gdbsupport: Add an event-pipe class.

This pulls out the implementation of an event pipe used to implement
target async support in both linux-low.cc (gdbserver) and linux-nat.c
(gdb).

This will be used to replace the existing event pipe in linux-low.cc
and linux-nat.c in future commits.

Co-Authored-By: Lancelot SIX <lsix@lancelotsix.com>

gdbsupport/Makefile.am

@@ -35,6 +35,10 @@ if SELFTEST
 selftest = selftest.cc
 endif
 
+if HAVE_PIPE_OR_PIPE2
+eventpipe = event-pipe.cc
+endif
+
 libgdbsupport_a_SOURCES = \
     agent.cc \
     btrace-common.cc \
@@ -74,6 +78,7 @@ libgdbsupport_a_SOURCES = \
     tdesc.cc \
     thread-pool.cc \
     xml-utils.cc \
+    ${eventpipe} \
     $(selftest)
 
 # Double-check that no defines are missing from our configury.

gdbsupport/Makefile.in

@@ -144,7 +144,8 @@ am__v_AR_0 = @echo "  AR      " $@;
 am__v_AR_1 = 
 libgdbsupport_a_AR = $(AR) $(ARFLAGS)
 libgdbsupport_a_LIBADD =
-@SELFTEST_TRUE@am__objects_1 = selftest.$(OBJEXT)
+@HAVE_PIPE_OR_PIPE2_TRUE@am__objects_1 = event-pipe.$(OBJEXT)
+@SELFTEST_TRUE@am__objects_2 = selftest.$(OBJEXT)
 am_libgdbsupport_a_OBJECTS = agent.$(OBJEXT) btrace-common.$(OBJEXT) \
 	buffer.$(OBJEXT) cleanups.$(OBJEXT) common-debug.$(OBJEXT) \
 	common-exceptions.$(OBJEXT) common-inferior.$(OBJEXT) \
@@ -160,7 +161,7 @@ am_libgdbsupport_a_OBJECTS = agent.$(OBJEXT) btrace-common.$(OBJEXT) \
 	safe-strerror.$(OBJEXT) scoped_mmap.$(OBJEXT) search.$(OBJEXT) \
 	signals.$(OBJEXT) signals-state-save-restore.$(OBJEXT) \
 	tdesc.$(OBJEXT) thread-pool.$(OBJEXT) xml-utils.$(OBJEXT) \
-	$(am__objects_1)
+	$(am__objects_1) $(am__objects_2)
 libgdbsupport_a_OBJECTS = $(am_libgdbsupport_a_OBJECTS)
 AM_V_P = $(am__v_P_@AM_V@)
 am__v_P_ = $(am__v_P_@AM_DEFAULT_V@)
@@ -338,7 +339,6 @@ pdfdir = @pdfdir@
 prefix = @prefix@
 program_transform_name = @program_transform_name@
 psdir = @psdir@
-runstatedir = @runstatedir@
 sbindir = @sbindir@
 sharedstatedir = @sharedstatedir@
 srcdir = @srcdir@
@@ -361,6 +361,7 @@ AM_CPPFLAGS = -I$(srcdir)/../include -I$(srcdir)/../gdb \
 AM_CXXFLAGS = $(WARN_CFLAGS) $(WERROR_CFLAGS)
 noinst_LIBRARIES = libgdbsupport.a
 @SELFTEST_TRUE@selftest = selftest.cc
+@HAVE_PIPE_OR_PIPE2_TRUE@eventpipe = event-pipe.cc
 libgdbsupport_a_SOURCES = \
     agent.cc \
     btrace-common.cc \
@@ -400,6 +401,7 @@ libgdbsupport_a_SOURCES = \
     tdesc.cc \
     thread-pool.cc \
     xml-utils.cc \
+    ${eventpipe} \
     $(selftest)
 
 all: config.h
@@ -482,6 +484,7 @@ distclean-compile:
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/environ.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/errors.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/event-loop.Po@am__quote@
+@AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/event-pipe.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/fileio.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/filestuff.Po@am__quote@
 @AMDEP_TRUE@@am__include@ @am__quote@./$(DEPDIR)/format.Po@am__quote@

gdbsupport/configure

@@ -626,6 +626,8 @@ LTLIBOBJS
 LIBOBJS
 WERROR_CFLAGS
 WARN_CFLAGS
+HAVE_PIPE_OR_PIPE2_FALSE
+HAVE_PIPE_OR_PIPE2_TRUE
 SELFTEST_FALSE
 SELFTEST_TRUE
 LTLIBIPT
@@ -10002,6 +10004,15 @@ else
 fi
 
 
+ if test x$ac_cv_func_pipe = xyes -o x$ac_cv_func_pipe2 = xyes ; then
+  HAVE_PIPE_OR_PIPE2_TRUE=
+  HAVE_PIPE_OR_PIPE2_FALSE='#'
+else
+  HAVE_PIPE_OR_PIPE2_TRUE='#'
+  HAVE_PIPE_OR_PIPE2_FALSE=
+fi
+
+
 # Check the return and argument types of ptrace.
 
