Make the back-deployed concurrency dylibs depend only on what's available in older runtimes

include/swift/Runtime/Exclusivity.h

@@ -72,6 +72,13 @@ void swift_dumpTrackedAccesses();
 
 #endif
 
+// When building the concurrency library for back deployment, we rename these
+// symbols unformly so they don't conflict with the real concurrency library.
+#ifdef SWIFT_CONCURRENCY_BACK_DEPLOYMENT
+#  define swift_task_enterThreadLocalContext swift_task_enterThreadLocalContextBackDeploy
+#  define swift_task_exitThreadLocalContext swift_task_exitThreadLocalContextBackDeploy
+#endif
+
 /// Called when a task inits, resumes and returns control to caller synchronous
 /// code to update any exclusivity specific state associated with the task.
 ///

stdlib/public/BackDeployConcurrency/CMakeLists.txt

@@ -42,5 +42,6 @@ set(swift_concurrency_install_component back-deployment)
 set(swift_concurrency_options
   BACK_DEPLOYMENT_LIBRARY 5.5
   DARWIN_INSTALL_NAME_DIR "@rpath")
+set(swift_concurrency_extra_sources "../BackDeployConcurrency/Exclusivity.cpp")
 
 add_subdirectory(../Concurrency stdlib/public/BackDeployConcurrency)

stdlib/public/BackDeployConcurrency/Exclusivity.cpp

@@ -0,0 +1,35 @@
+//===--- Exclusivity.cpp - Exclusivity tracking ---------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// This implements the runtime support for dynamically tracking exclusivity.
+//
+//===----------------------------------------------------------------------===//
+#include <cinttypes>
+
+#include "swift/Runtime/Exclusivity.h"
+#include "../runtime/ExclusivityPrivate.h"
+#include "../runtime/SwiftTLSContext.h"
+
+using namespace swift;
+using namespace swift::runtime;
+
+// Thread-local storage used by the back-deployed concurrency library.
+namespace {
+
+static thread_local SwiftTLSContext TLSContext;
+
+} // anonymous namespace
+
+SwiftTLSContext &SwiftTLSContext::get() { return TLSContext; }
+
+// Bring in the concurrency-specific exclusivity code.
+#include "../runtime/ConcurrencyExclusivity.inc"

stdlib/public/Concurrency/CMakeLists.txt

@@ -10,8 +10,9 @@
 #
 #===----------------------------------------------------------------------===#
 
-set(LLVM_OPTIONAL_SOURCES
-  ${swift_concurrency_objc_sources})
+if(NOT swift_concurrency_extra_sources)
+  set(swift_concurrency_extra_sources)
+endif()
 
 if(NOT swift_concurrency_install_component)
   set(swift_concurrency_install_component stdlib)
@@ -78,7 +79,7 @@ add_swift_target_library(swift_Concurrency ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} I
   AsyncThrowingStream.swift
   AsyncStream.cpp
   Deque.swift
-  ${swift_concurrency_objc_sources}
+  ${swift_concurrency_extra_sources}
 
   SWIFT_MODULE_DEPENDS_LINUX Glibc
   SWIFT_MODULE_DEPENDS_FREEBSD Glibc

stdlib/public/Concurrency/GlobalExecutor.cpp

@@ -239,8 +239,9 @@ static void initializeDispatchEnqueueFunc(dispatch_queue_t queue, void *obj,
                                           dispatch_qos_class_t qos) {
   dispatchEnqueueFuncType func = nullptr;
 
-  // Always fall back to plain dispatch_async_f on Windows for now.
-#if !defined(_WIN32)
+  // Always fall back to plain dispatch_async_f on Windows for now, and
+  // also for back-deployed concurrency.
+#if !defined(_WIN32) && !defined(SWIFT_CONCURRENCY_BACK_DEPLOYMENT)
   if (runtime::environment::concurrencyEnableJobDispatchIntegration())
     func = reinterpret_cast<dispatchEnqueueFuncType>(
         dlsym(RTLD_NEXT, "dispatch_async_swift_job"));