Bail out of processing stop if hook-stop resumes target / changes context

This patch, relative to a tree with
https://sourceware.org/ml/gdb-patches/2015-08/msg00295.html, fixes
issues/crashes that trigger if something unexpected happens during a
hook-stop.

E.g., if the inferior disappears while running the hook-stop, we hit
failed assertions:

 (gdb) define hook-stop
 Type commands for definition of "hook-stop".
 End with a line saying just "end".
 >kill
 >end
 (gdb) si
 Kill the program being debugged? (y or n) [answered Y; input not from terminal]
 /home/pedro/gdb/mygit/build/../src/gdb/thread.c:88: internal-error: inferior_thread: Assertion `tp' failed.
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.
 Quit this debugging session? (y or n)

I noticed that if a hook-stop issues a synchronous execution command,
we print the same stop event twice:

 (gdb) define hook-stop
 Type commands for definition of "hook-stop".
 End with a line saying just "end".
 >si
 >end
 (gdb) si
 0x000000000040074a      42          args[i] = 1; /* Init value.  */  <<<<<<< once
 0x000000000040074a      42          args[i] = 1; /* Init value.  */  <<<<<<< twice
 (gdb)

In MI:

 *stopped,reason="end-stepping-range",frame={addr="0x000000000040074a",func="main",args=[],file="threads.c",fullname="/home/pedro/gdb/tests/threads.c",line="42"},thread-id="1",stopped-threads="all",core="0"
 *stopped,reason="end-stepping-range",frame={addr="0x000000000040074a",func="main",args=[],file="threads.c",fullname="/home/pedro/gdb/tests/threads.c",line="42"},thread-id="1",stopped-threads="all",core="0"
 (gdb)

The fix has GDB stop processing the event if the context changed.  I
don't expect people to be doing crazy things from the hook-stop.
E.g., it gives me headaches to try to come up a proper behavior for
handling a thread change from a hook-stop... (E.g., imagine the
hook-stop does thread N; step, with scheduler-locing on).  I think the
most important bit here is preventing crashes.

The patch adds a new hook-stop.exp test that covers the above and also
merges in the old hook-stop-continue.exp and hook-stop-frame.exp into
the same framework.

gdb/ChangeLog:
2015-09-14  Pedro Alves  <palves@redhat.com>

	* infrun.c (current_stop_id): New global.
	(get_stop_id, new_stop_id): New functions.
	(fetch_inferior_event): Handle normal_stop proceeding the target.
	(struct stop_context): New.
	(save_stop_context, release_stop_context_cleanup)
	(stop_context_changed): New functions.
	(normal_stop): Return true if the hook-stop changes the stop
	context.
	* infrun.h (get_stop_id): Declare.
	(normal_stop): Now returns int.  Add documentation.

gdb/testsuite/ChangeLog:
2015-09-14  Pedro Alves  <palves@redhat.com>

	* gdb.base/hook-stop-continue.c: Delete.
	* gdb.base/hook-stop-continue.exp: Delete.
	* gdb.base/hook-stop-frame.c: Delete.
	* gdb.base/hook-stop-frame.exp: Delete.
	* gdb.base/hook-stop.c: New file.
	* gdb.base/hook-stop.exp: New file.

6 files changed, 183 insertions, 142 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 17b264d..d0bf156 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,12 @@
+2015-09-14  Pedro Alves  <palves@redhat.com>
+
+	* gdb.base/hook-stop-continue.c: Delete.
+	* gdb.base/hook-stop-continue.exp: Delete.
+	* gdb.base/hook-stop-frame.c: Delete.
+	* gdb.base/hook-stop-frame.exp: Delete.
+	* gdb.base/hook-stop.c: New file.
+	* gdb.base/hook-stop.exp: New file.
+
 2015-09-14  Pierre-Marie de Rodat  <derodat@adacore.com>
 
