[binutils-gdb] Cygwin: Fix putting inferior in foreground (fix input)

[binutils-gdb] Cygwin: Fix putting inferior in foreground (fix input)

[Pedro Alves]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=dfec66ffff42d8274132f6df20d5281e10f0e746

commit dfec66ffff42d8274132f6df20d5281e10f0e746
Author: Pedro Alves <pedro@palves.net>
Date:   Thu May 6 20:59:14 2021 +0100

    Cygwin: Fix putting inferior in foreground (fix input)
    
    gdb.base/interrupt.exp reveals that inferior input is
    broken on Cygwin:
    
      (gdb) continue
      Continuing.
      talk to me baby
      Input/output error                                   <<< BAD
      PASS: gdb.base/interrupt.exp: process is alive
      a
      [Thread 10688.0x2590 exited with code 1]
      [Thread 10688.0x248c exited with code 1]
      [Thread 10688.0x930 exited with code 1]
      [Thread 10688.0x2c98 exited with code 1]
    
      Program terminated with signal SIGHUP, Hangup.
      The program no longer exists.
      (gdb) FAIL: gdb.base/interrupt.exp: child process ate our char
      a
      Ambiguous command "a": actions, add-auto-load-safe-path, add-auto-load-scripts-directory, add-inferior...
      (gdb) ERROR: "" is not a unique command name.
    
    The problem is that inflow.c:child_terminal_inferior is failing to put
    the inferior in the foreground, because we're passing down the
    inferior's Windows PID instead of the Cygwin PID to Cygwin tcsetpgrp.
    
    That is fixed by this commit.  Afterwards we will get:
    
      (gdb) continue
      Continuing.
      talk to me baby
      PASS: gdb.base/interrupt.exp: process is alive
      a
      a                                                              <<< GOOD
      PASS: gdb.base/interrupt.exp: child process ate our char
      [New Thread 7236.0x1c58]
    
      Thread 6 received signal SIGINT, Interrupt.                    <<< new thread spawned for SIGINT
      [Switching to Thread 7236.0x1c58]
      0x00007ffa6643ea6b in TlsGetValue () from /cygdrive/c/Windows/System32/KERNELBASE.dll
      (gdb) FAIL: gdb.base/interrupt.exp: send_gdb control C
    
    We still have the FAIL seen above because this change has another
    consequence.  By failing to put the inferior in the foreground
    correctly, Ctrl-C was always reaching GDB first.  Now that the
    inferior is put in the foreground properly, Ctrl-C reaches the
    inferior first, which results in a Windows Ctrl-C event, which results
    in Windows injecting a new thread in the inferior to report the Ctrl-C
    exception => SIGINT.  That is, running the test manually:
    
    Before patch:
    
      (gdb) c
      Continuing.
      [New Thread 2352.0x1f5c]
      [New Thread 2352.0x1988]
      [New Thread 2352.0x18cc]
                                                               <<< Ctrl-C pressed.
      Thread 7 received signal SIGTRAP, Trace/breakpoint trap.
      [Switching to Thread 2352.0x18cc]
      0x00007ffa68930b11 in ntdll!DbgBreakPoint () from /cygdrive/c/Windows/SYSTEM32/ntdll.dll
      (gdb)
    
    Above, GDB got the SIGINT, and it manually passes it down the
    inferior, which reaches windows_nat_target::interrupt(), which
    interrupts the inferior with DebugBreakProcess (which injects a new
    thread in the inferior that executes an int3 instruction).
    
    After this patch, we have (with "set debugexceptions on" so
    DBG_CONTROL_C is visible):
    
      (gdb) c
      Continuing.
      [New Thread 9940.0x1168]
      [New Thread 9940.0x5f8]
      gdb: Target exception MS_VC_EXCEPTION at 0x7ffa6638cf19
      gdb: Target exception MS_VC_EXCEPTION at 0x7ffa6638cf19
      [New Thread 9940.0x3d8]
      gdb: Target exception DBG_CONTROL_C at 0x7ffa6643ea6b   <<< Ctrl-C
    
      Thread 7 received signal SIGINT, Interrupt.             <<< new injected thread
      [Switching to Thread 9940.0x3d8]
      0x00007ffa6643ea6b in TlsGetValue () from /cygdrive/c/Windows/System32/KERNELBASE.dll
      (gdb)
    
    This new behavior is exactly the same as what you see with a MinGW GDB
    build.  Also, SIGINT reaching inferior first is what you get on Linux
    as well currently.
    
    I wrote an initial fix for this before I discovered that Cygwin
    downstream had a similar change, so I then combined the patches.  I am
    adding a Co-Authored-By for that reason.
    
    Co-Authored-By: Takashi Yano <takashi.yano@nifty.ne.jp>
    Approved-By: Tom Tromey <tom@tromey.com>
    Change-Id: I3a8c3355784c6a817dbd345ba9dac24be06c4b3f

Diff:
---
 gdb/inflow.c | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/gdb/inflow.c b/gdb/inflow.c
index a413957a1dd..3dd70b97fe5 100644
--- a/gdb/inflow.c
+++ b/gdb/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
@@ -345,11 +349,25 @@ child_terminal_inferior (struct target_ops *self)
 	     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
-	  result = tcsetpgrp (0, getpgid (inf->pid));
-#else
-	  result = tcsetpgrp (0, tinfo->process_group);
+	  pid_t pgrp = tinfo->process_group;
+#ifdef __CYGWIN__
+	  /* The Windows native target uses Win32 routines to run or
+	     attach to processes (CreateProcess / DebugActiveProcess),
+	     so a Cygwin inferior has a Windows PID, rather than a
+	     Cygwin PID.  We want to pass the Cygwin PID to Cygwin
+	     tcsetpgrp if we have a Cygwin inferior, so try to convert
+	     first.  If we have a non-Cygwin inferior, we'll end up
+	     passing down the WINPID to tcsetpgrp, stored in
+	     terminal_state::process_group.  tcsetpgrp still succeeds
+	     in that case, and it seems preferable to switch the
+	     foreground pgrp away from GDB, for consistency.  */
+	  pid_t cygpid = cygwin_internal (CW_WINPID_TO_CYGWIN_PID, inf->pid);
+	  if (cygpid <= cygwin_internal (CW_MAX_CYGWIN_PID))
+	    pgrp = getpgid (cygpid);
+#elif defined (HAVE_GETPGID)
+	  pgrp = getpgid (inf->pid);
 #endif
+	  result = tcsetpgrp (0, pgrp);
 	  if (result == -1)
 	    {
 #if 0