[clangd] Use pthread instead of thread_local to support more runtimes.

[sammccall]

Summary

thread_local has nice syntax and semantics, but requires __cxa_thread_atexit,
and some not-ancient runtime libraries don't provide it.
The clang-x86_64-linux-selfhost-modules buildbot is one example :-)

It works on windows, and the other platforms clang-tools-extra supports should
all have the relevant pthread API. So we just use that if it's available,
falling back to thread_local (so if a platform has neither, we'll fail to link).

The fallback should really be the other way, that would require cmake changes.

[ilya-biryukov]

LGTM.

[jyknight]

Is there some way to figure out what's going on in clang-x86_64-linux-selfhost-modules?

I believe there should be no linux builders which are missing this function -- it was added to libgcc in 4.8, and we don't support older versions, so I think missing this function on a linux build must be a misconfiguration of some sort.

[FireBurn]

This breaks compilation for me on Gentoo's clang-9999.ebuild

The following patch fixes things for me

commit a4c071b16bbbc84bbb960000d90b51bed8a12127 (HEAD -> master)
Author: Mike Lothian <[e-mail]>
Date:   Tue Feb 6 09:55:41 2018 +0000

    [clangd] Fix include to use Clang's config.h

    This fixes:

    [clangd] Use pthread instead of thread_local to support more runtimes.

diff --git a/clangd/Context.cpp b/clangd/Context.cpp
index 23006778..aed832f4 100644
--- a/clangd/Context.cpp
+++ b/clangd/Context.cpp
@@ -8,7 +8,7 @@
 //===---------------------------------------------------------------------===//

 #include "Context.h"
-#include "llvm/Config/config.h"
+#include "clang/Config/config.h"
 #include <cassert>

 // The thread-local Context is scoped in a function to avoid init-order issues.

clangd/Context.cpp

 //===--- Context.cpp -----------------------------------------*- C++-*-===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===---------------------------------------------------------------------===//
 
 #include "Context.h"
+#include "llvm/Config/config.h"
 #include <cassert>
 
+// The thread-local Context is scoped in a function to avoid init-order issues.
+// It's created by currentContext() when first needed.
+
+#ifdef HAVE_PTHREAD_GETSPECIFIC
+// We'd love to use thread_local everywhere.
+// It requires support from the runtime: __cxa_thread_atexit.
+// Rather than detect this, we use the pthread API where available.
+#include <pthread.h>
+static clang::clangd::Context &currentContext() {
+  using clang::clangd::Context;
+  static pthread_key_t CtxKey;
+
+  // Once (across threads), set up pthread TLS for Context, and its destructor.
+  static int Dummy = [] { // Create key only once, for all threads.
+    if (auto Err = pthread_key_create(&CtxKey, /*destructor=*/+[](void *Ctx) {
+          delete reinterpret_cast<Context *>(Ctx);
+        }))
+      llvm_unreachable(strerror(Err));
+    return 0;
+  }();
+  (void)Dummy;
+
+  // Now grab the current context from TLS, and create it if it doesn't exist.
+  void *Ctx = pthread_getspecific(CtxKey);
+  if (!Ctx) {
+    Ctx = new Context();
+    if (auto Err = pthread_setspecific(CtxKey, Ctx))
+      llvm_unreachable(strerror(Err));
+  }
+  return *reinterpret_cast<Context *>(Ctx);
+}
+#else
+// Only supported platform without pthread is windows, and thread_local works.
+static clang::clangd::Context &currentContext() {
+  static thread_local auto C = clang::clangd::Context::empty();
+  return C;
+}
+#endif
+
 namespace clang {
 namespace clangd {
 
 Context Context::empty() { return Context(/*Data=*/nullptr); }
 
 Context::Context(std::shared_ptr<const Data> DataPtr)
     : DataPtr(std::move(DataPtr)) {}
 
 Context Context::clone() const { return Context(DataPtr); }
 
-// The thread-local Context is scoped in a function to avoid
-// initialization-order issues. It's created when first needed.
-static Context &currentContext() {
-  static thread_local Context C = Context::empty();
-  return C;
-}
-
 const Context &Context::current() { return currentContext(); }
 
 Context Context::swapCurrent(Context Replacement) {
   std::swap(Replacement, currentContext());
   return Replacement;
 }
 
 } // namespace clangd
 } // namespace clang