Move some Windows operations to worker thread

On Windows, certain debugging APIs can only be called from the thread
that started (or attached) to the inferior.  Also, there is no way on
Windows to wait for a debug event in addition to other events.
Therefore, in order to implement target async for Windows, gdb will
have to call some functions in a worker thread.

This patch implements the worker thread and moves the necessary
operations there.  Target async isn't yet implemented, so this patch
does not cause any visible changes.

1 files changed, 182 insertions, 75 deletions

diff --git a/gdb/windows-nat.c b/gdb/windows-nat.c
index 1e31037..ce458d2 100644
--- a/gdb/windows-nat.c
+++ b/gdb/windows-nat.c
@@ -43,6 +43,7 @@
 #endif
 #include <algorithm>
 #include <vector>
+#include <queue>
 
 #include "filenames.h"
 #include "symfile.h"
@@ -244,6 +245,8 @@ static const struct xlate_exception xlate[] =
 
 struct windows_nat_target final : public x86_nat_target<inf_child_target>
 {
+  windows_nat_target ();
+
   void close () override;
 
   void attach (const char *, int) override;
@@ -302,7 +305,8 @@ struct windows_nat_target final : public x86_nat_target<inf_child_target>
 
   const char *thread_name (struct thread_info *) override;
 
-  int get_windows_debug_event (int pid, struct target_waitstatus *ourstatus);
+  ptid_t get_windows_debug_event (int pid, struct target_waitstatus *ourstatus,
+				  target_wait_flags options);
 
   void do_initial_windows_stuff (DWORD pid, bool attaching);
 
@@ -317,6 +321,34 @@ private:
 				   bool main_thread_p);
   void delete_thread (ptid_t ptid, DWORD exit_code, bool main_thread_p);
   DWORD fake_create_process ();
+
+  BOOL windows_continue (DWORD continue_status, int id, int killed);
+
+  /* This function implements the background thread that starts
+     inferiors and waits for events.  */
+  void process_thread ();
+
+  /* Push FUNC onto the queue of requests for process_thread, and wait
+     until it has been called.  On Windows, certain debugging
+     functions can only be called by the thread that started (or
+     attached to) the inferior.  These are all done in the worker
+     thread, via calls to this method.  */
+  void do_synchronously (gdb::function_view<void ()> func);
+
+  /* This waits for a debug event, dispatching to the worker thread as
+     needed.  */
+  void wait_for_debug_event_main_thread (DEBUG_EVENT *event);
+
+  /* Queue used to send requests to process_thread.  This is
+     implicitly locked.  */
+  std::queue<gdb::function_view<void ()>> m_queue;
+
+  /* Event used to signal process_thread that an item has been
+     pushed.  */
+  HANDLE m_pushed_event;
+  /* Event used by process_thread to indicate that it has processed a
+     single function call.  */
+  HANDLE m_response_event;
 };
 
 static windows_nat_target the_windows_nat_target;
@@ -332,6 +364,74 @@ check (BOOL ok, const char *file, int line)
     }
 }
 
