Redesign the caching layer

The original caching layer in cgit has no upper bound on the number of
concurrent cache entries, so when cgit is traversed by a spider (like the
googlebot), the cache might end up filling your disk. Also, if any error
occurs in the cache layer, no content is returned to the client.

This patch redesigns the caching layer to avoid these flaws by
* giving the cache a bound number of slots
* disabling the cache for the current request when errors occur

The cache size limit is implemented by hashing the querystring (the cache
lookup key) and generating a cache filename based on this hash modulo the
cache size. In order to detect hash collisions, the full lookup key (i.e.
the querystring) is stored in the cache file (separated from its associated
content by ascii 0).

The cache filename is the reversed 8-digit hexadecimal representation of

  hash(key) % cache_size

which should make the filesystem lookup pretty fast (if directory content
is indexed/sorted); reversing the representation avoids the problem where
all keys have equal prefix.

There is a new config option, cache-size, which sets the upper bound for
the cache. Default value for this option is 0, which has the same effect
as setting nocache=1 (hence nocache is now deprecated).

Included in this patch is also a new testfile which verifies that the
new option works as intended.

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

1 files changed, 291 insertions, 68 deletions

diff --git a/cache.c b/cache.c
index 89f7ecd..e590d7b 100644
--- a/cache.c
+++ b/cache.c
@@ -4,117 +4,340 @@
  *
  * Licensed under GNU General Public License v2
  *   (see COPYING for full license text)
+ *
+ *
+ * The cache is just a directory structure where each file is a cache slot,
+ * and each filename is based on the hash of some key (e.g. the cgit url).
+ * Each file contains the full key followed by the cached content for that
+ * key.
+ *
  */
 
 #include "cgit.h"
 #include "cache.h"
 
-const int NOLOCK = -1;
+#define CACHE_BUFSIZE (1024 * 4)
+
+struct cache_slot {
+        const char *key;
+        int keylen;
+        int ttl;
+        cache_fill_fn fn;
+        void *cbdata;
+        int cache_fd;
+        int lock_fd;
+        const char *cache_name;
+        const char *lock_name;
+        int match;
+        struct stat cache_st;
+        struct stat lock_st;
+        int bufsize;
+        char buf[CACHE_BUFSIZE];
+};
 
-char *cache_safe_filename(const char *unsafe)
+/* Open an existing cache slot and fill the cache buffer with
+ * (part of) the content of the cache file. Return 0 on success
+ * and errno otherwise.
+ */
+static int open_slot(struct cache_slot *slot)
 {
-        static char buf[4][PATH_MAX];
-        static int bufidx;
-        char *s;
-        char c;
-
-        bufidx++;
-        bufidx &= 3;
-        s = buf[bufidx];
-
-        while(unsafe && (c = *unsafe++) != 0) {
-                if (c == '/' || c == ' ' || c == '&' || c == '|' ||
-                    c == '>' || c == '<' || c == '.')
-                        c = '_';
-                *s++ = c;
-        }
-        *s = '\0';
-        return buf[bufidx];
+        char *bufz;
+        int bufkeylen = -1;
+
+        slot->cache_fd = open(slot->cache_name, O_RDONLY);
+        if (slot->cache_fd == -1)
+                return errno;
+
+        if (fstat(slot->cache_fd, &slot->cache_st))
+                return errno;
+
+        slot->bufsize = read(slot->cache_fd, slot->buf, sizeof(slot->buf));
+        if (slot->bufsize == 0)
+                return errno;
+
+        bufz = memchr(slot->buf, 0, slot->bufsize);
+        if (bufz)
+                bufkeylen = bufz - slot->buf;
+
+        slot->match = bufkeylen == slot->keylen &&
+            !memcmp(slot->key, slot->buf, bufkeylen + 1);
+
+        return 0;
 }
 
-int cache_exist(struct cacheitem *item)
+/* Close the active cache slot */
+static void close_slot(struct cache_slot *slot)
 {
-        if (stat(item->name, &item->st)) {
-                item->st.st_mtime = 0;
-                return 0;
+        if (slot->cache_fd > 0) {
+                close(slot->cache_fd);
+                slot->cache_fd = -1;
         }
-        return 1;
 }
 
-int cache_create_dirs()
+/* Print the content of the active cache slot (but skip the key). */
+static int print_slot(struct cache_slot *slot)
 {
-        char *path;
+        ssize_t i, j = 0;
+
+        i = lseek(slot->cache_fd, slot->keylen + 1, SEEK_SET);
+        if (i != slot->keylen + 1)
+                return errno;
 
-        path = fmt("%s", ctx.cfg.cache_root);
-        if (mkdir(path, S_IRWXU) && errno!=EEXIST)
+        while((i=read(slot->cache_fd, slot->buf, sizeof(slot->buf))) > 0)
+                j = write(STDOUT_FILENO, slot->buf, i);
+
+        if (j < 0)
+                return errno;
+        else
                 return 0;
+}
 
