/* cache.c: cache management
*
* Copyright (C) 2006 Lars Hjemli
*
* 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"
#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];
};
/* 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)
{
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;
}
/* Close the active cache slot */
static void close_slot(struct cache_slot *slot)
{
if (slot->cache_fd > 0) {
close(slot->cache_fd);
slot->cache_fd = -1;
}
}
/* Print the content of the active cache slot (but skip the key). */
static int print_slot(struct cache_slot *slot)
{
ssize_t i, j = 0;
i = lseek(slot->cache_fd, slot->keylen + 1, SEEK_SET);
if (i != slot->keylen + 1)
return errno;
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;
}
/* 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);
}
/* 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 (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);
}
/* 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;
}
}
/* 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)
{
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;
}
/* 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
err = unlink(slot->lock_name);
return err;
}
/* 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 tmp;
/* Preserve stdout */
tmp = dup(STDOUT_FILENO);
if (tmp == -1)
return errno;
/* Redirect stdout to lockfile */
if (dup2(slot->lock_fd, STDOUT_FILENO) == -1)
return errno;
/* Generate cache content */
slot->fn(slot->cbdata);
/* Restore stdout */
if (dup2(tmp, STDOUT_FILENO) == -1)
return errno;
/* Close the temporary filedescriptor */
close(tmp);
return 0;
}
/* 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)
{
unsigned long h = FNV_OFFSET;
unsigned char *s = (unsigned char *)str;
if (!s)
return h;
while(*s) {
h *= FNV_PRIME;
h ^= *s++;
}
return h;
}
static int process_slot(struct cache_slot *slot)
{
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;
}
/* 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)
{
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;
}
/* 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);
}
