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Date: Thu, 14 May 2026 23:11:48 +0300
To: cygwin AT cygwin DOT com
Subject: Re: On exit, winsup/cygwin/fhandler/console.cc waits on closed
 handle, deadlock possible
Message-ID: <20260514231148.60c1743e@Tarkus>
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From: Ivan Krylov via Cygwin <cygwin AT cygwin DOT com>
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On Wed, 13 May 2026 12:59:59 +0300
Ivan Krylov <ikrylov AT disroot DOT org> wrote:

> I think it doesn't close the race window completely.

Here's a patch that I have more confidence in:

diff --git a/winsup/cygwin/fhandler/console.cc b/winsup/cygwin/fhandler/console.cc
index c76347f6f..a97df4cef 100644
--- a/winsup/cygwin/fhandler/console.cc
+++ b/winsup/cygwin/fhandler/console.cc
@@ -62,6 +62,7 @@ fhandler_console::console_state NO_COPY
 
 static bool NO_COPY inside_pcon_checked = false;
 static bool NO_COPY inside_pcon = false;
+static LONG NO_COPY inside_pcon_cleanup = 0;
 static int NO_COPY parent_pty;
 static HANDLE NO_COPY parent_pty_input_mutex = NULL;
 
@@ -530,8 +531,13 @@ fhandler_console::cons_master_thread (handle_set_t *p, tty *ttyp)
 	default: /* Error */
 	  free (input_rec);
 	  free (input_tmp);
-	  if (inside_pcon && parent_pty_input_mutex)
+	  if (inside_pcon && parent_pty_input_mutex) {
 	    ReleaseMutex (parent_pty_input_mutex);
+	    if (InterlockedCompareExchange(&inside_pcon_cleanup, 0, 0)) {
+	      ForceCloseHandle (parent_pty_input_mutex);
+	      inside_pcon = false;
+	    }
+	  }
 	  ReleaseMutex (p->input_mutex);
 	  return;
 	}
@@ -713,8 +719,13 @@ remove_record:
 	  while (true);
 	}
 skip_writeback:
-      if (inside_pcon && parent_pty_input_mutex)
+      if (inside_pcon && parent_pty_input_mutex) {
 	ReleaseMutex (parent_pty_input_mutex);
+	if (InterlockedCompareExchange(&inside_pcon_cleanup, 0, 0)) {
+	  ForceCloseHandle (parent_pty_input_mutex);
+	  inside_pcon = false;
+	}
+      }
       ReleaseMutex (p->input_mutex);
       cygwait (40);
     }
@@ -2048,7 +2059,7 @@ fhandler_console::pcon_hand_over_proc (void)
   else
     system_printf("Acquiring pcon_ho_mutex failed.");
   ReleaseMutex (mtx);
-  ForceCloseHandle (parent_pty_input_mutex);
+  InterlockedExchange(&inside_pcon_cleanup, 1);
 }
 
 bool

Instead of manipulating the mutex handle shared between threads and
leaving a race window between if(parent_pty_input_mutex) and
WaitForSingleObject(parent_pty_input_mutex, ...), tell the
cons_master_thread to close the handle and stop using it.

-- 
Best regards,
Ivan

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