[RFC 4/6] Introduce run_on_main_thread

[Tom Tromey <tom@tromey.com>]

This introduces a way for a callback to be run on the main thread.

2019-03-08  Tom Tromey  <tom@tromey.com>

	* ser-event.h (run_on_main_thread): Declare.
	* ser-event.c (runnable_event, runnables, runnable_mutex): New
	globals.
	(run_events, run_on_main_thread, _initialize_ser_event): New
	functions.
---
 gdb/ChangeLog   |  8 +++++++
 gdb/ser-event.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++++
 gdb/ser-event.h |  6 +++++
 3 files changed, 73 insertions(+)

diff --git a/gdb/ser-event.c b/gdb/ser-event.c
index d3956346246..821b2f169df 100644
--- a/gdb/ser-event.c
+++ b/gdb/ser-event.c
@@ -20,6 +20,8 @@
 #include "ser-event.h"
 #include "serial.h"
 #include "common/filestuff.h"
+#include <mutex>
+#include "event-loop.h"
 
 /* On POSIX hosts, a serial_event is basically an abstraction for the
    classical self-pipe trick.
@@ -217,3 +219,60 @@ serial_event_clear (struct serial_event *event)
   ResetEvent (state->event);
 #endif
 }
+
+\f
+
+/* The serial event used when posting runnables.  */
+
+static struct serial_event *runnable_event;
+
+/* Runnables that have been posted.  */
+
+static std::vector<std::function<void ()>> runnables;
+
+/* Mutex to hold when handling runnable_event or runnables.  */
+
+static std::mutex runnable_mutex;
+
+/* Run all the queued runnables.  */
+
+static void
+run_events (int error, gdb_client_data client_data)
+{
+  std::vector<std::function<void ()>> local;
+
+  /* Hold the lock while changing the globals, but not while running
+     the runnables.  */
+  {
+    std::lock_guard<std::mutex> lock (runnable_mutex);
+
+    /* Clear the event fd.  Do this before flushing the events list,
+       so that any new event post afterwards is sure to re-awaken the
+       event loop.  */
+    serial_event_clear (runnable_event);
+
+    /* Move the vector in case running a runnable pushes a new
+       runnable.  */
+    std::swap (local, runnables);
+  }
+
+  for (auto &item : local)
+    item ();
+}
+
+/* See ser-event.h.  */
+
+void
+run_on_main_thread (std::function<void ()> &&func)
+{
+  std::lock_guard<std::mutex> lock (runnable_mutex);
+  runnables.emplace_back (std::move (func));
+  serial_event_set (runnable_event);
+}
+
+void
+_initialize_ser_event ()
+{
+  runnable_event = make_serial_event ();
+  add_file_handler (serial_event_fd (runnable_event), run_events, nullptr);
+}
diff --git a/gdb/ser-event.h b/gdb/ser-event.h
index 137348557f9..61a84f9cc79 100644
--- a/gdb/ser-event.h
+++ b/gdb/ser-event.h
@@ -19,6 +19,8 @@
 #ifndef SER_EVENT_H
 #define SER_EVENT_H
 
+#include <functional>
+
 /* This is used to be able to signal the event loop (or any other
    select/poll) of events, in a race-free manner.
 
@@ -48,4 +50,8 @@ extern void serial_event_set (struct serial_event *event);
    call is made.  */
 extern void serial_event_clear (struct serial_event *event);
 
+/* Send a runnable to the main thread.  */
+
+extern void run_on_main_thread (std::function<void ()> &&);
+
 #endif
-- 
2.17.2