API function to check if a cache bin is empty

Why do we use the word _bin_ in the function name? I think this should be called cache_is_empty() for consistency.

Shouldn't this be a db_query_range($query, 0, 1);?

+++ includes/cache.inc	12 Sep 2009 13:41:07 -0000
@@ -260,6 +275,19 @@
+  
+  /**
+  * Check if a cache bin is empty.
+  *

Trailing white-space and misaligned PHPDoc comment here.

+++ includes/cache.inc	12 Sep 2009 13:41:07 -0000
@@ -449,4 +477,11 @@
+    $result = db_query_range("SELECT data FROM {" . $this->bin . "} ", 0, 1)->fetchField();

Shouldn't this be a count query or similar?

If that first row by coincidence happens to be big, then we process a lot of data for nothing.

+++ includes/cache.inc	12 Sep 2009 13:41:07 -0000
@@ -449,4 +477,11 @@
+  }
+
 }

Please remove the trailing blank line after ::isEmpty().

I'm on crack. Are you, too?

See http://drupal.org/update/modules/6/7#select_count for how this query should be written.

+++ includes/cache.inc	13 Sep 2009 11:30:24 -0000
@@ -260,6 +275,19 @@
+  /**
+   * Check if a cache bin is empty.
+   *
+   * A cache bin is considered empty if it does not contain any valid data for any
+   * cache ID.
+   *
+   * @param $bin
+   *   The cache bin to check.
+   * @return
+   *   TRUE if the cache bin specified is empty.
+   */
+  function isEmpty();

The docblock doesn't match the function parameters.

+++ includes/cache.inc	13 Sep 2009 11:30:24 -0000
@@ -449,4 +477,10 @@
+  function isEmpty() {
+    $this->garbageCollection();
+    $result = db_query_range("SELECT 1 FROM {" . $this->bin . "} ", 0, 1)->fetchField();
+    return empty($result);
+  }

This has to be a dynamic query, because the table changes.

Something like this should work:

db_select($this->bin)
  ->addExpression('1', 'count')
  ->limit(0, 1)
  ->execute()
  ->fetchField();

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Never write DB:TNG from memory... the correct query is probably more:

$query = db_select($this->bin);
$query->addExpression('1');
$query->range(0, 1)
  ->execute()
  ->fetchField();

Please add an implemenation to includes/cache-install.inc DrupalFakeCache class which was committed a minute ago...

+++ includes/cache.inc	13 Sep 2009 15:47:47 -0000
@@ -449,4 +475,14 @@
+  function isEmpty() {
+    $this->garbageCollection();

Hm - auto-running GC looks like some (perhaps?) unwanted magic to me. Not sure though.

+++ modules/simpletest/tests/cache.test	13 Sep 2009 15:47:49 -0000
@@ -312,3 +312,40 @@
+class CacheIsEmptyCase extends CacheTestCase {
...
+      'group' => 'Cache'

Missing trailing comma after last array element.

+++ modules/simpletest/tests/cache.test	13 Sep 2009 15:47:49 -0000
@@ -312,3 +312,40 @@
+  function testIsEmpty() {
+    // Clear the cache bin.
+    cache_clear_all('*', $this->default_bin);
+    $this->assertTrue(cache_is_empty($this->default_bin), t('The cache bin is empty'));
+    // Add some data to the cache bin.
+    cache_set($this->default_cid, $this->default_value, $this->default_bin);
+    $this->assertCacheExists(t('Cache was set.'), $this->default_value, $this->default_cid);
+    $this->assertFalse(cache_is_empty($this->default_bin), t('The cache bin is not empty'));
+    // Remove the cached data.
+    cache_clear_all($this->default_cid, $this->default_bin);
+    $this->assertCacheRemoved(t('Cache was removed.'), $this->default_cid);
+    $this->assertTrue(cache_is_empty($this->default_bin), t('The cache bin is empty'));
+  }

Can we squeeze a blank line between the chunks/chapters here? :)

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I'm happy with this now, and it cleans up some of the existing APIs which is Slush-worthy. Committed to CVS HEAD. Thanks.

Looks like this change is breaks alternative cache-backends (memcache, apc...) because it's not possible to count items in most of them.
Is this API change is really required?

We have wildcard cache clearing in our API as well, but neither support that, I think it's OK for them to just silently not work for things like this - better than querying cache tables directly when you are using the default backend.

or, to be one the safe side you can always report as nonempty, worst can happen is a test fail, big deal.