Re: [PATCH] Cygwin: pipe: Restore non-blocking mode which was reset for non-cygwin app.

[Takashi Yano <takashi.yano@nifty.ne.jp>]

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On Tue, 12 Mar 2024 08:03:16 +0900
Takashi Yano wrote:
> +  /* Set read pipe itself always non-blocking for cygwin process.
> +     Blocking/non-blocking is simulated in raw_read(). For write
> +     pipe, follow is_nonblocking(). */
> +  int fd;
> +  cygheap_fdenum cfd (false);
> +  while ((fd = cfd.next ()) >= 0)
> +    if (cfd->get_dev () == FH_PIPEW
> +	&& (fd == fileno_stdout || fd == fileno_stderr))
> +      {
> +	fhandler_pipe *pipe = (fhandler_pipe *)(fhandler_base *) cfd;
> +	pipe->set_pipe_non_blocking (false);

Sorry. Commenting here is not right. v4 patch attached.

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>

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From 4b536f7dda6c6000e8eccb6af6dbf6abd80f1020 Mon Sep 17 00:00:00 2001
From: Takashi Yano <takashi.yano@nifty.ne.jp>
Date: Mon, 11 Mar 2024 22:08:00 +0900
Subject: [PATCH v4] Cygwin: pipe: Make sure to set read pipe non-blocking for
 cygwin apps.

If pipe reader is a non-cygwin app first, and cygwin process reads
the same pipe after that, the pipe has been set to bclocking mode
for the cygwin app. However, the commit 9e4d308cd592 assumes the
pipe for cygwin process always is non-blocking mode. With this patch,
the pipe mode is reset to non-blocking when cygwin app is started.

Addresses: https://cygwin.com/pipermail/cygwin/2024-March/255644.html
Fixes: 9e4d308cd592 ("Cygwin: pipe: Adopt FILE_SYNCHRONOUS_IO_NONALERT flag for read pipe.")
Reported-by: wh <wh9692@protonmail.com>
Reviewed-by: Corinna Vinschen <corinna@vinschen.de>
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>
---
 winsup/cygwin/fhandler/pipe.cc          | 63 +++++++++++++++++++++++++
 winsup/cygwin/local_includes/fhandler.h |  3 ++
 winsup/cygwin/spawn.cc                  | 34 +------------
 3 files changed, 68 insertions(+), 32 deletions(-)

diff --git a/winsup/cygwin/fhandler/pipe.cc b/winsup/cygwin/fhandler/pipe.cc
index 29d3b41d9..ae43cbc00 100644
--- a/winsup/cygwin/fhandler/pipe.cc
+++ b/winsup/cygwin/fhandler/pipe.cc
@@ -18,6 +18,7 @@ details. */
 #include "pinfo.h"
 #include "shared_info.h"
 #include "tls_pbuf.h"
+#include "sigproc.h"
 #include <assert.h>
 
 /* This is only to be used for writing.  When reading,
@@ -602,6 +603,17 @@ fhandler_pipe::fixup_after_fork (HANDLE parent)
   ReleaseMutex (hdl_cnt_mtx);
 }
 
