Reset distributed context on request init (#2340)

ext/ddtrace.c

@@ -931,6 +931,8 @@ static void dd_rinit_once(void) {
 static pthread_once_t dd_rinit_once_control = PTHREAD_ONCE_INIT;
 
 static void dd_initialize_request(void) {
+    DDTRACE_G(distributed_trace_id) = (ddtrace_trace_id){0};
+    DDTRACE_G(distributed_parent_trace_id) = 0;
     DDTRACE_G(additional_global_tags) = zend_new_array(0);
     DDTRACE_G(default_priority_sampling) = DDTRACE_PRIORITY_SAMPLING_UNKNOWN;
     DDTRACE_G(propagated_priority_sampling) = DDTRACE_PRIORITY_SAMPLING_UNSET;

tests/ext/distributed_tracing/distributed_trace_reset_context_on_init.phpt

@@ -0,0 +1,63 @@
+--TEST--
+Reset distributed tracing context on request init
+--ENV--
+DD_TRACE_GENERATE_ROOT_SPAN=0
+--FILE--
+<?php
+// Initial state
+var_dump(DDTrace\current_context());
+
+// Update distributed tracing context 
+DDTrace\set_distributed_tracing_context("123", "321");
+var_dump(DDTrace\current_context());
+
+// Reinitialize request to clear the distributed context state
+ini_set("datadog.trace.enabled", 0);
+ini_set("datadog.trace.enabled", 1);
+
+// Reinitialized state
+var_dump(DDTrace\current_context());
+
+?>
+--EXPECT--
+array(5) {
+  ["trace_id"]=>
+  string(1) "0"
+  ["span_id"]=>
+  string(1) "0"
+  ["version"]=>
+  NULL
+  ["env"]=>
+  NULL
+  ["distributed_tracing_propagated_tags"]=>
+  array(0) {
+  }
+}
+array(6) {
+  ["trace_id"]=>
+  string(3) "123"
+  ["span_id"]=>
+  string(3) "321"
+  ["version"]=>
+  NULL
+  ["env"]=>
+  NULL
+  ["distributed_tracing_parent_id"]=>
+  string(3) "321"
+  ["distributed_tracing_propagated_tags"]=>
+  array(0) {
+  }
+}
+array(5) {
+  ["trace_id"]=>
+  string(1) "0"
+  ["span_id"]=>
+  string(1) "0"
+  ["version"]=>
+  NULL
+  ["env"]=>
+  NULL
+  ["distributed_tracing_propagated_tags"]=>
+  array(0) {
+  }
+}