 	* gdb.ada/access_to_packed_array.exp: New testcase.
diff --git a/gdb/testsuite/gdb.base/hook-stop-continue.c b/gdb/testsuite/gdb.base/hook-stop-continue.c
deleted file mode 100644
index 47090fc..0000000
--- a/gdb/testsuite/gdb.base/hook-stop-continue.c
+++ /dev/null
@@ -1,42 +0,0 @@
-/* This testcase is part of GDB, the GNU debugger.
-
-   Copyright 2008-2015 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/>.  */
-
-int
-funbreak (int i)
-{
-  i = i * 2; /* set breakpoint here */
-  i = i + 10;
-  return i;
-}
-
-int
-func (int i)
-{
-  return i * 2;
-}
-
-int
-main (int argc, char **argv, char **envp)
-{
-  func (1);
-  func (2);
-  func (3);
-  func (4);
-  funbreak (5);
-
-  return 0;
-}
diff --git a/gdb/testsuite/gdb.base/hook-stop-continue.exp b/gdb/testsuite/gdb.base/hook-stop-continue.exp
deleted file mode 100644
index 06c2fe6..0000000
--- a/gdb/testsuite/gdb.base/hook-stop-continue.exp
+++ /dev/null
@@ -1,52 +0,0 @@
-# Copyright 2008-2015 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/>.
-
-standard_testfile .c
-
-if { [prepare_for_testing ${testfile}.exp "${testfile}" "${testfile}.c" {debug nowarnings}] } {
-    return -1
-}
-
-if ![runto_main] then {
-    perror "Couldn't run to main"
-}
-
-set bp_location [gdb_get_line_number "set breakpoint here"]
-
-gdb_test "break $bp_location" \
-    "Breakpoint.*at.* file .*$srcfile, line $bp_location\\." \
-    "breakpoint line number"
-
-gdb_test "print \$do_continue = 1" "1"
-
-send_gdb "define hook-stop\n"
-gdb_expect {
-  -re "Type commands for definition of \"hook-stop\".\r\nEnd with a line saying just \"end\".\r\n>$"\
-          {send_gdb "if \$do_continue\nset \$do_continue = 0\ncontinue\nend\nend\n"
-           gdb_expect {
-             -re "$gdb_prompt $"\
-                       {pass "define hook-stop command"}
-             timeout {fail "(timeout) define hook-stop command"}
-           }
-          }
-  -re "$gdb_prompt $"\
-          {fail "define hook-stop command"}
-  timeout {fail "(timeout) define hook-stop command"}
-}
-
-gdb_test "next" "Breakpoint.*funbreak \\(i=5\\) at .*:$bp_location\r\n$bp_location.*set breakpoint here \\*/" \
-    "next triggering hook-stop"
-
-gdb_test "next" "i = i \\+ 10;" "next no hook-stop"
diff --git a/gdb/testsuite/gdb.base/hook-stop-frame.exp b/gdb/testsuite/gdb.base/hook-stop-frame.exp
deleted file mode 100644
index 5b443ff..0000000
--- a/gdb/testsuite/gdb.base/hook-stop-frame.exp
+++ /dev/null
@@ -1,48 +0,0 @@
-# Copyright 2009-2015 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/>.
-
-standard_testfile
-
-if { [prepare_for_testing ${testfile}.exp "${testfile}" $srcfile {debug nowarnings}] } {
-    return -1
-}
-
-if ![runto_main] then {
-    perror "Couldn't run to main"
-}
-
-set bp_location [gdb_get_line_number "set breakpoint here"]
-
-gdb_test "break func" \
-    "Breakpoint.*at.* file .*$srcfile.*\\." \
-    "breakpoint line number"
-
-set test "define hook-stop command"
-gdb_test_multiple "define hook-stop" "$test" {
-    -re "Type commands for definition of \"hook-stop\".\r\nEnd with a line saying just \"end\".\r\n>$" {
-	gdb_test "echo \"Hello.\"\nend" "" "$test"
-    }
-}
-
-set test "hook-stop runs before frame print"
-gdb_test_multiple "continue" "$test" {
-    -re "\"Hello\\.\"\r\nBreakpo.*func.*set breakpoint here.*${gdb_prompt} $" {
-	pass $test
-    }
-
-    -re "Breakpo.*func.*set breakpoint here.*\"Hello\\.\".*${gdb_prompt} $" {
-	fail $test
-    }
-}
diff --git a/gdb/testsuite/gdb.base/hook-stop-frame.c b/gdb/testsuite/gdb.base/hook-stop.c
index 8dbb8f1..4152f28 100644
--- a/gdb/testsuite/gdb.base/hook-stop-frame.c
+++ b/gdb/testsuite/gdb.base/hook-stop.c
@@ -21,11 +21,17 @@ func (void)
   int a;
 
   a = 1; /* set breakpoint here */
+  a = 2;
 }
 
 int
 main (int argc, char **argv)
 {
+  int i;
+
+  i = 1;
+  i = 2;
+  i = 3;
   func ();
 