 
@@ -10597,6 +10608,10 @@ if test -z "${SELFTEST_TRUE}" && test -z "${SELFTEST_FALSE}"; then
   as_fn_error $? "conditional \"SELFTEST\" was never defined.
 Usually this means the macro was only invoked conditionally." "$LINENO" 5
 fi
+if test -z "${HAVE_PIPE_OR_PIPE2_TRUE}" && test -z "${HAVE_PIPE_OR_PIPE2_FALSE}"; then
+  as_fn_error $? "conditional \"HAVE_PIPE_OR_PIPE2\" was never defined.
+Usually this means the macro was only invoked conditionally." "$LINENO" 5
+fi
 
 : "${CONFIG_STATUS=./config.status}"
 ac_write_fail=0

gdbsupport/configure.ac

@@ -53,6 +53,9 @@ GDB_AC_COMMON
 GDB_AC_SELFTEST
 AM_CONDITIONAL(SELFTEST, $enable_unittests)
 
+AM_CONDITIONAL(HAVE_PIPE_OR_PIPE2,
+   [test x$ac_cv_func_pipe = xyes -o x$ac_cv_func_pipe2 = xyes ])
+
 # Check the return and argument types of ptrace.
 GDB_AC_PTRACE
 

gdbsupport/event-pipe.cc

@@ -0,0 +1,101 @@
+/* Event pipe for GDB, the GNU debugger.
+
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "gdbsupport/common-defs.h"
+#include "gdbsupport/event-pipe.h"
+#include "gdbsupport/filestuff.h"
+
+#include <errno.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+event_pipe::~event_pipe ()
+{
+  if (is_open ())
+    close ();
+}
+
+/* See event-pipe.h.  */
+
+bool
+event_pipe::open ()
+{
+  if (is_open ())
+    return false;
+
+  if (gdb_pipe_cloexec (m_fds) == -1)
+    return false;
+
+  if (fcntl (m_fds[0], F_SETFL, O_NONBLOCK) == -1
+      || fcntl (m_fds[1], F_SETFL, O_NONBLOCK) == -1)
+    {
+      close ();
+      return false;
+    }
+
+  return true;
+}
+
+/* See event-pipe.h.  */
+
+void
+event_pipe::close ()
+{
+  ::close (m_fds[0]);
+  ::close (m_fds[1]);
+  m_fds[0] = -1;
+  m_fds[1] = -1;
+}
+
+/* See event-pipe.h.  */
+
+void
+event_pipe::flush ()
+{
+  int ret;
+  char buf;
+
+  do
+    {
+      ret = read (m_fds[0], &buf, 1);
+    }
+  while (ret >= 0 || (ret == -1 && errno == EINTR));
+}
+
+/* See event-pipe.h.  */
+
+void
+event_pipe::mark ()
+{
+  int ret;
+
+  /* It doesn't really matter what the pipe contains, as long we end
+     up with something in it.  Might as well flush the previous
+     left-overs.  */
+  flush ();
+
+  do
+    {
+      ret = write (m_fds[1], "+", 1);
+    }
+  while (ret == -1 && errno == EINTR);
+
+  /* Ignore EAGAIN.  If the pipe is full, the event loop will already
+     be awakened anyway.  */
+}

gdbsupport/event-pipe.h

@@ -0,0 +1,60 @@
+/* Event pipe for GDB, the GNU debugger.
+
+   Copyright (C) 2021 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef COMMON_EVENT_PIPE_H
+#define COMMON_EVENT_PIPE_H
+
+/* An event pipe used as a waitable file in the event loop in place of
+   some other event associated with a signal.  The handler for the
+   signal marks the event pipe to force a wakeup in the event loop.
+   This uses the well-known self-pipe trick.  */
+
+class event_pipe
+{
+public:
+  event_pipe() = default;
+  ~event_pipe();
+
+  DISABLE_COPY_AND_ASSIGN (event_pipe);
+
+  /* Create a new pipe.  */
+  bool open ();
+
+  /* Close the pipe.  */
+  void close ();
+
+  /* True if the event pipe has been opened.  */
+  bool is_open () const
+  { return m_fds[0] != -1; }
+
+  /* The file descriptor of the waitable file to use in the event
+     loop.  */
+  int event_fd () const
+  { return m_fds[0]; }
+
+  /* Flush the event pipe.  */
+  void flush ();
+
+  /* Put something in the pipe, so the event loop wakes up.  */
+  void mark ();
+private:
+  int m_fds[2] = { -1, -1 };
+};
+
+#endif /* COMMON_EVENT_PIPE_H */