Test gdb.threads/forking-threads-plus-breakpoint.exp with displaced stepping off

This exposes the internal error Don mentioned in PR19496:

  (1) internal error --  gdb/target.c:2713: internal-error: Can't determine the current address space of thread

More analysis here:

  https://sourceware.org/ml/gdb-patches/2016-01/msg00685.html

The (now kfailed) internal error looks like:

 continue &
 Continuing.
 (gdb) PASS: gdb.threads/forking-threads-plus-breakpoint.exp: cond_bp_target=1: detach_on_fork=on: displaced=off: continue &
 [New Thread 2846.2847]
 (...)
 [New Thread 2867.2867]
 /home/pedro/gdb/mygit/src/gdb/target.c:2723: internal-error: Can't determine the current address space of thread Thread 2846.2846

 A problem internal to GDB has been detected,
 further debugging may prove unreliable.
 Quit this debugging session? (y or n) KFAIL: gdb.threads/forking-threads-plus-breakpoint.exp: cond_bp_target=1: detach_on_fork=on: displaced=off: inferior 1 exited (GDB internal error) (PRMS: remote/19496)
 Resyncing due to internal error.

gdb/testsuite/ChangeLog:
2016-02-01  Pedro Alves  <palves@redhat.com>

	PR remote/19496
	* gdb.threads/forking-threads-plus-breakpoint.exp
	(displaced_stepping_supported): New global.
	(probe_displaced_stepping_support): New procedure.
	(do_test): Add 'displaced' parameter, and use it.
	(top level): Check for displaced stepping support.  Add displaced
	stepping on/off testing axis.

1 files changed, 58 insertions, 5 deletions

diff --git a/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp b/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp
index b5f7c21..6c72061 100644
--- a/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp
+++ b/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp
@@ -24,13 +24,49 @@ if {[build_executable "failed to prepare" $testfile $srcfile {debug pthreads}] =
     return -1
 }
 
+# Assume yes.
+set displaced_stepping_supported 1
+
+# "set displaced on" only tells gdb to use displaced stepping if
+# possible.  Probe for actual support.
+
+proc probe_displaced_stepping_support {} {
+    global displaced_stepping_supported
+    global binfile gdb_prompt
+
+    with_test_prefix "probe displaced-stepping support" {
+	clean_restart $binfile
+
+	gdb_test_no_output "set displaced on"
+	if ![runto_main] then {
+	    fail "Can't run to main"
+	    return 0
+	}
+
+	# We're stopped at the main breakpoint.  If displaced stepping is
+	# supported, we'll see related debug output while we step past
+	# that breakpoint.
+	gdb_test_no_output "set debug displaced 1"
+	gdb_test_multiple "next" "probe" {
+	    -re "displaced pc to.*$gdb_prompt $" {
+		pass "supported"
+	    }
+	    -re ".*$gdb_prompt $" {
+		set displaced_stepping_supported 0
+		pass "not supported"
+	    }
+	}
+    }
+}
+
 # The test proper.  If COND_BP_TARGET is true, then test with
 # conditional breakpoints evaluated on the target side, if possible.
 # DETACH_ON_FORK is used as value for the "set detach-on-fork"
 # setting.  If "on", this exercises GDB explicitly continuing the fork
 # child until exit.  If "off", this exercises GDB detaching the fork
-# child.
-proc do_test { cond_bp_target detach_on_fork } {
+# child.  DISPLACED indicates whether to use displaced stepping or
+# not.
+proc do_test { cond_bp_target detach_on_fork displaced } {
     global GDBFLAGS
     global srcfile testfile binfile
     global decimal gdb_prompt
@@ -48,6 +84,7 @@ proc do_test { cond_bp_target detach_on_fork } {
     }
 
     gdb_test_no_output "set detach-on-fork $detach_on_fork"
+    gdb_test_no_output "set displaced $displaced"
 
     gdb_test "break $linenum if zero == 1" \
 	"Breakpoint .*" \
@@ -63,6 +100,12 @@ proc do_test { cond_bp_target detach_on_fork } {
     set fork_count 0
     set ok 0
 
+    if {$displaced == "off"
+	&& [target_info exists gdb_protocol]
+	&& ([target_info gdb_protocol] == "remote"
+	    || [target_info gdb_protocol] == "extended-remote")} {
+	setup_kfail "remote/19496" *-*-*
+    }
     set test "inferior 1 exited"
     gdb_test_multiple "" $test {
 	-re "Inferior 1 \(\[^\r\n\]+\) exited normally" {
@@ -92,14 +135,24 @@ proc do_test { cond_bp_target detach_on_fork } {
 	"only inferior 1 left"
 }
 
+probe_displaced_stepping_support
+
 foreach_with_prefix cond_bp_target {1 0} {
     foreach_with_prefix detach_on_fork {"on" "off"} {
-	do_test $cond_bp_target $detach_on_fork
-
 	# Disable "off" for now.  The test does pass with
 	# detach-on-fork off (at the time of writing), but gdb seems
 	# to slow down quadratically as inferiors are created, and
 	# then the test takes annoyingly long to complete...
-	break
+	if {$detach_on_fork == "off"} {
+	    continue
+	}
+
+	foreach_with_prefix displaced {"on" "off"} {
+	    if {$displaced == "on" && !$displaced_stepping_supported} {
+		continue
+	    }
+
+	    do_test $cond_bp_target $detach_on_fork $displaced
+	}
     }
 }