Fix ID generation after fork.

3rd_party/randutils/include/lightstep/randutils.h

@@ -540,13 +540,13 @@ class auto_seeded : public SeedSeq {
         return *this;
     }
 
-    auto_seeded(default_seeds seeds)
+    auto_seeded(default_seeds seeds) noexcept
         : SeedSeq(seeds.begin(), seeds.end())
     {
         // Nothing else to do
     }
 
-    auto_seeded()
+    auto_seeded() noexcept
         : auto_seeded(local_entropy())
     {
         // Nothing else to do
@@ -650,7 +650,7 @@ class random_generator {
 public:
     template <typename Seeding = default_seed_type,
               typename... Params>
-    random_generator(Seeding&& seeding = default_seed_type{})
+    random_generator(Seeding&& seeding = default_seed_type{}) noexcept
         : engine_{seed_seq_cast(std::forward<Seeding>(seeding))}
     {
         // Nothing (else) to do
@@ -662,7 +662,7 @@ class random_generator {
     //     https://llvm.org/bugs/show_bug.cgi?id=23029
     template <typename Seeding,
               typename... Params>
-    random_generator(Seeding&& seeding, Params&&... params)
+    random_generator(Seeding&& seeding, Params&&... params) noexcept
         : engine_{seed_seq_cast(std::forward<Seeding>(seeding)),
                   std::forward<Params>(params)...}
     {
@@ -671,7 +671,7 @@ class random_generator {
 
     template <typename Seeding = default_seed_type,
               typename... Params>
-    void seed(Seeding&& seeding = default_seed_type{})
+    void seed(Seeding&& seeding = default_seed_type{}) noexcept
     {
         engine_.seed(seed_seq_cast(seeding));
     }

src/utility.cpp

@@ -2,13 +2,16 @@
 #include <lightstep/randutils.h>
 #include <opentracing/string_view.h>
 #include <opentracing/value.h>
+#include "lightstep-tracer-common/collector.pb.h"
+
 #include <unistd.h>
 #include <cmath>
 #include <iomanip>
 #include <random>
 #include <sstream>
 #include <stdexcept>
-#include "lightstep-tracer-common/collector.pb.h"
+
+#include <pthread.h>
 
 namespace lightstep {
 //------------------------------------------------------------------------------
@@ -25,12 +28,37 @@ google::protobuf::Timestamp ToTimestamp(
   return ts;
 }
 
+//------------------------------------------------------------------------------
+// TlsRandomNumberGenerator
+//------------------------------------------------------------------------------
+// Wraps a thread_local random number generator, but adds a fork handler so that
+// the generator will be correctly seeded after forking.
+//
+// See https://stackoverflow.com/q/51882689/4447365 and
+//     https://github.com/opentracing-contrib/nginx-opentracing/issues/52
+namespace {
+class TlsRandomNumberGenerator {
+ public:
+  TlsRandomNumberGenerator() { pthread_atfork(nullptr, nullptr, OnFork); }
+
+  static std::mt19937_64& engine() { return base_generator_.engine(); }
+
+ private:
+  static thread_local lightstep::randutils::mt19937_64_rng base_generator_;
+
+  static void OnFork() { base_generator_.seed(); }
+};
+
+thread_local lightstep::randutils::mt19937_64_rng
+    TlsRandomNumberGenerator::base_generator_{};
+}  // namespace
+
 //------------------------------------------------------------------------------
 // GenerateId
 //------------------------------------------------------------------------------
 uint64_t GenerateId() {
-  static thread_local lightstep::randutils::mt19937_64_rng rand_source{};
-  return rand_source.engine()();
+  static TlsRandomNumberGenerator random_number_generator;
+  return TlsRandomNumberGenerator::engine()();
 }
 
 //------------------------------------------------------------------------------

test/CMakeLists.txt

@@ -9,6 +9,7 @@ _lightstep_test(tracer_test tracer_test.cpp
                             utility.cpp)
 _lightstep_test(utility_test utility_test.cpp)
 _lightstep_test(logger_test logger_test.cpp)
+_lightstep_test(fork_id_test fork_id_test.cpp)
 _lightstep_test(propagation_test propagation_test.cpp)
 _lightstep_test(auto_recorder_test auto_recorder_test.cpp
                                    utility.cpp

test/fork_id_test.cpp

@@ -0,0 +1,42 @@
+// Verifies that IDs don't clash after forking the process.
+//
+// See https://github.com/opentracing-contrib/nginx-opentracing/issues/52
+#include "../src/utility.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <iostream>
+using namespace lightstep;
+
+static uint64_t* child_id;
+
+int main() {
+  GenerateId();
+
+  // Set up shared memory to communicate between parent and child processes.
+  //
+  // See https://stackoverflow.com/a/13274800/4447365
+  child_id = static_cast<uint64_t*>(mmap(nullptr, sizeof(*child_id),
+                                         PROT_READ | PROT_WRITE,
+                                         MAP_SHARED | MAP_ANONYMOUS, -1, 0));
+  *child_id = 0;
+  if (fork() == 0) {
+    *child_id = GenerateId();
+    exit(EXIT_SUCCESS);
+  } else {
+    wait(nullptr);
+    auto parent_id = GenerateId();
+    auto child_id_copy = *child_id;
+    munmap(static_cast<void*>(child_id), sizeof(*child_id));
+    if (parent_id == child_id_copy) {
+      std::cerr << "Child and parent ids are the same value " << parent_id
+                << "\n";
+      return -1;
+    }
+  }
+  return 0;
+}