         cgit_parse_query(cgit_querystring, cgit_querystring_cb);
         cgit_refresh_cache(&cacheitem);
         cgit_print_cache(&cacheitem);
         return 0;
 }
diff --git a/cgit.h b/cgit.h
index 3b0994a..7e4bfef 100644
--- a/cgit.h
+++ b/cgit.h
@@ -1,65 +1,66 @@
 #ifndef CGIT_H
 #define CGIT_H
 
 #include "git.h"
 #include <openssl/sha.h>
 #include <ctype.h>
 #include <sched.h>
 
 typedef void (*configfn)(const char *name, const char *value);
 
 struct cacheitem {
         char *name;
         struct stat st;
         int ttl;
         int fd;
 };
 
 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 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 void cache_prepare(struct cacheitem *item);
 extern int cache_lock(struct cacheitem *item);
 extern int cache_unlock(struct cacheitem *item);
 extern int cache_exist(struct cacheitem *item);
 extern int cache_expired(struct cacheitem *item);
 
 #endif /* CGIT_H */