Disable multithreaded linking for Windows LLD.

This only affects ndk-build because CMake can't currently check if it
is using LLD since our toolchain file has finished running by the
time the user can add ldflags.

Not enabling this with standalone toolchains since we can't enable it
for the built in toolchains without recompiling Clang and those are
supposed to match. Will disable in the driver in a follow up if a
workaround is not found.

Test: ./checkbuild.py
Bug: android/ndk#855
Change-Id: I4a24166adce463bea47ca4bf0e97d5375c69f40e

build/core/build-binary.mk

@@ -533,6 +533,15 @@ ifneq ($(filter armeabi%,$(TARGET_ARCH_ABI)),)
     my_ldflags += $(TARGET_$(my_link_arm_mode)_LDFLAGS)
 endif
 
+# https://github.com/android-ndk/ndk/issues/855
+ifeq ($(HOST_OS),windows)
+    ndk_fuse_ld_flags := $(filter -fuse-ld=%,$(my_ldflags))
+    ndk_used_linker := $(lastword $(ndk_fuse_ld_flags))
+    ifeq ($(ndk_used_linker),-fuse-ld=lld)
+        my_ldflags += -Wl,--no-threads
+    endif
+endif
+
 # When LOCAL_SHORT_COMMANDS is defined to 'true' we are going to write the
 # list of all object files and/or static/shared libraries that appear on the
 # command line to a file, then use the @<listfile> syntax to invoke it.

docs/BuildSystemMaintainers.md

@@ -210,7 +210,13 @@ installed to the triple directory and is not triple-prefixed, but rather is only
 installed as `<NDK>/toolchains/llvm/prebuilt/<host-tag>/bin/ld.lld` because the
 one binary supports all ABIs.
 
+
+Warning: Multithreaded linking with LLD may hang on Windows. This feature is on
+by default. To workaround this, `-Wl,--no-threads` must be passed to Clang when
+using LLD on Windows. See [Issue 855].
+
 [Issue 70838247]: https://issuetracker.google.com/70838247
+[Issue 855]: https://github.com/android-ndk/ndk/issues/855
 
 ## Sysroot
 

docs/changelogs/Changelog-r19.md

@@ -61,6 +61,10 @@ Announcements
 r19b
 ----
 
+ * [Issue 855]: ndk-build automatically disables multithreaded linking for LLD
+   on Windows, where it may hang. It is not possible for the NDK to detect this
+   situation for CMake, so CMake users and custom build systems must pass
+   `-Wl,--no-threads` when linking with LLD on Windows.
  * [Issue 849]: Fixed unused command line argument warning when using standalone
    toolchains to compile C code.
  * [Issue 890]: Fixed `CMAKE_FIND_ROOT_PATH`. CMake projects will no longer
@@ -124,6 +128,10 @@ Known Issues
    to not call `dlclose`.
  * [Issue 70838247]: Gold emits broken debug information for AArch64. AArch64
    still uses BFD by default.
+ * [Issue 855]: LLD may hang on Windows when using multithreaded linking.
+   ndk-build will automatically disable multithreaded linking in this situation,
+   but CMake users and custom build systems should pass `-Wl,--no-threads` when
+   using LLD on Windows. The other linkers and operating systems are unaffected.
  * [Issue 884]: Third-party build systems must pass `-fno-addrsig` to Clang for
    compatibility with binutils. ndk-build, CMake, and standalone toolchains
    handle this automatically.
@@ -135,6 +143,7 @@ Known Issues
 
 [Issue 360]: https://github.com/android-ndk/ndk/issues/360
 [Issue 70838247]: https://issuetracker.google.com/70838247
+[Issue 855]: https://github.com/android-ndk/ndk/issues/855
 [Issue 884]: https://github.com/android-ndk/ndk/issues/884
 [Issue 888]: https://github.com/android-ndk/ndk/issues/888
 [use plugin version 3.1 or newer]: https://developer.android.com/studio/releases/gradle-plugin#updating-plugin

tests/build/lld_no_threads/project/jni/Android.mk

@@ -0,0 +1,6 @@
+LOCAL_PATH := $(call my-dir)
+
+include $(CLEAR_VARS)
+LOCAL_MODULE := libfoo
+LOCAL_SRC_FILES := foo.cpp
+include $(BUILD_SHARED_LIBRARY)

tests/build/lld_no_threads/project/jni/foo.cpp

@@ -0,0 +1 @@
+void foo() {}

tests/build/lld_no_threads/test.py

@@ -0,0 +1,67 @@
+#
+# Copyright (C) 2019 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+"""Check that Windows LLD does not use threads with ndk-build.
+
+https://github.com/android-ndk/ndk/issues/855 reports that this sometimes hangs
+on Windows.
+"""
+import os
+import subprocess
+import sys
+
+
+def check_ndk_build(ndk_path, abi, platform, build_flags, use_lld):
+    """Checks --no-threads behavior with ndk-build.
+
+    If use_lld is used, the test will build the ndk-build project with
+    APP_LDFLAGS=-fuse-ld=lld to force the project to use lld. In this case, we
+    expect to see -Wl,--no-threads passed to the linker on Windows.
+    """
+    ndk_build = os.path.join(ndk_path, 'ndk-build')
+    is_win = sys.platform == 'win32'
+    if is_win:
+        ndk_build += '.cmd'
+    if use_lld:
+        build_flags.append('APP_LDFLAGS=-fuse-ld=lld')
+    project_path = 'project'
+    ndk_args = build_flags + [
+        'APP_ABI=' + abi,
+        'APP_PLATFORM=android-{}'.format(platform),
+        'V=1',
+        '-B',
+    ]
+    proc = subprocess.Popen([ndk_build, '-C', project_path] + ndk_args,
+                            stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
+    out, _ = proc.communicate()
+    out = out.decode('utf-8')
+    if proc.returncode != 0:
+        return proc.returncode == 0, out
+
+    out_words = out.split(' ')
+    if is_win and use_lld:
+        result = '-Wl,--no-threads' in out_words
+    else:
+        result = '-Wl,--no-threads' not in out_words
+
+    return result, out
+
+
+def run_test(ndk_path, abi, platform, build_flags):
+    """Checks --no-threads-behavior."""
+    result, out = check_ndk_build(ndk_path, abi, platform, build_flags, False)
+    if not result:
+        return result, out
+    return check_ndk_build(ndk_path, abi, platform, build_flags, True)