Add MultiSampler implementation (#147)

* Add MultiSampler implementation

* Add MultiSampler to README table

Author: chingor13

README.md

@@ -87,6 +87,7 @@ The provided samplers are:
 | ----- | ----------- |
 | [NeverSampleSampler][never-sampler] | Never trace any requests |
 | [AlwaysSampleSampler][always-sampler] | Trace all requests |
+| [MultiSampler][multi-sampler] | Check multiple samplers |
 | [QpsSampler][qps-sampler] | Trace X requests per second. Requires a PSR-6 cache implementation |
 | [ProbabilitySampler][probability-sampler] | Trace X percent of requests. |
 
@@ -159,6 +160,7 @@ This is not an official Google product.
 [pecl]: https://pecl.php.net/
 [never-sampler]: https://census-instrumentation.github.io/opencensus-php/api/OpenCensus/Trace/Sampler/NeverSampleSampler.html
 [always-sampler]: https://census-instrumentation.github.io/opencensus-php/api/OpenCensus/Trace/Sampler/NeverSampleSampler.html
+[multi-sampler]: https://census-instrumentation.github.io/opencensus-php/api/OpenCensus/Trace/Sampler/MultiSampler.html
 [qps-sampler]: https://census-instrumentation.github.io/opencensus-php/api/OpenCensus/Trace/Sampler/NeverSampleSampler.html
 [probability-sampler]: https://census-instrumentation.github.io/opencensus-php/api/OpenCensus/Trace/Sampler/NeverSampleSampler.html
 [echo-exporter]: https://census-instrumentation.github.io/opencensus-php/api/OpenCensus/Trace/Exporter/EchoExporter.html

src/Trace/Sampler/MultiSampler.php

@@ -0,0 +1,68 @@
+<?php
+/**
+ * Copyright 2018 OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+namespace OpenCensus\Trace\Sampler;
+
+/**
+ * This implementation of the SamplerInterface wraps any number of child
+ * SamplerInterface implementations. All provided implementations must return
+ * true in order for the request to be sampled.
+ *
+ * Example:
+ * ```
+ * use OpenCensus\Trace\Sampler\AlwaysSampleSampler;
+ * use OpenCensus\Trace\Sampler\MultiSampler;
+ *
+ * $sampler = new MultiSampler([
+ *   new AlwaysSampleSampler()
+ * ]);
+ * ```
+ */
+class MultiSampler implements SamplerInterface
+{
+
+    /**
+     * @var SamplerInterface[]
+     */
+    private $samplers;
+
+    /**
+     * Create a new MultiSampler.
+     *
+     * @param SamplerInterface[] $samplers The samplers to consult.
+     */
+    public function __construct(array $samplers = [])
+    {
+        $this->samplers = $samplers;
+    }
+
+    /**
+     * Returns false if any provided sampler chooses not to sample this
+     * request.
+     *
+     * @return bool
+     */
+    public function shouldSample()
+    {
+        foreach ($this->samplers as $sampler) {
+            if ($sampler->shouldSample() === false) {
+                return false;
+            }
+        }
+        return true;
+    }
+}

tests/unit/Trace/Sampler/MultiSamplerTest.php

@@ -0,0 +1,72 @@
+<?php
+/**
+ * Copyright 2018 OpenCensus Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+namespace OpenCensus\Tests\Unit\Trace\Sampler;
+
+use OpenCensus\Trace\Sampler\MultiSampler;
+use OpenCensus\Trace\Sampler\SamplerInterface;
+
+/**
+ * @group trace
+ */
+class MultiSamplerTest extends \PHPUnit_Framework_TestCase
+{
+    public function testNoSamplers()
+    {
+        $sampler = new MultiSampler();
+        $this->assertTrue($sampler->shouldSample());
+    }
+
+    public function testSingleSampler()
+    {
+        $innerSampler = $this->prophesize(SamplerInterface::class);
+        $innerSampler->shouldSample()->willReturn(true)->shouldBeCalled();
+
+        $sampler = new MultiSampler([
+            $innerSampler->reveal()
+        ]);
+        $this->assertTrue($sampler->shouldSample());
+    }
+
+    public function testMultipleSamplers()
+    {
+        $innerSampler = $this->prophesize(SamplerInterface::class);
+        $innerSampler->shouldSample()->willReturn(true)->shouldBeCalled();
+        $innerSampler2 = $this->prophesize(SamplerInterface::class);
+        $innerSampler2->shouldSample()->willReturn(true)->shouldBeCalled();
+
+        $sampler = new MultiSampler([
+            $innerSampler->reveal(),
+            $innerSampler2->reveal()
+        ]);
+        $this->assertTrue($sampler->shouldSample());
+    }
+
+    public function testInnerSamplerFails()
+    {
+        $innerSampler = $this->prophesize(SamplerInterface::class);
+        $innerSampler->shouldSample()->willReturn(true)->shouldBeCalled();
+        $innerSampler2 = $this->prophesize(SamplerInterface::class);
+        $innerSampler2->shouldSample()->willReturn(false)->shouldBeCalled();
+
+        $sampler = new MultiSampler([
+            $innerSampler->reveal(),
+            $innerSampler2->reveal()
+        ]);
+        $this->assertFalse($sampler->shouldSample());
+    }
+}