gNjSjQX1

1diff --git a/src/NW/Lib/TracingRequestHeaders.php b/src/NW/Lib/TracingRequestHeaders.php
2new file mode 100644
3index 000000000..79703ad48
4--- /dev/null
5+++ b/src/NW/Lib/TracingRequestHeaders.php
6@@ -0,0 +1,20 @@
7+<?php
8+
9+namespace NW\Lib;
10+
11+use Zipkin\Propagation\Getter;
12+
13+// This class implements the Zipkin getter interface to extract the zipkin
14+// headers out of the $_SERVER variable.
15+// Due to how the api is written, $_SERVER gets passed in as $carrier rather
16+// than get accessed here directly.
17+final class TracingRequestHeaders implements Getter
18+{
19+    public function get($carrier, string $key): ?string
20+    {
21+        $key = strtoupper($key);
22+        $key = str_replace('-', '_', $key);
23+
24+        return $carrier['HTTP_' . $key] ?? null;
25+    }
26+}
27diff --git a/src/NW/Lib/YelpTracingUtils.php b/src/NW/Lib/YelpTracingUtils.php
28index a94d8bb91..5ed17fd66 100644
29--- a/src/NW/Lib/YelpTracingUtils.php
30+++ b/src/NW/Lib/YelpTracingUtils.php
31@@ -5,6 +5,7 @@ namespace NW\Lib;
32 use NW\Container;
33 use NW\Lib\Paasta\PaastaEnvironment;
34 use Zipkin\Kind;
35+use Zipkin\Propagation\TraceContext;
36 use Zipkin\Tags;
37 use Zipkin\Tracing;
38 
39@@ -41,7 +42,20 @@ class YelpTracingUtils
40     public function serverSpan($endpoint)
41     {
42         $tracer = $this->tracing->getTracer();
43-        $span = $tracer->newTrace();
44+        $extractor = $this->tracing->getPropagation()->getExtractor(new TracingRequestHeaders);
45+
46+        // Try to extract the zipkin context from the incoming headers. If this works, extractor
47+        // returns a TraceContext object that we can use to continue the trace.
48+        // If the headers weren't set, it returns a DefaultSamplingFlags object in which case we
49+        // start a new trace. Afaict the only way to differentiate between the 2 is by checking
50+        // the return type and we need to do so because there's no zipkin-php API that correctly
51+        // handles passing either in so we need to write the if/else ourselves.
52+        $extractedContext = $extractor($_SERVER);
53+        if ($extractedContext instanceof TraceContext) {
54+            $span = $tracer->joinSpan($extractedContext);
55+        } else {
56+            $span = $tracer->newTrace();
57+        }
58         $span->setName($_SERVER['REQUEST_METHOD'] . ' ' . $endpoint);
59         $span->setKind(Kind\SERVER);
60 
61diff --git a/tests/Integration/Src/NW/Lib/TracingTest.php b/tests/Integration/Src/NW/Lib/TracingTest.php
62index 59f13e40d..2c1178c71 100644
63--- a/tests/Integration/Src/NW/Lib/TracingTest.php
64+++ b/tests/Integration/Src/NW/Lib/TracingTest.php
65@@ -4,6 +4,7 @@ namespace NWTests\Integration\Src\NW\Lib\HTTP\Client;
66 
67 use GuzzleHttp\Client as GuzzleClient;
68 use NWTests\Helpers\TestCases\NWTestCase;
69+use function Zipkin\Propagation\Id\generateNextId;
70 
71 class TracingTest extends NWTestCase
72 {
73@@ -21,4 +22,20 @@ class TracingTest extends NWTestCase
74         $this->assertEquals(200, $nwResponse->getStatusCode());
75         $this->assertTrue($nwResponse->hasHeader('X-Zipkin-Id'));
76     }
77+
78+    public function testWeReuseIncomingTraceId()
79+    {
80+        $client = new GuzzleClient();
81+        $traceId = generateNextId();
82+        $nwResponse = $client->get('http://web:80/ready.php', [
83+            'headers' => [
84+                'X-B3-TraceId' => $traceId,
85+                'X-B3-SpanId' => generateNextId(),
86+                'X-B3-ParentSpanId' => generateNextId(),
87+                'X-B3-Sampled' => 0
88+            ]
89+        ]);
90+        $this->assertEquals(200, $nwResponse->getStatusCode());
91+        $this->assertEquals($traceId, $nwResponse->getHeader('X-Zipkin-Id')[0]);
92+    }
93 }
94diff --git a/tests/Unit/Src/NW/Lib/YelpTracingUtilsTest.php b/tests/Unit/Src/NW/Lib/YelpTracingUtilsTest.php
