Fix rare corruption in StaticMeta::head_ list around fork

Summary:
In a rare case, the current thread's would be inserted in the StaticMeta linked
list twice, causing the list to be corrupted, leading to code spinning forever.

After a fork, in the child, only the current thread survives, so all other threads
must be forcefully removed from StaticMeta. We do that by clearing the list
and re-adding the current thread, but we didn't check whether the current thread
was already there. It is possible for the current thread to not be in the list
if it never used any ThreadLocalPtr objects with the same tag.

Now, when the thread in the child tries to use a ThreadLocalPtr with the same
tag, it adds itself to the list (##if (prevCapacity == 0)
meta.push_back(threadEntry)##), but it had already been added by the post-fork
handler, boom.

Fix by adding the necessary check in onForkChild.

Test Plan: @durham's test case, also added new test for this

Reviewed By: delong.j@fb.com

FB internal diff: D1158672

@override-unit-failures

folly/detail/ThreadLocalDetail.h

@@ -213,7 +213,11 @@ struct StaticMeta {
   static void onForkChild(void) {
     // only the current thread survives
     inst_->head_.next = inst_->head_.prev = &inst_->head_;
-    inst_->push_back(getThreadEntry());
+    ThreadEntry* threadEntry = getThreadEntry();
+    // If this thread was in the list before the fork, add it back.
+    if (threadEntry->elementsCapacity != 0) {
+      inst_->push_back(threadEntry);
+    }
     inst_->lock_.unlock();
   }
 

folly/test/ThreadLocalTest.cpp

@@ -476,6 +476,35 @@ TEST(ThreadLocal, Fork) {
   EXPECT_EQ(1, totalValue());
 }
 
+struct HoldsOneTag2 {};
+
+TEST(ThreadLocal, Fork2) {
+  // A thread-local tag that was used in the parent from a *different* thread
+  // (but not the forking thread) would cause the child to hang in a
+  // ThreadLocalPtr's object destructor. Yeah.
+  ThreadLocal<HoldsOne, HoldsOneTag2> p;
+  {
+    // use tag in different thread
+    std::thread t([&p] { p.get(); });
+    t.join();
+  }
+  pid_t pid = fork();
+  if (pid == 0) {
+    {
+      ThreadLocal<HoldsOne, HoldsOneTag2> q;
+      q.get();
+    }
+    _exit(0);
+  } else if (pid > 0) {
+    int status;
+    EXPECT_EQ(pid, waitpid(pid, &status, 0));
+    EXPECT_TRUE(WIFEXITED(status));
+    EXPECT_EQ(0, WEXITSTATUS(status));
+  } else {
+    EXPECT_TRUE(false) << "fork failed";
+  }
+}
+
 // Simple reference implementation using pthread_get_specific
 template<typename T>
 class PThreadGetSpecific {