[core] Only require trace id for distributed tracing headers (#708)

* [core] Only require trace id for distributed tracing headers

* Ignore parent_id if origin is 'synthetics'

* Update lib/ddtrace/propagation/distributed_headers.rb

* missing expected value

* ignore method length of test

lib/ddtrace/propagation/distributed_headers.rb

@@ -11,6 +11,10 @@ def initialize(env)
     end
 
     def valid?
+      # Synthetics sends us `X-Datadog-Parent-Id: 0` which normally we would want
+      # to filter out, but is ok in this context since there is no parent from Synthetics
+      return true if origin == 'synthetics' && trace_id
+
       # Sampling priority and origin are optional.
       trace_id && parent_id
     end

test/propagation/distributed_headers_test.rb

@@ -4,7 +4,7 @@
 require 'ddtrace/propagation/distributed_headers'
 
 class DistributedHeadersTest < Minitest::Test
-  def test_valid_without_sampling_priority
+  def test_valid_without_sampling_priority # rubocop:disable Metrics/MethodLength
     test_cases = {
       { 'HTTP_X_DATADOG_TRACE_ID' => '123',
         'HTTP_X_DATADOG_PARENT_ID' => '456' } => true,
@@ -20,9 +20,6 @@ def test_valid_without_sampling_priority
       { 'HTTP_X_DATADOG_TRACE_ID' => 'a',
         'HTTP_X_DATADOG_PARENT_ID' => '456',
         'HTTP_X_DATADOG_SAMPLING_PRIORITY' => '0' } => false,
-      { 'HTTP_X_DATADOG_TRACE_ID' => '123',
-        'HTTP_X_DATADOG_PARENT_ID' => 'b',
-        'HTTP_X_DATADOG_SAMPLING_PRIORITY' => '0' } => false,
       { 'HTTP_X_DATADOG_TRACE_ID' => '123',
         'HTTP_X_DATADOG_PARENT_ID' => '456',
         'HTTP_X_DATADOG_SAMPLING_PRIORITY' => 'ooops' } =>  true, # corner case, 0 is valid for a sampling priority
@@ -31,7 +28,17 @@ def test_valid_without_sampling_priority
       { 'HTTP_X_DATADOG_TRACE_TYPO' => '123',
         'HTTP_X_DATADOG_PARENT_ID' => '456' } => false,
       { 'HTTP_X_DATADOG_TRACE_ID' => '123',
-        'HTTP_X_DATADOG_PARENT_TYPO' => '456' } => false
+        'HTTP_X_DATADOG_PARENT_ID' => 'b',
+        'HTTP_X_DATADOG_SAMPLING_PRIORITY' => '0' } => false,
+      { 'HTTP_X_DATADOG_TRACE_ID' => '123',
+        'HTTP_X_DATADOG_PARENT_TYPO' => '456' } => false,
+      # Parent id is not required when origin is synthetics
+      { 'HTTP_X_DATADOG_TRACE_ID' => '123',
+        'HTTP_X_DATADOG_PARENT_ID' => '0',
+        'HTTP_X_DATADOG_ORIGIN' => 'not-synthetics' } => false,
+      { 'HTTP_X_DATADOG_TRACE_ID' => '123',
+        'HTTP_X_DATADOG_PARENT_ID' => '0',
+        'HTTP_X_DATADOG_ORIGIN' => 'synthetics' } => true
     }
 
     test_cases.each do |env, expected|
@@ -70,6 +77,8 @@ def test_parent_id
     test_cases = {
       { 'HTTP_X_DATADOG_PARENT_ID' => '123' } => 123,
       { 'HTTP_X_DATADOG_PARENT_ID' => '0' } => nil,
+      { 'HTTP_X_DATADOG_PARENT_ID' => 'a' } => nil,
+      { 'HTTP_X_DATADOG_PARENT_ID' => '' } => nil,
       { 'HTTP_X_DATADOG_PARENT_ID' => '-1' } => 18446744073709551615,
       { 'HTTP_X_DATADOG_PARENT_ID' => '-8809075535603237910' } => 9637668538106313706,
       { 'HTTP_X_DATADOG_PARENT_ID' => 'ooops' } => nil,