Re: GDB looses pgrp setting in the terminal for debugged process after break.

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

On Fri, 5 Feb 2021 19:34:57 +0900
Takashi Yano wrote:
> On Tue, 26 Jan 2021 12:14:02 +0900
> Takashi Yano wrote:
> > Hi GDB maintainer,
> > 
> > In GDB, debugged process cannot continue execution after break
> > if it reads stdin.
> > 
> > With the following steps, cat is terminated with error.
> > 
> > 1) Install coreutils-debuginfo package.
> > 2) Run "gdb cat" in console (command prompt), not in mintty.
> > 3) Enter "start" in gdb.
> > 4) Enter "cont" in gdb.
> > 
> > This results in:
> > /usr/bin/cat: -: Input/output error
> > 
> > Both gdb-9.2-1 and gdb-10.1-1(TEST) have this problem.
> > 
> > I looked into this problem and found the cause is that the pgid
> > setting for /usr/bin/cat is lost after break. The following patch
> > for GDB source resolves the issue. In the following section,
> > winpid is passed to getpgid() rather than cygwin pid. Also, winpid
> > is passed to other POSIX system calls such as kill() elsewhere. 
> > 
> > --- inflow.c.orig	2020-05-24 06:10:29.000000000 +0900
> > +++ inflow.c	2021-01-23 17:48:27.963609500 +0900
> > @@ -364,11 +364,11 @@
> >  #ifdef HAVE_TERMIOS_H
> >  	  /* If we can't tell the inferior's actual process group,
> >  	     then restore whatever was the foreground pgrp the last
> >  	     time the inferior was running.  See also comments
> >  	     describing terminal_state::process_group.  */
> > -#ifdef HAVE_GETPGID
> > +#if defined (HAVE_GETPGID) && !defined (__CYGWIN__)
> >  	  result = tcsetpgrp (0, getpgid (inf->pid));
> >  #else
> >  	  result = tcsetpgrp (0, tinfo->process_group);
> >  #endif
> >  	  if (result == -1)
> > 
> > 
> > I hope the GDB maintainer will check it out.
> > 
> > Addresses: https://cygwin.com/pipermail/cygwin-patches/2021q1/011018.html
> 
> I have noticed that cygwin gdb essentially has the problem
> regarding terminal process group.
> 
> Cygwin gdb uses CreateProcessW() to execute debugging process
> rather than exec(). If the debugging process is a cygwin process,
> cygwin pid is assigned, however, if the debugging process is a
> non cygwin process, cygwin pid is not assigned. Therefore, there
> is no appropriate process group ID to set.
> 
> I wonder what is the right thing under that situation. Any idea?

The patch below seems to be better than the previous one a bit.

diff --git a/inflow.c.orig b/inflow.c
index 00b2235..fa7b408 100644
--- a/inflow.c.orig
+++ b/inflow.c
@@ -40,6 +40,10 @@
 #include <sys/ioctl.h>
 #endif
 
+#ifdef __CYGWIN__
+#include <sys/cygwin.h>
+#endif
+
 #ifndef O_NOCTTY
 #define O_NOCTTY 0
 #endif
@@ -367,7 +371,13 @@ child_terminal_inferior (struct target_ops *self)
 	     time the inferior was running.  See also comments
 	     describing terminal_state::process_group.  */
 #ifdef HAVE_GETPGID
+#ifdef __CYGWIN__
+	  pid_t cygpid = cygwin_internal (CW_WINPID_TO_CYGWIN_PID, inf->pid);
+	  if (cygpid <= cygwin_internal (CW_MAX_CYGWIN_PID))
+	    result = tcsetpgrp (0, getpgid (cygpid));
+#else
 	  result = tcsetpgrp (0, getpgid (inf->pid));
+#endif
 #else
 	  result = tcsetpgrp (0, tinfo->process_group);
 #endif

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