-        if (!ctx.repo)
+/* Check if the slot has expired */
+static int is_expired(struct cache_slot *slot)
+{
+        if (slot->ttl < 0)
                 return 0;
+        else
+                return slot->cache_st.st_mtime + slot->ttl*60 < time(NULL);
+}
 
-        path = fmt("%s/%s", ctx.cfg.cache_root,
-                   cache_safe_filename(ctx.repo->url));
+/* Check if the slot has been modified since we opened it.
+ * NB: If stat() fails, we pretend the file is modified.
+ */
+static int is_modified(struct cache_slot *slot)
+{
+        struct stat st;
 
-        if (mkdir(path, S_IRWXU) && errno!=EEXIST)
-                return 0;
+        if (stat(slot->cache_name, &st))
+                return 1;
+        return (st.st_ino != slot->cache_st.st_ino ||
+                st.st_mtime != slot->cache_st.st_mtime ||
+                st.st_size != slot->cache_st.st_size);
+}
 
-        if (ctx.qry.page) {
-                path = fmt("%s/%s/%s", ctx.cfg.cache_root,
-                           cache_safe_filename(ctx.repo->url),
-                           ctx.qry.page);
-                if (mkdir(path, S_IRWXU) && errno!=EEXIST)
-                        return 0;
+/* Close an open lockfile */
+static void close_lock(struct cache_slot *slot)
+{
+        if (slot->lock_fd > 0) {
+                close(slot->lock_fd);
+                slot->lock_fd = -1;
         }
-        return 1;
 }
 
-int cache_refill_overdue(const char *lockfile)
+/* Create a lockfile used to store the generated content for a cache
+ * slot, and write the slot key + \0 into it.
+ * Returns 0 on success and errno otherwise.
+ */
+static int lock_slot(struct cache_slot *slot)
 {
-        struct stat st;
+        slot->lock_fd = open(slot->lock_name, O_RDWR|O_CREAT|O_EXCL,
+                             S_IRUSR|S_IWUSR);
+        if (slot->lock_fd == -1)
+                return errno;
+        write(slot->lock_fd, slot->key, slot->keylen + 1);
+        return 0;
+}
 
-        if (stat(lockfile, &st))
-                return 0;
+/* Release the current lockfile. If `replace_old_slot` is set the
+ * lockfile replaces the old cache slot, otherwise the lockfile is
+ * just deleted.
+ */
+static int unlock_slot(struct cache_slot *slot, int replace_old_slot)
+{
+        int err;
+
+        if (replace_old_slot)
+                err = rename(slot->lock_name, slot->cache_name);
         else
-                return (time(NULL) - st.st_mtime > ctx.cfg.cache_max_create_time);
+                err = unlink(slot->lock_name);
+        return err;
 }
 
-int cache_lock(struct cacheitem *item)
+/* Generate the content for the current cache slot by redirecting
+ * stdout to the lock-fd and invoking the callback function
+ */
+static int fill_slot(struct cache_slot *slot)
 {
-        int i = 0;
-        char *lockfile = xstrdup(fmt("%s.lock", item->name));
+        int tmp;
 
- top:
-        if (++i > ctx.cfg.max_lock_attempts)
-                die("cache_lock: unable to lock %s: %s",
-                    item->name, strerror(errno));
+        /* Preserve stdout */
+        tmp = dup(STDOUT_FILENO);
+        if (tmp == -1)
+                return errno;
 
-        item->fd = open(lockfile, O_WRONLY|O_CREAT|O_EXCL, S_IRUSR|S_IWUSR);
+        /* Redirect stdout to lockfile */
+        if (dup2(slot->lock_fd, STDOUT_FILENO) == -1)
+                return errno;
 
-        if (item->fd == NOLOCK && errno == ENOENT && cache_create_dirs())
-                goto top;
+        /* Generate cache content */
+        slot->fn(slot->cbdata);
 
-        if (item->fd == NOLOCK && errno == EEXIST &&
-            cache_refill_overdue(lockfile) && !unlink(lockfile))
-                        goto top;
+        /* Restore stdout */
+        if (dup2(tmp, STDOUT_FILENO) == -1)
+                return errno;
 
-        free(lockfile);
-        return (item->fd > 0);
+        /* Close the temporary filedescriptor */
+        close(tmp);
+        return 0;
 }
 
-int cache_unlock(struct cacheitem *item)
+/* Crude implementation of 32-bit FNV-1 hash algorithm,
+ * see http://www.isthe.com/chongo/tech/comp/fnv/ for details
+ * about the magic numbers.
+ */
+#define FNV_OFFSET 0x811c9dc5
+#define FNV_PRIME  0x01000193
+
+unsigned long hash_str(const char *str)
 {
-        close(item->fd);
-        return (rename(fmt("%s.lock", item->name), item->name) == 0);
+        unsigned long h = FNV_OFFSET;
+        unsigned char *s = (unsigned char *)str;
+
+        if (!s)
+                return h;
+
+        while(*s) {
+                h *= FNV_PRIME;
+                h ^= *s++;
+        }
+        return h;
 }
 