+windows_nat_target::windows_nat_target ()
+  : m_pushed_event (CreateEvent (nullptr, false, false, nullptr)),
+    m_response_event (CreateEvent (nullptr, false, false, nullptr))
+{
+  auto fn = [] (LPVOID self) -> DWORD
+    {
+      ((windows_nat_target *) self)->process_thread ();
+      return 0;
+    };
+
+  HANDLE bg_thread = CreateThread (nullptr, 64 * 1024, fn, this, 0, nullptr);
+  CloseHandle (bg_thread);
+}
+
+/* A wrapper for WaitForSingleObject that issues a warning if
+   something unusual happens.  */
+static void
+wait_for_single (HANDLE handle, DWORD howlong)
+{
+  while (true)
+    {
+      DWORD r = WaitForSingleObject (handle, howlong);
+      if (r == WAIT_OBJECT_0)
+	return;
+      if (r == WAIT_FAILED)
+	{
+	  unsigned err = (unsigned) GetLastError ();
+	  warning ("WaitForSingleObject failed (code %u): %s",
+		   err, strwinerror (err));
+	}
+      else
+	warning ("unexpected result from WaitForSingleObject: %u",
+		 (unsigned) r);
+    }
+}
+
+void
+windows_nat_target::process_thread ()
+{
+  while (true)
+    {
+      wait_for_single (m_pushed_event, INFINITE);
+
+      gdb::function_view<void ()> func = std::move (m_queue.front ());
+      m_queue.pop ();
+
+      func ();
+      SetEvent (m_response_event);
+   }
+}
+
+void
+windows_nat_target::do_synchronously (gdb::function_view<void ()> func)
+{
+  m_queue.emplace (std::move (func));
+  SetEvent (m_pushed_event);
+  wait_for_single (m_response_event, INFINITE);
+}
+
+void
+windows_nat_target::wait_for_debug_event_main_thread (DEBUG_EVENT *event)
+{
+  do_synchronously ([&] ()
+    {
+      wait_for_debug_event (event, INFINITE);
+    });
+}
+
 /* See nat/windows-nat.h.  */
 
 windows_thread_info *
@@ -1079,11 +1179,9 @@ windows_per_inferior::handle_access_violation
    threads, if we are continuing execution.  KILLED non-zero means we
    have killed the inferior, so we should ignore weird errors due to
    threads shutting down.  */
-static BOOL
-windows_continue (DWORD continue_status, int id, int killed)
+BOOL
+windows_nat_target::windows_continue (DWORD continue_status, int id, int killed)
 {
-  BOOL res;
-
   windows_process.desired_stop_thread_id = id;
 
   if (windows_process.matching_pending_stop (debug_events))
@@ -1160,17 +1258,19 @@ windows_continue (DWORD continue_status, int id, int killed)
 	th->suspend ();
       }
 
-  res = continue_last_debug_event (continue_status, debug_events);
-
-  if (!res)
+  gdb::optional<unsigned> err;
+  do_synchronously ([&] ()
     {
-      unsigned err = (unsigned) GetLastError ();
-      error (_("Failed to resume program execution"
-	       " (ContinueDebugEvent failed, error %u: %s)"),
-	     err, strwinerror (err));
-    }
+      if (!continue_last_debug_event (continue_status, debug_events))
+	err = (unsigned) GetLastError ();
+    });
+
+  if (err.has_value ())
+    error (_("Failed to resume program execution"
+	     " (ContinueDebugEvent failed, error %u: %s)"),
+	   *err, strwinerror (*err));
 
-  return res;
+  return TRUE;
 }
 
 /* Called in pathological case where Windows fails to send a
@@ -1376,14 +1476,12 @@ ctrl_c_handler (DWORD event_type)
   return TRUE;
 }
 
-/* Get the next event from the child.  Returns a non-zero thread id if the event
-   requires handling by WFI (or whatever).  */
+/* Get the next event from the child.  Returns the thread ptid.  */
 
-int
-windows_nat_target::get_windows_debug_event (int pid,
-					     struct target_waitstatus *ourstatus)
+ptid_t
+windows_nat_target::get_windows_debug_event
+     (int pid, struct target_waitstatus *ourstatus, target_wait_flags options)
 {
-  BOOL debug_event;
   DWORD continue_status, event_code;
   DWORD thread_id = 0;
 
@@ -1402,15 +1500,13 @@ windows_nat_target::get_windows_debug_event (int pid,
 	= windows_process.thread_rec (ptid, INVALIDATE_CONTEXT);
       th->reload_context = true;
 
-      return thread_id;
+      return ptid;
     }
 
   windows_process.last_sig = GDB_SIGNAL_0;
   DEBUG_EVENT *current_event = &windows_process.current_event;
 
