(dev/core#174) CRM_Utils_Cache_SqlGroup - Refine trivial TTL handling to stabilize...

Since incorporting `E2E_Cache_SqlGroupTest`, we've observed periodic flakiness in some
of the test-runs around code like this:

```php
  $cache->set("key", "value", 1);
  $get = $cache->get("key");
```

This makes the tests more reliable by refining the way `SqlGroup` handles trivially short TTLs.

Before
----------------------------------------
Cache items with `$ttl=1` may be expired before you ever get a chance to read them.

After
----------------------------------------
Cache items with `$ttl=1` may sometimes last slightly longer than a second
(if recorded late in the clock's current second), which improves the odds that
you'll be able to read the cache.

Technical Details
----------------------------------------
To reproduce the problem, run this script a few times via `cv scr`:

```php
<?php
$cache = CRM_Utils_Cache::create([
  'name' => 'tmp-cache',
  'type' => ['SqlGroup']
]);

for ($i = 0; $i < 500; $i++) {
  $cache->set("foo_$i", 'hi', 1);
  $get = $cache->get("foo_$i");
  if ($get !== 'hi') {
    echo "miss! ($i)\n";
  } else {
    echo ".";
  }
}
```

Given that `$cache->set()` and `$cache->get()` are right next to each other,
you would expect (and the test authors did evidently expect) that cache would be valid
long enough to read content. And yet we periodically have a cache-miss (particularly with
`SqlGroup`, which is a bit slower to handle writes).

For example, suppose `microtime()===100.95`.  If you compute expiration as
`$expires=time()+$ttl`, then `$expires===101`.  However, this means that
the cache's effective validity period is more like `~0.05` seconds rather
than a full second.

A more correct solution would be to track expiration times as microseconds.
However, that's a bigger patch, and we don't really need that level of
precision -- typical TTLs for us are more like "a week" or "a day" or "an
hour" or *maybe* "a minute".  The `$ttl=1` is a trivial scenario that only
makes sense for unit-testing.

This patch just rounds up the calculation of `$expires` -- in our
hypothetical at `microtime()===100.95`, that makes `$expires===102` and an
effective validity period of `~1.05` seconds.

diff --git a/CRM/Utils/Cache/SqlGroup.php b/CRM/Utils/Cache/SqlGroup.php
index aa4d15e47249b1f7e2f670bb62857b176637ae3d..beaf5fd1ddb3d27113e894049faabb231a04317c 100644 (file)
--- a/CRM/Utils/Cache/SqlGroup.php
+++ b/CRM/Utils/Cache/SqlGroup.php
@@ -118,8 +118,12 @@ class CRM_Utils_Cache_SqlGroup implements CRM_Utils_Cache_Interface {
       throw new \CRM_Utils_Cache_CacheException("SqlGroup: Failed to acquire lock on cache key.");
     }
 
+    if (is_int($ttl) && $ttl <= 0) {
+      return $this->delete($key);
+    }
+
     $dataExists = CRM_Core_DAO::singleValueQuery("SELECT COUNT(*) FROM {$this->table} WHERE {$this->where($key)}");
-    $expires = CRM_Utils_Date::convertCacheTtlToExpires($ttl, self::DEFAULT_TTL);
+    $expires = round(microtime(1)) + CRM_Utils_Date::convertCacheTtl($ttl, self::DEFAULT_TTL);
 
     $dataSerialized = CRM_Core_BAO_Cache::encode($value);