Avoid possible infinite loops on APCu eviction (#136)

TobiasBengtsson · LKaemmerling · web-flow · commit a09ea80ec1ec · 2024-02-01T14:28:34.000+01:00
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 &lt;me@tobiasbengtsson.se&gt;
Co-authored-by: Lukas Kämmerling &lt;lukas.kaemmerling@hetzner-cloud.de&gt;
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)));
             }
         }
     }

Original file line number	Diff line number	Diff line change
`@@ -72,6 +72,13 @@ public function updateHistogram(array $data): void`
`72`	`72`	`$old = apcu_fetch($sumKey);`
`73`	`73`	`if ($old !== false) {`
`74`	`74`	`$done = apcu_cas($sumKey, $old, $this->toBinaryRepresentationAsInteger($this->fromBinaryRepresentationAsInteger($old) + $data['value']));`
	`75`	`+ } else {`
	`76`	`+ $new = apcu_add($sumKey, $this->toBinaryRepresentationAsInteger(0));`
	`77`	`+`
	`78`	`+ // If sum does not exist, assume a new histogram and store the metadata`
	`79`	`+ if ($new) {`
	`80`	`+ apcu_store($this->metaKey($data), json_encode($this->metaData($data)));`
	`81`	`+ }`
`75`	`82`	`}`
`76`	`83`	`}`
`77`	`84`
`@@ -160,6 +167,11 @@ public function updateGauge(array $data): void`
`160`	`167`	`$old = apcu_fetch($valueKey);`
`161`	`168`	`if ($old !== false) {`
`162`	`169`	`$done = apcu_cas($valueKey, $old, $this->toBinaryRepresentationAsInteger($this->fromBinaryRepresentationAsInteger($old) + $data['value']));`
	`170`	`+ } else {`
	`171`	`+ $new = apcu_add($valueKey, $this->toBinaryRepresentationAsInteger(0));`
	`172`	`+ if ($new) {`
	`173`	`+ apcu_store($this->metaKey($data), json_encode($this->metaData($data)));`
	`174`	`+ }`
`163`	`175`	`}`
`164`	`176`	`}`
`165`	`177`	`}`
`@@ -183,6 +195,9 @@ public function updateCounter(array $data): void`
`183`	`195`	`$old = apcu_fetch($valueKey);`
`184`	`196`	`if ($old !== false) {`
`185`	`197`	`$done = apcu_cas($valueKey, $old, $this->toBinaryRepresentationAsInteger($this->fromBinaryRepresentationAsInteger($old) + $data['value']));`
	`198`	`+ } else {`
	`199`	`+ apcu_add($this->valueKey($data), 0);`
	`200`	`+ apcu_store($this->metaKey($data), json_encode($this->metaData($data)));`
`186`	`201`	`}`
`187`	`202`	`}`
`188`	`203`	`}`