   return 0;
diff --git a/gdb/testsuite/gdb.base/hook-stop.exp b/gdb/testsuite/gdb.base/hook-stop.exp
new file mode 100644
index 0000000..cdee7db
--- /dev/null
+++ b/gdb/testsuite/gdb.base/hook-stop.exp
@@ -0,0 +1,168 @@
+# Copyright 2009-2015 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/>.
+
+standard_testfile
+
+if { [build_executable ${testfile}.exp "${testfile}" $srcfile {debug nowarnings}] } {
+    return -1
+}
+
+# Define the hook-stop that runs COMMANDS.
+
+proc define_hook_stop {commands} {
+    set test "define hook-stop command"
+    gdb_test_multiple "define hook-stop" "$test" {
+	-re "Type commands for definition of \"hook-stop\".\r\nEnd with a line saying just \"end\".\r\n>$" {
+	    gdb_test "$commands\nend" "" "$test"
+	}
+    }
+}
+
+# Restart GDB, run to main, set a breakpoint, and define a hook-stop
+# that runs COMMANDS.  If running to main fails, this returns to the
+# caller's caller directly.
+
+proc setup {commands} {
+    global srcfile binfile
+
+    clean_restart $binfile
+
+    if ![runto_main] then {
+	fail "Can't run to main"
+	return -code return
+    }
+
+    gdb_test "break func" \
+	"Breakpoint.*at.* file .*$srcfile.*\\." \
+	"breakpoint line number"
+
+    define_hook_stop $commands
+}
+
+# Check that the hook-stop runs before the frame is printed.
+
+proc hook_stop_before_frame {} {
+    with_test_prefix "hook-stop runs before frame print" {
+	global gdb_prompt
+
+	setup "echo \"Hello.\""
+
+	set test "run hook-stop"
+	gdb_test_multiple "continue" "$test" {
+	    -re "\"Hello\\.\"\r\nBreakpo.*func.*set breakpoint here.*${gdb_prompt} $" {
+		pass $test
+	    }
+
+	    -re "Breakpo.*func.*set breakpoint here.*\"Hello\\.\".*${gdb_prompt} $" {
+		fail $test
+	    }
+	}
+    }
+}
+
+# Check that GDB gracefully handles the case of the inferior dying
+# while running the hook-stop.
+
+proc hook_stop_kill {} {
+    with_test_prefix "hook-stop kills inferior" {
+	global gdb_prompt
+
+	setup "kill"
+
+	gdb_test_no_output "set confirm off"
+
+	set test "run hook-stop"
+	gdb_test_multiple "continue" "$test" {
+	    -re "Continuing.\r\n${gdb_prompt} $" {
+		pass $test
+	    }
+	}
+
+	gdb_test "info threads" "No threads.*"
+    }
+}
+
+# Check that GDB gracefully handles the case of the hook-stop
+# continuing the inferior in the foreground.
+
+proc hook_stop_continue_fg {} {
+    with_test_prefix "hook-stop runs continue" {
+	global gdb_prompt
+
+	setup "if \$do_continue\nset \$do_continue = 0\ncontinue\nend"
+
+	gdb_test "print \$do_continue = 1" " = 1"
+
+	gdb_test "next" "Breakpoint.*func \\(\\) at .*set breakpoint here \\*/" \
+	    "next triggering hook-stop"
+
+	gdb_test "next" "a = 2;" "next no hook-stop"
+    }
+}
+
+# Check that GDB gracefully handles the case of the hook-stop
+# continuing the inferior in the background.
+
+proc hook_stop_continue_bg {} {
+    with_test_prefix "hook-stop runs continue&" {
+	global gdb_prompt
+
+	setup "if \$do_continue\nset \$do_continue = 0\ncontinue&\nend"
+
+	gdb_test "print \$do_continue = 1" " = 1"
+
+	set test "run hook-stop"
+	gdb_test_multiple "continue" "$test" {
+	    -re "Continuing.\r\n.*${gdb_prompt} " {
+		pass $test
+	    }
+	}
+
+	set test "inferior exits normally"
+	gdb_test_multiple "" "$test" {
+	    -re "exited normally" {
+		pass $test
+	    }
+	}
+	gdb_test "info threads" "No threads.*"
+    }
+}
+
+# Check that GDB gracefully handles the case of the hook-stop running
+# "next".  GDB used to print the stop event twice.
+
+proc hook_stop_next {} {
+    with_test_prefix "hook-stop runs next" {
+	global gdb_prompt
+
+	setup "next"
+
+	set test "run hook-stop"
+	gdb_test_multiple "continue" "$test" {
+	    -re "a = 2.*a = 2${gdb_prompt} $" {
+		fail $test
+	    }
+	    -re "a = 2.*${gdb_prompt} $" {
+		pass $test
+	    }
+	}
+    }
+}
+
+hook_stop_before_frame
+hook_stop_kill
+hook_stop_continue_fg
+hook_stop_continue_bg
+hook_stop_next