Fix inferior deadlock with "target remote | CMD"

Comparing test results between

  --target_board=native-gdbserver
  --target_board=native-stdio-gdbserver

I noticed that gdb.base/bigcore.exp is failing with native-stdio-gdbserver:

  Running src/gdb/testsuite/gdb.base/bigcore.exp ...
  FAIL: gdb.base/bigcore.exp: continue (timeout)
  ...

The problem is that:

  1. When debugging with "target remote | CMD", the inferior's
     stdout/stderr streams are connected to a pipe.

  2. The bigcore.c program prints a lot to the screen before it
     reaches the breakpoint location that the "continue" shown above
     wants to reach.

  3. GDB is not flushing the inferior's output pipe while the inferior
     is running.

  4. The pipe becomes full.

  5. The inferior thus deadlocks.

The bug is #3 above, which is what this commit fixes.  A new test is
added, that specifically exercises this scenario.  The test fails
before the fix, and passes after, and gdb.base/bigcore.exp also starts
passing.

gdb/ChangeLog:
2017-10-19  Pedro Alves  <palves@redhat.com>

	* ser-base.c (ser_base_read_error_fd): Delete the file handler if
	async.
	(handle_error_fd): New function.
	(ser_base_async): Add/delete an event loop file handler for
	error_fd.

gdb/testsuite/ChangeLog:
2017-10-19  Pedro Alves  <palves@redhat.com>

	* gdb.base/long-inferior-output.c: New file.
	* gdb.base/long-inferior-output.exp: New file.

gdb/ChangeLog

@@ -1,3 +1,11 @@
+2017-10-19  Pedro Alves  <palves@redhat.com>
+
+	* ser-base.c (ser_base_read_error_fd): Delete the file handler if
+	async.
+	(handle_error_fd): New function.
+	(ser_base_async): Add/delete an event loop file handler for
+	error_fd.
+
 2017-10-19  Pedro Alves  <palves@redhat.com>
 
 	* xml-support.c (xml_fetch_content_from_file): Don't read in

gdb/ser-base.c

@@ -288,6 +288,8 @@ ser_base_read_error_fd (struct serial *scb, int close_fd)
 	  if (s == 0 && close_fd)
 	    {
 	      /* End of file.  */
+	      if (serial_is_async_p (scb))
+		delete_file_handler (scb->error_fd);
 	      close (scb->error_fd);
 	      scb->error_fd = -1;
 	      break;
@@ -313,6 +315,17 @@ ser_base_read_error_fd (struct serial *scb, int close_fd)
     }
 }
 
+/* Event-loop callback for a serial's error_fd.  Flushes any error
+   output we might have.  */
+
+static void
+handle_error_fd (int error, gdb_client_data client_data)
+{
+  serial *scb = (serial *) client_data;
+
+  ser_base_read_error_fd (scb, 0);
+}
+
 /* Read a character with user-specified timeout.  TIMEOUT is number of
    seconds to wait, or -1 to wait forever.  Use timeout of 0 to effect
    a poll.  Returns char if successful.  Returns SERIAL_TIMEOUT if
@@ -589,6 +602,9 @@ ser_base_async (struct serial *scb,
 	fprintf_unfiltered (gdb_stdlog, "[fd%d->asynchronous]\n",
 			    scb->fd);
       reschedule (scb);
+
+      if (scb->error_fd != -1)
+	add_file_handler (scb->error_fd, handle_error_fd, scb);
     }
   else
     {
@@ -607,5 +623,8 @@ ser_base_async (struct serial *scb,
 	  delete_timer (scb->async_state);
 	  break;
 	}
+
+      if (scb->error_fd != -1)
+	delete_file_handler (scb->error_fd);
     }
 }

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2017-10-19  Pedro Alves  <palves@redhat.com>
+
+	* gdb.base/long-inferior-output.c: New file.
+	* gdb.base/long-inferior-output.exp: New file.
+
 2017-11-18  Keith Seitz  <keiths@redhat.com>
 
 	* gdb.cp/cpexprs.cc (base) <operator fluff const* const*>: New

gdb/testsuite/gdb.base/long-inferior-output.c

@@ -0,0 +1,38 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2017 Free Software Foundation, Inc.
+
+   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 <stdio.h>
+
+static size_t total_bytes = 0;
+
+int
+main ()
+{
+  int i = 0;
+
+  /* When testing with "target remote |", stdout is a pipe, and thus
+     block buffered by default.  Force it to be line buffered.  */
+  setvbuf (stdout, NULL, _IOLBF, 0);
+
+  /* This outputs > 70 KB which is larger than the default pipe buffer
+     size on most systems (typically 16 KB or 64 KB).  */
+  for (i = 0; i < 3000; i++)
+    total_bytes += printf ("this is line number %d\n", i);
+
+  printf ("total bytes written = %u\n", (unsigned) total_bytes);
+  return 0; /* printing done */
+}

gdb/testsuite/gdb.base/long-inferior-output.exp

@@ -0,0 +1,75 @@
+# Copyright 2017 Free Software Foundation, Inc.
+
+# 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/>.
+
+# When debugging with "target remote |", the inferior's output is
+# connected to a pipe, and if GDB doesn't flush the pipe while the
+# inferior is running and the pipe becomes full, then the inferior
+# deadlocks:
+#
+#  1. User sets breakpoint, and types "continue"
+#
+#  2. Inferior prints to stdout/stderr before reaching breakpoint
+#     location.
+#
+#  3. The output pipe becomes full, so the inferior blocks forever in
+#     the printf/write call.
+#
+#  4. The breakpoint is never reached.
+
+if [target_info exists gdb,noinferiorio] {
+    verbose "Skipping because of noinferiorio."
+    return
+}
+
+standard_testfile
+
+if [prepare_for_testing "failed to prepare" $testfile {} {debug}] {
+    return -1
+}
+
+if { ![runto_main] } then {
+    fail "run to main"
+    return
+}
+
+set printing_done_line [gdb_get_line_number "printing done"]
+gdb_test "break $printing_done_line" ".*" "set breakpoint after printing"
+
+send_gdb "continue\n"
+
+set expected_lines 3000
+set more 1
+set i 0
+while {$more} {
+    set more 0
+    gdb_expect {
+	-i $inferior_spawn_id
+	-ex "this is line number $i" {
+	    incr i
+           if {$i != $expected_lines} {
+		set more 1
+	    }
+	}
+    }
+}
+
+gdb_assert {$i == $expected_lines} "saw all lines"
+
+set test "breakpoint reached"
+gdb_test_multiple "" $test {
+    -re "Breakpoint .* main .*$srcfile:$printing_done_line.*$gdb_prompt $" {
+	pass $test
+    }
+}