When one cache table is cleared, cached items from other tables invalidated

In Drush or in Devel's PHP execution form, run the following:

(No, you idiot! Don't run arbitrary PHP code that some jerk you've never met tells you to run in some random forum on the internet! But if you're adventurous…)

variable_set('cache_lifetime', '600');

$expire = time() + 3600;

// Set some cached data in the standard "cache" table with an expiry
cache_set('foo', array('bar' => 'baz'), 'cache', $expire);
// …and in the "cache_block" table
cache_set('foo', array('bar' => 'baz'), 'cache_block', $expire);

// Print it back out:
$cache = cache_get('foo', 'cache');
print_r($cache);
$cache = cache_get('foo', 'cache_block');
print_r($cache);

That being executed, in a separate page load, execute this:

cache_clear_all(NULL, 'cache_page');

// Wait two seconds…
usleep(2000000);

// …And try getting the cache data again.
$cache = cache_get('foo', 'cache');
print_r($cache);
$cache = cache_get('foo', 'cache_block');
print_r($cache);

The output will be 00.

The relevant code, in cache_clear_all():

function cache_clear_all($cid = NULL, $table = NULL, $wildcard = FALSE) {
  global $user;
  
  // ...

  if (empty($cid)) {
    if (variable_get('cache_lifetime', 0)) {
      // We store the time in the current user's $user->cache variable which
      // will be saved into the sessions table by sess_write(). We then
      // simulate that the cache was flushed for this user by not returning
      // cached data that was cached before the timestamp.
      $user->cache = time();
      // ...

And in cache_get():

function cache_get($cid, $table = 'cache') {
  global $user;

  // ...

  if (isset($cache->data)) {
    // If the data is permanent or we're not enforcing a minimum cache lifetime
    // always return the cached data.
    if ($cache->expire == CACHE_PERMANENT || !variable_get('cache_lifetime', 0)) {
      // ...
    }
    // If enforcing a minimum cache lifetime, validate that the data is
    // currently valid for this user before we return it by making sure the
    // cache entry was created before the timestamp in the current session's
    // cache timer. The cache variable is loaded into the $user object by
    // sess_read() in session.inc.
    else {
      if (isset($user->cache) && $user->cache > $cache->created) {
        // This cache data is too old and thus not valid for us, ignore it.
        return 0;
      }
    // ...

Thus: cache_clear_all(), when called for a particular table, sets $user->cache; then, if cached data is not permanent, cache_get() checks if $user->cache is older than the created timestamp of the cache, regardless of whether that cached data is in the same table that was cleared or not.

We ran into this problem when submission of a node form in one tab was causing issues with AHAH widgets in another, if I'm understanding my colleague correctly.

Not sure if there's a clean way to fix this… also haven't tested this in D7 yet.