1 file 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);
+}
+