Support base::LaunchOptions.current_directory when using posix_spawn on Mac.

This uses the undocumented pthread_chdir_np() to change the launching
thread's PWD for the duration of the posix_spawn() syscall. There is no
other way to change directories when using posix_spawn, other than an
exec trampoline.

Bug: 179923
Change-Id: Ic6ab10cd1cb5be2f008b434b3546c05f080ef9b1
Reviewed-on: https://chromium-review.googlesource.com/c/1311258Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604799}

base/process/launch_mac.cc

@@ -8,12 +8,23 @@
 #include <mach/mach.h>
 #include <spawn.h>
 #include <string.h>
+#include <sys/syscall.h>
 #include <sys/wait.h>
 
 #include "base/logging.h"
+#include "base/mac/availability.h"
 #include "base/posix/eintr_wrapper.h"
 #include "base/threading/scoped_blocking_call.h"
 
+// Changes the current thread's directory to a path or directory file
+// descriptor. libpthread only exposes a syscall wrapper starting in
+// macOS 10.12, but the system call dates back to macOS 10.5. On older OSes,
+// the syscall is issued directly.
+extern "C" {
+int pthread_chdir_np(const char*) API_AVAILABLE(macosx(10.12));
+int pthread_fchdir_np(int fd) API_AVAILABLE(macosx(10.12));
+}
+
 namespace base {
 
 namespace {
@@ -71,6 +82,24 @@ class PosixSpawnFileActions {
   DISALLOW_COPY_AND_ASSIGN(PosixSpawnFileActions);
 };
 
+int ChangeCurrentThreadDirectory(const char* path) {
+  if (__builtin_available(macOS 10.12, *)) {
+    return pthread_chdir_np(path);
+  } else {
+    return syscall(SYS___pthread_chdir, path);
+  }
+}
+
+// The recommended way to unset a per-thread cwd is to set a new value to an
+// invalid file descriptor, per libpthread-218.1.3/private/private.h.
+int ResetCurrentThreadDirectory() {
+  if (__builtin_available(macOS 10.12, *)) {
+    return pthread_fchdir_np(-1);
+  } else {
+    return syscall(SYS___pthread_fchdir, -1);
+  }
+}
+
 }  // namespace
 
 void RestoreDefaultExceptionHandler() {
@@ -92,9 +121,6 @@ void RestoreDefaultExceptionHandler() {
 
 Process LaunchProcessPosixSpawn(const std::vector<std::string>& argv,
                                 const LaunchOptions& options) {
-  DCHECK(options.current_directory.empty())
-      << "LaunchProcessPosixSpawn does not support current_directory";
-
   PosixSpawnAttr attr;
 
   short flags = POSIX_SPAWN_CLOEXEC_DEFAULT;
@@ -153,11 +179,27 @@ Process LaunchProcessPosixSpawn(const std::vector<std::string>& argv,
                                     ? options.real_path.value().c_str()
                                     : argv_cstr[0];
 
+  // If the new program has specified its PWD, change the thread-specific
+  // working directory. The new process will inherit it during posix_spawn().
+  if (!options.current_directory.empty()) {
+    int rv =
+        ChangeCurrentThreadDirectory(options.current_directory.value().c_str());
+    if (rv != 0) {
+      DPLOG(ERROR) << "pthread_chdir_np";
+      return Process();
+    }
+  }
+
   // Use posix_spawnp as some callers expect to have PATH consulted.
   pid_t pid;
   int rv = posix_spawnp(&pid, executable_path, file_actions.get(), attr.get(),
                         &argv_cstr[0], new_environ);
 
+  // Restore the thread's working directory if it was changed.
+  if (!options.current_directory.empty()) {
+    ResetCurrentThreadDirectory();
+  }
+
   if (rv != 0) {
     DLOG(ERROR) << "posix_spawnp(" << executable_path << "): -" << rv << " "
                 << strerror(rv);

base/process/launch_posix.cc

@@ -303,9 +303,8 @@ Process LaunchProcess(const std::vector<std::string>& argv,
                       const LaunchOptions& options) {
   TRACE_EVENT0("base", "LaunchProcess");
 #if defined(OS_MACOSX)
+  // TODO(rsesek): Remove this feature. https://crbug.com/179923.
   if (FeatureList::IsEnabled(kMacLaunchProcessPosixSpawn)) {
-    // TODO(rsesek): Do this unconditionally. https://crbug.com/179923.
-    if (options.current_directory.empty())
     return LaunchProcessPosixSpawn(argv, options);
   }
 #endif