-  if (!(debug_event = wait_for_debug_event (&windows_process.current_event,
-					    1000)))
-    goto out;
+  wait_for_debug_event_main_thread (&windows_process.current_event);
 
   continue_status = DBG_CONTINUE;
 
@@ -1632,7 +1728,9 @@ windows_nat_target::get_windows_debug_event (int pid,
     }
 
 out:
-  return thread_id;
+  if (thread_id == 0)
+    return null_ptid;
+  return ptid_t (windows_process.current_event.dwProcessId, thread_id, 0);
 }
 
 /* Wait for interesting events to occur in the target process.  */
@@ -1650,8 +1748,6 @@ windows_nat_target::wait (ptid_t ptid, struct target_waitstatus *ourstatus,
 
   while (1)
     {
-      int retval;
-
       /* If the user presses Ctrl-c while the debugger is waiting
 	 for an event, he expects the debugger to interrupt his program
 	 and to get the prompt back.  There are two possible situations:
@@ -1679,14 +1775,11 @@ windows_nat_target::wait (ptid_t ptid, struct target_waitstatus *ourstatus,
 	     the user tries to resume the execution in the inferior.
 	     This is a classic race that we should try to fix one day.  */
       SetConsoleCtrlHandler (&ctrl_c_handler, TRUE);
-      retval = get_windows_debug_event (pid, ourstatus);
+      ptid_t result = get_windows_debug_event (pid, ourstatus, options);
       SetConsoleCtrlHandler (&ctrl_c_handler, FALSE);
 
-      if (retval)
+      if (result != null_ptid)
 	{
-	  ptid_t result = ptid_t (windows_process.current_event.dwProcessId,
-				  retval, 0);
-
 	  if (ourstatus->kind () != TARGET_WAITKIND_EXITED
 	      && ourstatus->kind () !=  TARGET_WAITKIND_SIGNALLED)
 	    {
@@ -1869,7 +1962,6 @@ out:
 void
 windows_nat_target::attach (const char *args, int from_tty)
 {
-  BOOL ok;
   DWORD pid;
 
   pid = parse_pid_to_attach (args);
@@ -1879,26 +1971,31 @@ windows_nat_target::attach (const char *args, int from_tty)
 	     "This can cause attach to fail on Windows NT/2K/XP");
 
   windows_init_thread_list ();
-  ok = DebugActiveProcess (pid);
   windows_process.saw_create = 0;
 
-#ifdef __CYGWIN__
-  if (!ok)
+  gdb::optional<unsigned> err;
+  do_synchronously ([&] ()
     {
-      /* Try fall back to Cygwin pid.  */
-      pid = cygwin_internal (CW_CYGWIN_PID_TO_WINPID, pid);
+      BOOL ok = DebugActiveProcess (pid);
 
-      if (pid > 0)
-	ok = DebugActiveProcess (pid);
-  }
+#ifdef __CYGWIN__
+      if (!ok)
+	{
+	  /* Try fall back to Cygwin pid.  */
+	  pid = cygwin_internal (CW_CYGWIN_PID_TO_WINPID, pid);
+
+	  if (pid > 0)
+	    ok = DebugActiveProcess (pid);
+	}
 #endif
 
-  if (!ok)
-    {
-      unsigned err = (unsigned) GetLastError ();
-      error (_("Can't attach to process %u (error %u: %s)"),
-	     (unsigned) pid, err, strwinerror (err));
-    }
+      if (!ok)
+	err = (unsigned) GetLastError ();
+    });
+
+  if (err.has_value ())
+    error (_("Can't attach to process %u (error %u: %s)"),
+	   (unsigned) pid, *err, strwinerror (*err));
 
   DebugSetProcessKillOnExit (FALSE);
 
@@ -1925,14 +2022,19 @@ windows_nat_target::detach (inferior *inf, int from_tty)
   ptid_t ptid = minus_one_ptid;
   resume (ptid, 0, GDB_SIGNAL_0);
 
