From 49018bf5774fc90ddddc762a8462030c5d528643 Mon Sep 17 00:00:00 2001
From: Takashi Yano <takashi.yano@nifty.ne.jp>
Date: Mon, 1 Jul 2024 17:44:53 +0900
Subject: [PATCH] Cygwin: pty: Avoid client deadlock when pty master stops to
 read.

Previsouly, the following commands hangs:
  mintty -e timeout 1 dash -c 'yes aaaaaaaaaaaaaaaaaaaaaaaaa | cat'

The mechanism is as follows.

When the child process (timeout) is terminated, mintty seems to stop
reading pty master even if yes or cat still alive.

If the the pipe used to transfer output from pty slave to pty master
is full due to lack of master reader, WriteFile() to the pipe is
blocked. WriteFile() cannot be canceled by cygwin signal, therefore,
pty slave hangs.

This patch avoids hanging by checking pipe space before calling
WriteFile() and prevents writing data more than space.

Addresses: https://cygwin.com/pipermail/cygwin/2024-June/256178.html
Reported-by: jojelino <jojelino@gmail.com>
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>
---
 winsup/cygwin/fhandler/pty.cc | 25 ++++++++++++++++++++++---
 winsup/cygwin/release/3.5.4   |  4 ++++
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/winsup/cygwin/fhandler/pty.cc b/winsup/cygwin/fhandler/pty.cc
index 9d7ef3c9d..fa6bf1096 100644
--- a/winsup/cygwin/fhandler/pty.cc
+++ b/winsup/cygwin/fhandler/pty.cc
@@ -3118,8 +3118,22 @@ fhandler_pty_common::process_opost_output (HANDLE h, const void *ptr,
     return res; /* Discard write data */
   while (towrite)
     {
+      ssize_t space = towrite;
       if (!is_echo)
 	{
+	  IO_STATUS_BLOCK iosb = {{0}, 0};
+	  FILE_PIPE_LOCAL_INFORMATION fpli = {0};
+	  NTSTATUS status;
+
+	  status = NtQueryInformationFile (h, &iosb, &fpli, sizeof (fpli),
+					   FilePipeLocalInformation);
+	  if (!NT_SUCCESS (status))
+	    {
+	      if (towrite < len)
+		break;
+	      len = -1;
+	      return FALSE;
+	    }
 	  if (ttyp->output_stopped && is_nonblocking)
 	    {
 	      if (towrite < len)
@@ -3131,13 +3145,18 @@ fhandler_pty_common::process_opost_output (HANDLE h, const void *ptr,
 		  return TRUE;
 		}
 	    }
-	  while (ttyp->output_stopped)
-	    cygwait (10);
+	  if (ttyp->output_stopped || fpli.WriteQuotaAvailable == 0)
+	    {
+	      cygwait (1);
+	      continue;
+	    }
+	  space = fpli.WriteQuotaAvailable;
 	}
 
       if (!(ttyp->ti.c_oflag & OPOST))	// raw output mode
 	{
 	  DWORD n = MIN (OUT_BUFFER_SIZE, towrite);
+	  n = MIN (n, space);
 	  res = WriteFile (h, ptr, n, &n, NULL);
 	  if (!res)
 	    break;
@@ -3150,7 +3169,7 @@ fhandler_pty_common::process_opost_output (HANDLE h, const void *ptr,
 	  char *buf = (char *)ptr;
 	  DWORD n = 0;
 	  ssize_t rc = 0;
-	  while (n < OUT_BUFFER_SIZE && rc < towrite)
+	  while (n < OUT_BUFFER_SIZE && n < space && rc < towrite)
 	    {
 	      switch (buf[rc])
 		{
diff --git a/winsup/cygwin/release/3.5.4 b/winsup/cygwin/release/3.5.4
index 2a5f2b1bd..17db61146 100644
--- a/winsup/cygwin/release/3.5.4
+++ b/winsup/cygwin/release/3.5.4
@@ -15,3 +15,7 @@ Fixes:
 
 - Fix a problem that ldd command against cygwin DLLs sometimes hangs.
   Addresses: https://cygwin.com/pipermail/cygwin/2024-May/255991.html
+
+- Fix a problem that pty slave hangs on writing when pty master stops
+  to read.
+  Addresses: https://cygwin.com/pipermail/cygwin/2024-June/256178.html