adb: make fdevent_run_on_main_thread's fd nonblocking.

If we get a ton of fdevent_run_on_main_thread calls while running one
of the handlers, the socket might become full, which will result in a
deadlock in fdevent_run_on_main_thread when a write to the fd blocks
with the mutex taken. Resolve this by making the fd nonblocking, which
is safe because we always write after appending to the list, and read
before emptying the list, which guarantees that if the byte we write is
consumed, the std::function we appended will be run.

Bug: http://b/74616284
Test: adb_test
Test: python test_device.py
Change-Id: I29319bda2ad7b5a5cdcd91d1d0ddf39f7ab7d115
(cherry picked from commit 1222abc75b7854979e6a9f982004985179bcbede)

2 files changed, 19 insertions, 4 deletions

diff --git a/adb/fdevent.cpp b/adb/fdevent.cpp
index d2855619b..45d86bc1a 100644
--- a/adb/fdevent.cpp
+++ b/adb/fdevent.cpp
@@ -400,6 +400,10 @@ static void fdevent_run_setup() {
             PLOG(FATAL) << "failed to create run queue notify socketpair";
         }
 
+        if (!set_file_block_mode(s[0], false) || !set_file_block_mode(s[1], false)) {
+            PLOG(FATAL) << "failed to make run queue notify socket nonblocking";
+        }
+
         run_queue_notify_fd.reset(s[0]);
         fdevent* fde = fdevent_create(s[1], fdevent_run_func, nullptr);
         CHECK(fde != nullptr);
@@ -416,7 +420,12 @@ void fdevent_run_on_main_thread(std::function<void()> fn) {
     // run_queue_notify_fd could still be -1 if we're called before fdevent has finished setting up.
     // In that case, rely on the setup code to flush the queue without a notification being needed.
     if (run_queue_notify_fd != -1) {
-        if (adb_write(run_queue_notify_fd.get(), "", 1) != 1) {
+        int rc = adb_write(run_queue_notify_fd.get(), "", 1);
+
+        // It's possible that we get EAGAIN here, if lots of notifications came in while handling.
+        if (rc == 0) {
+            PLOG(FATAL) << "run queue notify fd was closed?";
+        } else if (rc == -1 && errno != EAGAIN) {
             PLOG(FATAL) << "failed to write to run queue notify fd";
         }
     }
diff --git a/adb/fdevent_test.cpp b/adb/fdevent_test.cpp
index dadae5ab7..95dc4c248 100644
--- a/adb/fdevent_test.cpp
+++ b/adb/fdevent_test.cpp
@@ -180,7 +180,13 @@ TEST_F(FdeventTest, run_on_main_thread) {
     PrepareThread();
     std::thread thread(fdevent_loop);
 
-    for (int i = 0; i < 100; ++i) {
+    // Block the main thread for a long time while we queue our callbacks.
+    fdevent_run_on_main_thread([]() {
+        check_main_thread();
+        std::this_thread::sleep_for(std::chrono::seconds(1));
+    });
+
+    for (int i = 0; i < 1000000; ++i) {
         fdevent_run_on_main_thread([i, &vec]() {
             check_main_thread();
             vec.push_back(i);
@@ -189,8 +195,8 @@ TEST_F(FdeventTest, run_on_main_thread) {
 
     TerminateThread(thread);
 
-    ASSERT_EQ(100u, vec.size());
-    for (int i = 0; i < 100; ++i) {
+    ASSERT_EQ(1000000u, vec.size());
+    for (int i = 0; i < 1000000; ++i) {
         ASSERT_EQ(i, vec[i]);
     }
 }