-  if (!DebugActiveProcessStop (windows_process.current_event.dwProcessId))
+  gdb::optional<unsigned> err;
+  do_synchronously ([&] ()
     {
-      unsigned err = (unsigned) GetLastError ();
-      error (_("Can't detach process %u (error %u: %s)"),
-	     (unsigned) windows_process.current_event.dwProcessId,
-	     err, strwinerror (err));
-    }
-  DebugSetProcessKillOnExit (FALSE);
+      if (!DebugActiveProcessStop (windows_process.current_event.dwProcessId))
+	err = (unsigned) GetLastError ();
+      else
+	DebugSetProcessKillOnExit (FALSE);
+    });
+
+  if (err.has_value ())
+    error (_("Can't detach process %u (error %u: %s)"),
+	   (unsigned) windows_process.current_event.dwProcessId,
+	   *err, strwinerror (*err));
 
   target_announce_detach (from_tty);
 
@@ -2378,7 +2480,7 @@ windows_nat_target::create_inferior (const char *exec_file,
 #endif	/* !__CYGWIN__ */
   const char *allargs = origallargs.c_str ();
   PROCESS_INFORMATION pi;
-  BOOL ret;
+  gdb::optional<unsigned> ret;
   DWORD flags = 0;
   const std::string &inferior_tty = current_inferior ()->tty ();
 
@@ -2485,10 +2587,15 @@ windows_nat_target::create_inferior (const char *exec_file,
     }
 
   windows_init_thread_list ();
-  ret = create_process (nullptr, args, flags, w32_env,
-			inferior_cwd != nullptr ? infcwd : nullptr,
-			disable_randomization,
-			&si, &pi);
+  do_synchronously ([&] ()
+    {
+      if (!create_process (nullptr, args, flags, w32_env,
+			   inferior_cwd != nullptr ? infcwd : nullptr,
+			   disable_randomization,
+			   &si, &pi))
+	ret = (unsigned) GetLastError ();
+    });
+
   if (w32_env)
     /* Just free the Win32 environment, if it could be created. */
     free (w32_env);
@@ -2605,14 +2712,18 @@ windows_nat_target::create_inferior (const char *exec_file,
   *temp = 0;
 
   windows_init_thread_list ();
-  ret = create_process (nullptr, /* image */
-			args,	/* command line */
-			flags,	/* start flags */
-			w32env,	/* environment */
-			inferior_cwd, /* current directory */
-			disable_randomization,
-			&si,
-			&pi);
+  do_synchronously ([&] ()
+    {
+      if (!create_process (nullptr, /* image */
+			   args,	/* command line */
+			   flags,	/* start flags */
+			   w32env,	/* environment */
+			   inferior_cwd, /* current directory */
+			   disable_randomization,
+			   &si,
+			   &pi))
+	ret = (unsigned) GetLastError ();
+    });
   if (tty != INVALID_HANDLE_VALUE)
     CloseHandle (tty);
   if (fd_inp >= 0)
@@ -2623,12 +2734,9 @@ windows_nat_target::create_inferior (const char *exec_file,
     _close (fd_err);
 #endif	/* !__CYGWIN__ */
 
-  if (!ret)
-    {
-      unsigned err = (unsigned) GetLastError ();
-      error (_("Error creating process %s, (error %u: %s)"),
-	     exec_file, err, strwinerror (err));
-    }
+  if (ret.has_value ())
+    error (_("Error creating process %s, (error %u: %s)"),
+	   exec_file, *ret, strwinerror (*ret));
 
 #ifdef __x86_64__
   BOOL wow64;
@@ -2724,8 +2832,7 @@ windows_nat_target::kill ()
     {
       if (!windows_continue (DBG_CONTINUE, -1, 1))
 	break;
-      if (!wait_for_debug_event (&windows_process.current_event, INFINITE))
-	break;
+      wait_for_debug_event_main_thread (&windows_process.current_event);
       if (windows_process.current_event.dwDebugEventCode
 	  == EXIT_PROCESS_DEBUG_EVENT)
 	break;