[RenderTextFormat] Allow value errors to be rendered as comments (#142)

* [RenderTextFormat] Allow value errors to be rendered as comments.

Redis and RedisNg allow samples with mismatching labels to be stored, which could cause ValueError to be thrown when
rendering. Rendering would fail as a result, which is not ideal.

- Change render() to accept an optional $silent parameter. When set to true, render the errors as comments instead of
  throwing them and failing the whole operation.

Signed-off-by: Joe Cai <joe.cai@bigcommerce.com>
---------

Signed-off-by: Joe Cai <joe.cai@bigcommerce.com>
Co-authored-by: Lukas Kämmerling <github@lukas-kaemmerling.de>

Author: joec4i

src/Prometheus/RenderTextFormat.php

@@ -5,16 +5,18 @@
 namespace Prometheus;
 
 use RuntimeException;
+use Throwable;
 
 class RenderTextFormat implements RendererInterface
 {
     const MIME_TYPE = 'text/plain; version=0.0.4';
 
     /**
      * @param MetricFamilySamples[] $metrics
+     * @param bool $silent If true, render value errors as comments instead of throwing them.
      * @return string
      */
-    public function render(array $metrics): string
+    public function render(array $metrics, bool $silent = false): string
     {
         usort($metrics, function (MetricFamilySamples $a, MetricFamilySamples $b): int {
             return strcmp($a->getName(), $b->getName());
@@ -25,7 +27,20 @@ public function render(array $metrics): string
             $lines[] = "# HELP " . $metric->getName() . " {$metric->getHelp()}";
             $lines[] = "# TYPE " . $metric->getName() . " {$metric->getType()}";
             foreach ($metric->getSamples() as $sample) {
-                $lines[] = $this->renderSample($metric, $sample);
+                try {
+                    $lines[] = $this->renderSample($metric, $sample);
+                } catch (Throwable $e) {
+                    // Redis and RedisNg allow samples with mismatching labels to be stored, which could cause ValueError
+                    // to be thrown when rendering. If this happens, users can decide whether to ignore the error or not.
+                    // These errors will normally disappear after the storage is flushed.
+                    if (!$silent) {
+                        throw $e;
+                    }
+
+                    $lines[] = "# Error: {$e->getMessage()}";
+                    $lines[] = "#   Labels: " . json_encode(array_merge($metric->getLabelNames(), $sample->getLabelNames()));
+                    $lines[] = "#   Values: " . json_encode(array_merge($sample->getLabelValues()));
+                }
             }
         }
         return implode("\n", $lines) . "\n";

tests/Test/Prometheus/RenderTextFormatTest.php

@@ -10,6 +10,8 @@
 use Prometheus\RenderTextFormat;
 use PHPUnit\Framework\TestCase;
 use Prometheus\Storage\InMemory;
+use Prometheus\Storage\Redis;
+use ValueError;
 
 class RenderTextFormatTest extends TestCase
 {
@@ -70,4 +72,57 @@ private function getExpectedOutput(): string
 
 TEXTPLAIN;
     }
+
+    public function testValueErrorThrownWithInvalidSamples(): void
+    {
+        $namespace = 'foo';
+        $counter = 'bar';
+        $storage = new Redis(['host' => REDIS_HOST]);
+        $storage->wipeStorage();
+
+        $registry = new CollectorRegistry($storage, false);
+        $registry->registerCounter($namespace, $counter, 'counter-help-text', ['label1', 'label2'])
+            ->inc(['bob', 'alice']);
+
+        // Reload the registry with an updated counter config
+        $registry = new CollectorRegistry($storage, false);
+        $registry->registerCounter($namespace, $counter, 'counter-help-text', ['label1', 'label2', 'label3'])
+            ->inc(['bob', 'alice', 'eve']);
+
+        $this->expectException(ValueError::class);
+
+        $renderer = new RenderTextFormat();
+        $renderer->render($registry->getMetricFamilySamples());
+    }
+
+    public function testOutputWithInvalidSamplesSkipped(): void
+    {
+        $namespace = 'foo';
+        $counter = 'bar';
+        $storage = new Redis(['host' => REDIS_HOST]);
+        $storage->wipeStorage();
+
+        $registry = new CollectorRegistry($storage, false);
+        $registry->registerCounter($namespace, $counter, 'counter-help-text', ['label1', 'label2'])
+            ->inc(['bob', 'alice']);
+
+        // Reload the registry with an updated counter config
+        $registry = new CollectorRegistry($storage, false);
+        $registry->registerCounter($namespace, $counter, 'counter-help-text', ['label1', 'label2', 'label3'])
+            ->inc(['bob', 'alice', 'eve']);
+
+        $expectedOutput = '
+# HELP foo_bar counter-help-text
+# TYPE foo_bar counter
+foo_bar{label1="bob",label2="alice"} 1
+# Error: array_combine(): Argument #1 ($keys) and argument #2 ($values) must have the same number of elements
+#   Labels: ["label1","label2"]
+#   Values: ["bob","alice","eve"]
+';
+
+        $renderer = new RenderTextFormat();
+        $output = $renderer->render($registry->getMetricFamilySamples(), true);
+
+        self::assertSame(trim($expectedOutput), trim($output));
+    }
 }