base/process/process_util_unittest.cc

@@ -9,6 +9,7 @@
 
 #include <limits>
 
+#include "base/bind.h"
 #include "base/command_line.h"
 #include "base/debug/alias.h"
 #include "base/debug/stack_trace.h"
@@ -32,6 +33,7 @@
 #include "base/test/scoped_task_environment.h"
 #include "base/test/test_timeouts.h"
 #include "base/threading/platform_thread.h"
+#include "base/threading/simple_thread.h"
 #include "base/threading/thread.h"
 #include "build/build_config.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ -478,19 +480,78 @@ TEST_F(ProcessUtilTest, HandlesToTransferClosedOnBadPathToMapFailure) {
 // On Android SpawnProcess() doesn't use LaunchProcess() and doesn't support
 // LaunchOptions::current_directory.
 #if !defined(OS_ANDROID)
-MULTIPROCESS_TEST_MAIN(CheckCwdProcess) {
-  FilePath expected;
-  CHECK(GetTempDir(&expected));
-  expected = MakeAbsoluteFilePath(expected);
-  CHECK(!expected.empty());
-
+static void CheckCwdIsExpected(FilePath expected) {
   FilePath actual;
   CHECK(GetCurrentDirectory(&actual));
   actual = MakeAbsoluteFilePath(actual);
   CHECK(!actual.empty());
 
-  CHECK(expected == actual) << "Expected: " << expected.value()
-                            << "  Actual: " << actual.value();
+  CHECK_EQ(expected, actual);
+}
+
+// N.B. This test does extra work to check the cwd on multiple threads, because
+// on macOS a per-thread cwd is set when using LaunchProcess().
+MULTIPROCESS_TEST_MAIN(CheckCwdProcess) {
+  // Get the expected cwd.
+  FilePath temp_dir;
+  CHECK(GetTempDir(&temp_dir));
+  temp_dir = MakeAbsoluteFilePath(temp_dir);
+  CHECK(!temp_dir.empty());
+
+  // Test that the main thread has the right cwd.
+  CheckCwdIsExpected(temp_dir);
+
+  // Create a non-main thread.
+  Thread thread("CheckCwdThread");
+  thread.Start();
+  auto task_runner = thread.task_runner();
+
+  // A synchronization primitive used to wait for work done on the non-main
+  // thread.
+  WaitableEvent event(WaitableEvent::ResetPolicy::AUTOMATIC);
+  RepeatingClosure signal_event =
+      BindRepeating(&WaitableEvent::Signal, Unretained(&event));
+
+  // Test that a non-main thread has the right cwd.
+  task_runner->PostTask(FROM_HERE, Bind(&CheckCwdIsExpected, temp_dir));
+  task_runner->PostTask(FROM_HERE, signal_event);
+
+  event.Wait();
+
+  // Get a new cwd for the process.
+  FilePath home_dir;
+  CHECK(PathService::Get(DIR_HOME, &home_dir));
+
+  // Change the cwd on the secondary thread. IgnoreResult is used when setting
+  // because it is checked immediately after.
+  task_runner->PostTask(FROM_HERE,
+                        Bind(IgnoreResult(&SetCurrentDirectory), home_dir));
+  task_runner->PostTask(FROM_HERE, Bind(&CheckCwdIsExpected, home_dir));
+  task_runner->PostTask(FROM_HERE, signal_event);
+
+  event.Wait();
+
+  // Make sure the main thread sees the cwd from the secondary thread.
+  CheckCwdIsExpected(home_dir);
+
+  // Change the directory back on the main thread.
+  CHECK(SetCurrentDirectory(temp_dir));
+  CheckCwdIsExpected(temp_dir);
+
+  // Ensure that the secondary thread sees the new cwd too.
+  task_runner->PostTask(FROM_HERE, Bind(&CheckCwdIsExpected, temp_dir));
+  task_runner->PostTask(FROM_HERE, signal_event);
+
+  event.Wait();
+
+  // Change the cwd on the secondary thread one more time and join the thread.
+  task_runner->PostTask(FROM_HERE,
+                        Bind(IgnoreResult(&SetCurrentDirectory), home_dir));
+  thread.Stop();
+
+  // Make sure that the main thread picked up the new cwd.
+  CheckCwdIsExpected(home_dir);
+
   return kSuccess;
 }