stepi/nexti: skip signal handler if "handle nostop" signal arrives

I noticed that "si" behaves differently when a "handle nostop" signal
arrives while the step is in progress, depending on whether the
program was stopped at a breakpoint when "si" was entered.
Specifically, in case GDB needs to step off a breakpoint, the handler
is skipped and the program stops in the next "mainline" instruction.
Otherwise, the "si" stops in the first instruction of the signal
handler.

I was surprised the testsuite doesn't catch this difference.  Turns
out gdb.base/sigstep.exp covers a bunch of cases related to stepping
and signal handlers, but does not test stepi nor nexti, only
step/next/continue.

My first reaction was that stopping in the signal handler was the
correct thing to do, as it's where the next user-visible instruction
that is executed is.  I considered then "nexti" -- a signal handler
could be reasonably considered a subroutine call to step over, it'd
seem intuitive to me that "nexti" would skip it.

But then, I realized that signals that arrive while a plain/line
"step" is in progress _also_ have their handler skipped.  A user might
well be excused for being confused by this, given:

  (gdb) help step
  Step program until it reaches a different source line.

And the signal handler's sources will be in different source lines,
after all.

I think that having to explain that "stepi" steps into handlers, (and
that "nexti" wouldn't according to my reasoning above), while "step"
does not, is a sign of an awkward interface.

E.g., if a user truly is interested in stepping into signal handlers,
then it's odd that she has to either force the signal to "handle
stop", or recall to do "stepi" whenever such a signal might be
delivered.  For that use case, it'd seem nicer to me if "step" also
stepped into handlers.

This suggests to me that we either need a global "step-into-handlers"
setting, or perhaps better, make "handle pass/nopass stop/nostop
print/noprint" have have an additional axis - "handle
stepinto/nostepinto", so that the user could configure whether
handlers for specific signals should be stepped into.

In any case, I think it's simpler (and thus better) for all step
commands to behave the same.  This commit thus makes "si/ni" skip
handlers for "handle nostop" signals that arrive while the command was
already in progress, like step/next do.

To be clear, nothing changes if the program was stopped for a signal,
and the user enters a stepping command _then_ -- GDB still steps into
the handler.  The change concerns signals that don't cause a stop and
that arrive while the step is in progress.

Tested on x86_64 Fedora 20, native and gdbserver.

gdb/
2014-10-27  Pedro Alves  <palves@redhat.com>

	* infrun.c (handle_signal_stop): Also skip handlers when a random
	signal arrives while handling a "stepi" or a "nexti".  Set the
	thread's 'step_after_step_resume_breakpoint' flag.

gdb/doc/
2014-10-27  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Continuing and Stepping): Add cross reference to
	info on stepping and signal handlers.
	(Signals): Explain stepping and signal handlers.  Add context
	index entry, and cross references.

gdb/testsuite/
2014-10-27  Pedro Alves  <palves@redhat.com>

	* gdb.base/sigstep.c (dummy): New global.
	(main): Issue a couple writes to the new global.
	* gdb.base/sigstep.exp (get_next_pc, test_skip_handler): New
	procedures.
	(skip_over_handler): Use test_skip_handler.
	(top level): Call skip_over_handler for stepi and nexti too.
	(breakpoint_over_handler): Use test_skip_handler.
	(top level): Call breakpoint_over_handler for stepi and nexti too.

2 files changed, 47 insertions, 15 deletions

diff --git a/gdb/testsuite/gdb.base/sigstep.c b/gdb/testsuite/gdb.base/sigstep.c
index aa2384a..cc69184 100644
--- a/gdb/testsuite/gdb.base/sigstep.c
+++ b/gdb/testsuite/gdb.base/sigstep.c
@@ -24,6 +24,7 @@
 #include <errno.h>
 
 static volatile int done;
+static volatile int dummy;
 
 static void
 handler (int sig)
@@ -74,8 +75,10 @@ main ()
 	      return 1;
 	    }
 	}
-      /* Wait.  */
-      while (!done);
+      /* Wait.  Issue a couple writes to a dummy volatile var to be
+	 reasonably sure our simple "get-next-pc" logic doesn't
+	 stumble on branches.  */
+      dummy = 0; dummy = 0; while (!done);
       done = 0;
     }
   return 0;
diff --git a/gdb/testsuite/gdb.base/sigstep.exp b/gdb/testsuite/gdb.base/sigstep.exp
index 184d46e..53152b8 100644
--- a/gdb/testsuite/gdb.base/sigstep.exp
+++ b/gdb/testsuite/gdb.base/sigstep.exp
@@ -269,9 +269,38 @@ proc skip_to_handler_entry { i } {
     gdb_test "clear *handler" ".*" "$prefix; clear handler"
 }
 
-skip_to_handler_entry step
-skip_to_handler_entry next
-skip_to_handler_entry continue
+foreach cmd {"stepi" "nexti" "step" "next" "continue"} {
+    skip_to_handler_entry $cmd
+}
+
+# Get the address of where a single-step should land.
+
+proc get_next_pc {test} {
+    global gdb_prompt
+    global hex
+
+    set next ""
+    gdb_test_multiple "x/2i \$pc" $test {
+	-re "$hex .*:\[^\r\n\]+\r\n\[ \]+($hex).*\.\r\n$gdb_prompt $" {
+	    set next $expect_out(1,string)
+	    pass $test
+	}
+    }
+
+    return $next
+}
+
+# Test that the command skipped over the handler.
+
+proc test_skip_handler {prefix i} {
+    if {$i == "stepi" || $i == "nexti"} {
+	set next_pc [get_next_pc "$prefix; get next PC"]
+	gdb_test "$i" "dummy = 0.*" "$prefix; performing $i"
+	gdb_test "p /x \$pc" " = $next_pc" "$prefix; advanced"
+    } else {
+	gdb_test "$i" "done = 0.*" "$prefix; performing $i"
+    }
+}
 
 # Try stepping when there's a signal pending but no breakpoints.
 # Should skip the handler advancing to the next line.
@@ -295,13 +324,13 @@ proc skip_over_handler { i } {
 
     # Make the signal pending
     sleep 1
-    
-    gdb_test "$i" "done = 0.*" "$prefix; performing $i"
+
+    test_skip_handler $prefix $i
 }
 
-skip_over_handler step
-skip_over_handler next
-skip_over_handler continue
+foreach cmd {"stepi" "nexti" "step" "next" "continue"} {
+    skip_over_handler $cmd
+}
 
 # Try stepping when there's a signal pending, a pre-existing
 # breakpoint at the current instruction, and a breakpoint in the
@@ -385,7 +414,7 @@ breakpoint_to_handler_entry continue
 
 # Try stepping when there's a signal pending, and a pre-existing
 # breakpoint at the current instruction, and no breakpoint in the
-# handler.  Should advance to the next line.
+# handler.  Should advance to the next line/instruction.
 
 proc breakpoint_over_handler { i } {
     global gdb_prompt
@@ -409,10 +438,10 @@ proc breakpoint_over_handler { i } {
     # Make the signal pending
     sleep 1
     
-    gdb_test "$i" "done = 0.*" "$prefix; performing $i"
+    test_skip_handler $prefix $i
     gdb_test "clear $infinite_loop" ".*" "$prefix; clear infinite loop"
 }
 
-breakpoint_over_handler step
-breakpoint_over_handler next
-breakpoint_over_handler continue
+foreach cmd {"stepi" "nexti" "step" "next" "continue"} {
+    breakpoint_over_handler $cmd
+}