95index 75b0935b1..43700759a 100644
96--- a/tests/Unit/Src/NW/Lib/YelpTracingUtilsTest.php
97+++ b/tests/Unit/Src/NW/Lib/YelpTracingUtilsTest.php
98@@ -9,6 +9,7 @@ use NWTests\Helpers\TestCases\NWTestCase;
99 use Zipkin\Reporters\InMemory as InMemoryReporter;
100 use Zipkin\Samplers\BinarySampler;
101 use Zipkin\TracingBuilder;
102+use function Zipkin\Propagation\Id\generateNextId;
103 
104 class YelpTracingUtilsTest extends NWTestCase
105 {
106@@ -45,21 +46,21 @@ class YelpTracingUtilsTest extends NWTestCase
107             $tracing->getTracer()->flush();
108             return array_map(function ($span) {return $span->toArray();}, $reporter->flush());
109         };
110-        return [$tracing, $flusher];
111-    }
112 
113-    /**
114-     *  @test
115-     **/
116-    public function server_span()
117-    {
118-        list($tracing, $flusher) = $this->createTestTracer();
119         $paastaEnv = Mockery::mock(PaastaEnvironment::class);
120         $paastaEnv->shouldReceive('getEcosystem')->andReturn('devc');
121         $paastaEnv->shouldReceive('getRegion')->andReturn('uswest1-devc');
122         $paastaEnv->shouldReceive('getHabitat')->andReturn('uswest1adevc');
123         $ytu = new YelpTracingUtils($paastaEnv, $tracing);
124+        return [$ytu, $flusher];
125+    }
126 
127+    /**
128+     *  @test
129+     **/
130+    public function server_span()
131+    {
132+        list($ytu, $flusher) = $this->createTestTracer();
133         $ytu->serverSpan('/consumerAPI/public/getCurrentWait');
134 
135         $spans = $flusher();
136@@ -79,15 +80,48 @@ class YelpTracingUtilsTest extends NWTestCase
137     /**
138      *  @test
139      **/
140-    public function client_span()
141+    public function server_span_uses_incoming_headers()
142     {
143-        list($tracing, $flusher) = $this->createTestTracer();
144-        $paastaEnv = Mockery::mock(PaastaEnvironment::class);
145-        $paastaEnv->shouldReceive('getEcosystem')->andReturn('devc');
146-        $paastaEnv->shouldReceive('getRegion')->andReturn('uswest1-devc');
147-        $paastaEnv->shouldReceive('getHabitat')->andReturn('uswest1adevc');
148-        $ytu = new YelpTracingUtils($paastaEnv, $tracing);
149+        $_SERVER['HTTP_X_B3_TRACEID'] = generateNextId();
150+        $_SERVER['HTTP_X_B3_SPANID'] = generateNextId();
151+        $_SERVER['HTTP_X_B3_PARENTSPANID'] = generateNextId();
152+        $_SERVER['HTTP_X_B3_SAMPLED'] = '1';
153 
154+        list($ytu, $flusher) = $this->createTestTracer();
155+        $ytu->serverSpan('/consumerAPI/public/getCurrentWait');
156+
157+        $spans = $flusher();
158+        $this->assertCount(1, $spans);
159+        $serverSpan = $spans[0];
160+
161+        $this->assertEquals($_SERVER['HTTP_X_B3_TRACEID'], $serverSpan['traceId']);
162+        $this->assertEquals($_SERVER['HTTP_X_B3_PARENTSPANID'], $serverSpan['parentId']);
163+        $this->assertEquals($_SERVER['HTTP_X_B3_SPANID'], $serverSpan['id']);
164+    }
165+
166+    /**
167+     *  @test
168+     **/
169+    public function server_span_respects_sampling()
170+    {
171+        $_SERVER['HTTP_X_B3_TRACEID'] = generateNextId();
172+        $_SERVER['HTTP_X_B3_SPANID'] = generateNextId();
173+        $_SERVER['HTTP_X_B3_PARENTSPANID'] = generateNextId();
174+        $_SERVER['HTTP_X_B3_SAMPLED'] = '0';
175+
176+        list($ytu, $flusher) = $this->createTestTracer();
177+        $ytu->serverSpan('/consumerAPI/public/getCurrentWait');
178+
179+        $spans = $flusher();
180+        $this->assertCount(0, $spans);
181+    }
182+
183+    /**
184+     *  @test
185+     **/
186+    public function client_span()
187+    {
188+        list($ytu, $flusher) = $this->createTestTracer();
189         $ytu->clientSpan('SELECT * from users;');
190 
191         $spans = $flusher();