Avoid possible infinite loops on APCu eviction (#136)

If we are unlucky with an APCu eviction at the wrong moment, and no other thread writing to the same metric, we can get stuck in an infinite loop where apcu_fetch will always return false. Fix by applying the same strategy as if apcu_fetch was false in the first place (before doing compare-and-swap) Signed-off-by: Tobias Bengtsson <[email protected]> Co-authored-by: Lukas Kämmerling <[email protected]>
PromPHP · Feb 1, 2024 · a09ea80 · a09ea80
1 parent 3f915df
commit a09ea80
Showing 1 changed file with 15 additions and 0 deletions.
diff --git a/src/Prometheus/Storage/APC.php b/src/Prometheus/Storage/APC.php
@@ -72,6 +72,13 @@ public function updateHistogram(array $data): void
             $old = apcu_fetch($sumKey);
             if ($old !== false) {
                 $done = apcu_cas($sumKey, $old, $this->toBinaryRepresentationAsInteger($this->fromBinaryRepresentationAsInteger($old) + $data['value']));
+            } else {
+                $new = apcu_add($sumKey, $this->toBinaryRepresentationAsInteger(0));
+
+                // If sum does not exist, assume a new histogram and store the metadata
+                if ($new) {
+                    apcu_store($this->metaKey($data), json_encode($this->metaData($data)));
+                }
             }
         }
 
@@ -160,6 +167,11 @@ public function updateGauge(array $data): void
                 $old = apcu_fetch($valueKey);
                 if ($old !== false) {
                     $done = apcu_cas($valueKey, $old, $this->toBinaryRepresentationAsInteger($this->fromBinaryRepresentationAsInteger($old) + $data['value']));
+                } else {
+                    $new = apcu_add($valueKey, $this->toBinaryRepresentationAsInteger(0));
+                    if ($new) {
+                        apcu_store($this->metaKey($data), json_encode($this->metaData($data)));
+                    }
                 }
             }
         }
@@ -183,6 +195,9 @@ public function updateCounter(array $data): void
             $old = apcu_fetch($valueKey);
             if ($old !== false) {
                 $done = apcu_cas($valueKey, $old, $this->toBinaryRepresentationAsInteger($this->fromBinaryRepresentationAsInteger($old) + $data['value']));
+            } else {
+                apcu_add($this->valueKey($data), 0);
+                apcu_store($this->metaKey($data), json_encode($this->metaData($data)));
             }
         }
     }