Avoid possible infinite loops on APCu eviction

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]>
PromPHP · Nov 9, 2023 · 8cf901d · 8cf901d
1 parent 735ace7
commit 8cf901d
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)));
+                }
             }
         }
 
@@ -139,6 +146,11 @@ 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)));
+                    }
                 }
             }
         }
@@ -162,6 +174,9 @@ 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)));
             }
         }
     }