-int cache_cancel_lock(struct cacheitem *item)
+static int process_slot(struct cache_slot *slot)
 {
-        return (unlink(fmt("%s.lock", item->name)) == 0);
+        int err;
+
+        err = open_slot(slot);
+        if (!err && slot->match) {
+                if (is_expired(slot)) {
+                        if (!lock_slot(slot)) {
+                                /* If the cachefile has been replaced between
+                                 * `open_slot` and `lock_slot`, we'll just
+                                 * serve the stale content from the original
+                                 * cachefile. This way we avoid pruning the
+                                 * newly generated slot. The same code-path
+                                 * is chosen if fill_slot() fails for some
+                                 * reason.
+                                 *
+                                 * TODO? check if the new slot contains the
+                                 * same key as the old one, since we would
+                                 * prefer to serve the newest content.
+                                 * This will require us to open yet another
+                                 * file-descriptor and read and compare the
+                                 * key from the new file, so for now we're
+                                 * lazy and just ignore the new file.
+                                 */
+                                if (is_modified(slot) || fill_slot(slot)) {
+                                        unlock_slot(slot, 0);
+                                        close_lock(slot);
+                                } else {
+                                        close_slot(slot);
+                                        unlock_slot(slot, 1);
+                                        slot->cache_fd = slot->lock_fd;
+                                }
+                        }
+                }
+                print_slot(slot);
+                close_slot(slot);
+                return 0;
+        }
+
+        /* If the cache slot does not exist (or its key doesn't match the
+         * current key), lets try to create a new cache slot for this
+         * request. If this fails (for whatever reason), lets just generate
+         * the content without caching it and fool the caller to belive
+         * everything worked out (but print a warning on stdout).
+         */
+
+        close_slot(slot);
+        if ((err = lock_slot(slot)) != 0) {
+                cache_log("[cgit] Unable to lock slot %s: %s (%d)\n",
+                          slot->lock_name, strerror(err), err);
+                slot->fn(slot->cbdata);
+                return 0;
+        }
+
+        if ((err = fill_slot(slot)) != 0) {
+                cache_log("[cgit] Unable to fill slot %s: %s (%d)\n",
+                          slot->lock_name, strerror(err), err);
+                unlock_slot(slot, 0);
+                close_lock(slot);
+                slot->fn(slot->cbdata);
+                return 0;
+        }
+        // We've got a valid cache slot in the lock file, which
+        // is about to replace the old cache slot. But if we
+        // release the lockfile and then try to open the new cache
+        // slot, we might get a race condition with a concurrent
+        // writer for the same cache slot (with a different key).
+        // Lets avoid such a race by just printing the content of
+        // the lock file.
+        slot->cache_fd = slot->lock_fd;
+        unlock_slot(slot, 1);
+        err = print_slot(slot);
+        close_slot(slot);
+        return err;
 }
 
-int cache_expired(struct cacheitem *item)
+/* Print cached content to stdout, generate the content if necessary. */
+int cache_process(int size, const char *path, const char *key, int ttl,
+                  cache_fill_fn fn, void *cbdata)
 {
-        if (item->ttl < 0)
+        unsigned long hash;
+        int len, i;
+        char filename[1024];
+        char lockname[1024 + 5];  /* 5 = ".lock" */
+        struct cache_slot slot;
+
+        /* If the cache is disabled, just generate the content */
+        if (size <= 0) {
+                fn(cbdata);
                 return 0;
-        return item->st.st_mtime + item->ttl * 60 < time(NULL);
+        }
+
+        /* Verify input, calculate filenames */
+        if (!path) {
+                cache_log("[cgit] Cache path not specified, caching is disabled\n");
+                fn(cbdata);
+                return 0;
+        }
+        len = strlen(path);
+        if (len > sizeof(filename) - 10) { /* 10 = "/01234567\0" */
+                cache_log("[cgit] Cache path too long, caching is disabled: %s\n",
+                          path);
+                fn(cbdata);
+                return 0;
+        }
+        if (!key)
+                key = "";
+        hash = hash_str(key) % size;
+        strcpy(filename, path);
+        if (filename[len - 1] != '/')
+                filename[len++] = '/';
+        for(i = 0; i < 8; i++) {
+                sprintf(filename + len++, "%x",
+                        (unsigned char)(hash & 0xf));
+                hash >>= 4;
+        }
+        filename[len] = '\0';
+        strcpy(lockname, filename);
+        strcpy(lockname + len, ".lock");
+        slot.fn = fn;
+        slot.cbdata = cbdata;
+        slot.ttl = ttl;
+        slot.cache_name = filename;
+        slot.lock_name = lockname;
+        slot.key = key;
+        slot.keylen = strlen(key);
+        return process_slot(&slot);
 }
+
+/* Print a message to stdout */
+void cache_log(const char *format, ...)
+{
+        va_list args;
+        va_start(args, format);
+        vfprintf(stderr, format, args);
+        va_end(args);
+}
+