+void
+fhandler_pipe::fixup_after_exec ()
+{
+  /* Set read pipe itself always non-blocking for cygwin process.
+     Blocking/non-blocking is simulated in raw_read(). For write
+     pipe, follow is_nonblocking(). */
+  bool mode = get_device () == FH_PIPEW ? is_nonblocking () : true;
+  set_pipe_non_blocking (mode);
+  fhandler_base::fixup_after_exec ();
+}
+
 int
 fhandler_pipe::dup (fhandler_base *child, int flags)
 {
@@ -1288,3 +1300,54 @@ close_proc:
     }
   return NULL;
 }
+
+void
+fhandler_pipe::spawn_worker (int fileno_stdin, int fileno_stdout,
+			     int fileno_stderr)
+{
+  bool need_send_noncygchld_sig = false;
+
+  /* spawn_worker() is called from spawn.cc only when non-cygwin app
+     is started. Set pipe mode blocking for the non-cygwin process. */
+  int fd;
+  cygheap_fdenum cfd (false);
+  while ((fd = cfd.next ()) >= 0)
+    if (cfd->get_dev () == FH_PIPEW
+	&& (fd == fileno_stdout || fd == fileno_stderr))
+      {
+	fhandler_pipe *pipe = (fhandler_pipe *)(fhandler_base *) cfd;
+	pipe->set_pipe_non_blocking (false);
+
+	/* Setup for query_ndl stuff. Read the comment below. */
+	if (pipe->request_close_query_hdl ())
+	  need_send_noncygchld_sig = true;
+      }
+    else if (cfd->get_dev () == FH_PIPER && fd == fileno_stdin)
+      {
+	fhandler_pipe *pipe = (fhandler_pipe *)(fhandler_base *) cfd;
+	pipe->set_pipe_non_blocking (false);
+      }
+
+  /* If multiple writers including non-cygwin app exist, the non-cygwin
+     app cannot detect pipe closure on the read side when the pipe is
+     created by system account or the pipe creator is running as service.
+     This is because query_hdl which is held in write side also is a read
+     end of the pipe, so the pipe is still alive for the non-cygwin app
+     even after the reader is closed.
+
+     To avoid this problem, let all processes in the same process
+     group close query_hdl using internal signal __SIGNONCYGCHLD when
+     non-cygwin app is started.  */
+  if (need_send_noncygchld_sig)
+    {
+      tty_min dummy_tty;
+      dummy_tty.ntty = (fh_devices) myself->ctty;
+      dummy_tty.pgid = myself->pgid;
+      tty_min *t = cygwin_shared->tty.get_cttyp ();
+      if (!t) /* If tty is not allocated, use dummy_tty instead. */
+	t = &dummy_tty;
+      /* Emit __SIGNONCYGCHLD to let all processes in the
+	 process group close query_hdl. */
+      t->kill_pgrp (__SIGNONCYGCHLD);
+    }
+}
diff --git a/winsup/cygwin/local_includes/fhandler.h b/winsup/cygwin/local_includes/fhandler.h
index 8729eb276..d9e0a011b 100644
--- a/winsup/cygwin/local_includes/fhandler.h
+++ b/winsup/cygwin/local_includes/fhandler.h
@@ -1234,6 +1234,7 @@ public:
   int open (int flags, mode_t mode = 0);
   bool open_setup (int flags);
   void fixup_after_fork (HANDLE);
+  void fixup_after_exec ();
   int dup (fhandler_base *child, int);
   void set_close_on_exec (bool val);
   int close ();
@@ -1295,6 +1296,8 @@ public:
 	}
       return false;
     }
+  static void spawn_worker (int fileno_stdin, int fileno_stdout,
+			    int fileno_stderr);
 };
 
 #define CYGWIN_FIFO_PIPE_NAME_LEN     47
diff --git a/winsup/cygwin/spawn.cc b/winsup/cygwin/spawn.cc
index 71d75bbf4..3da77088d 100644
--- a/winsup/cygwin/spawn.cc
+++ b/winsup/cygwin/spawn.cc
@@ -580,38 +580,8 @@ child_info_spawn::worker (const char *prog_arg, const char *const *argv,
       int fileno_stderr = 2;
 
       if (!iscygwin ())
-	{
-	  bool need_send_sig = false;
-	  int fd;
-	  cygheap_fdenum cfd (false);
-	  while ((fd = cfd.next ()) >= 0)
-	    if (cfd->get_dev () == FH_PIPEW
-		     && (fd == fileno_stdout || fd == fileno_stderr))
-	      {
-		fhandler_pipe *pipe = (fhandler_pipe *)(fhandler_base *) cfd;
-		pipe->set_pipe_non_blocking (false);
-		if (pipe->request_close_query_hdl ())
-		  need_send_sig = true;
-	      }
-	    else if (cfd->get_dev () == FH_PIPER && fd == fileno_stdin)
-	      {
-		fhandler_pipe *pipe = (fhandler_pipe *)(fhandler_base *) cfd;
-		pipe->set_pipe_non_blocking (false);
-	      }
-
-	  if (need_send_sig)
-	    {
-	      tty_min dummy_tty;
-	      dummy_tty.ntty = (fh_devices) myself->ctty;
-	      dummy_tty.pgid = myself->pgid;
-	      tty_min *t = cygwin_shared->tty.get_cttyp ();
-	      if (!t) /* If tty is not allocated, use dummy_tty instead. */
-		t = &dummy_tty;
-	      /* Emit __SIGNONCYGCHLD to let all processes in the
-		 process group close query_hdl. */
-	      t->kill_pgrp (__SIGNONCYGCHLD);
-	    }
-	}
+	fhandler_pipe::spawn_worker (fileno_stdin, fileno_stdout,
+				     fileno_stderr);
 
       bool no_pcon = mode != _P_OVERLAY && mode != _P_WAIT;
       term_spawn_worker.setup (iscygwin (), handle (fileno_stdin, false),
-- 
2.43.0