Put single-step breakpoints on the bp_location chain

This patch makes single-step breakpoints "real" breakpoints on the
global location list.

There are several benefits to this:

- It removes the currently limitation that only 2 single-step
  breakpoints can be inserted.  See an example here of a discussion
  around a case that wants more than 2, possibly unbounded:

  https://sourceware.org/ml/gdb-patches/2014-03/msg00663.html

- makes software single-step work on read-only code regions.

  The logic to convert a software breakpoint to a hardware breakpoint
  if the memory map says the breakpoint address is in read only memory
  is in insert_bp_location.  Because software single-step breakpoints
  bypass all that go and straight to target_insert_breakpoint, we
  can't software single-step over read only memory.  This patch
  removes that limitation, and adds a test that makes sure that works,
  by forcing a code region to read-only with "mem LOW HIGH ro" and
  then stepping through that.

- Fixes PR breakpoints/9649

  This is an assertion failure in insert_single_step_breakpoint in
  breakpoint.c, because we may leave stale single-step breakpoints
  behind on error.

  The tests for stepping through read-only regions exercise the root
  cause of the bug, which is that we leave single-step breakpoints
  behind if we fail to insert any single-step breakpoint.  Deleting
  the single-step breakpoints in resume_cleanups,
  delete_just_stopped_threads_infrun_breakpoints, and
  fetch_inferior_event fixes this.  Without that, we'd no longer hit
  the assertion, as that code is deleted, but we'd instead run into
  errors/warnings trying to insert/remove the stale breakpoints on
  next resume.

- Paves the way to have multiple threads software single-stepping at
  the same time, leaving update_global_location_list to worry about
  duplicate locations.

- Makes the moribund location machinery aware of software single-step
  breakpoints, paving the way to enable software single-step on
  non-stop, instead of forcing serialized displaced stepping for all
  single steps.

- It's generaly cleaner.

  We no longer have to play games with single-step breakpoints
  inserted at the same address as regular breakpoints, like we
  recently had to do for 7.8.  See this discussion:

  https://sourceware.org/ml/gdb-patches/2014-06/msg00052.html.

Tested on x86_64 Fedora 20, on top of my 'single-step breakpoints on
x86' series.

gdb/
2014-10-15  Pedro Alves  <palves@redhat.com>

	PR breakpoints/9649
	* breakpoint.c (single_step_breakpoints, single_step_gdbarch):
	Delete array globals.
	(single_step_breakpoints): New global.
	(breakpoint_xfer_memory): Remove special handling for single-step
	breakpoints.
	(update_breakpoints_after_exec): Delete bp_single_step
	breakpoints.
	(detach_breakpoints): Remove special handling for single-step
	breakpoints.
	(breakpoint_init_inferior): Delete bp_single_step breakpoints.
	(bpstat_stop_status): Add comment.
	(bpstat_what, bptype_string, print_one_breakpoint_location)
	(adjust_breakpoint_address, init_bp_location): Handle
	bp_single_step.
	(new_single_step_breakpoint): New function.
	(set_momentary_breakpoint, bkpt_remove_location): Remove special
	handling for single-step breakpoints.
	(insert_single_step_breakpoint, single_step_breakpoints_inserted)
	(remove_single_step_breakpoints, cancel_single_step_breakpoints):
	Rewrite.
	(detach_single_step_breakpoints, find_single_step_breakpoint):
	Delete functions.
	(breakpoint_has_location_inserted_here): New function.
	(single_step_breakpoint_inserted_here_p): Rewrite.
	* breakpoint.h: Remove FIXME.
	(enum bptype) <bp_single_step>: New enum value.
	(insert_single_step_breakpoint): Update comment.
	* infrun.c (resume_cleanups)
	(delete_step_thread_step_resume_breakpoint): Remove single-step
	breakpoints.
	(fetch_inferior_event): Install a cleanup that removes infrun
	breakpoints.
	(switch_back_to_stepped_thread) <expect thread advanced also>:
	Clear step-over info.

gdb/testsuite/
2014-10-15  Pedro Alves  <palves@redhat.com>

	PR breakpoints/9649
	* gdb.base/breakpoint-in-ro-region.c (main): Add more instructions.
	* gdb.base/breakpoint-in-ro-region.exp
	(probe_target_hardware_step): New procedure.
	(top level): Probe hardware stepping and hardware breakpoint
	support.  Test stepping through a read-only region, with both
	"breakpoint auto-hw" on and off and both "always-inserted" on and
	off.

2 files changed, 113 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.base/breakpoint-in-ro-region.c b/gdb/testsuite/gdb.base/breakpoint-in-ro-region.c
index 696a67d..2a999b1 100644
--- a/gdb/testsuite/gdb.base/breakpoint-in-ro-region.c
+++ b/gdb/testsuite/gdb.base/breakpoint-in-ro-region.c
@@ -23,6 +23,15 @@ main (void)
   i = 0;
   i = 0;
   i = 0;
+  i = 0;
+  i = 0;
+  i = 0;
+  i = 0;
+  i = 0;
+  i = 0;
+  i = 0;
+  i = 0;
+  i = 0;
 
   return 0;
 }
diff --git a/gdb/testsuite/gdb.base/breakpoint-in-ro-region.exp b/gdb/testsuite/gdb.base/breakpoint-in-ro-region.exp
index 8eacefa..1991aa5 100644
--- a/gdb/testsuite/gdb.base/breakpoint-in-ro-region.exp
+++ b/gdb/testsuite/gdb.base/breakpoint-in-ro-region.exp
@@ -27,6 +27,28 @@ if ![runto main] {
 
 delete_breakpoints
 
+# Probe for hardware stepping.
+
+proc probe_target_hardware_step {} {
+    global gdb_prompt
+
+    set hw_step 0
+
+    gdb_test_no_output "set debug target 1"
+    set test "probe target hardware step"
+    gdb_test_multiple "si" $test {
+	-re "to_resume \\(\[^\r\n\]+, step, .*$gdb_prompt $" {
+	    set hw_step 1
+	    pass $test
+	}
+	-re "$gdb_prompt $" {
+	    pass $test
+	}
+    }
+    gdb_test "set debug target 0" "->to_log_command.*\\).*"
+    return $hw_step
+}
+
 # Get the bounds of a function, and write them to FUNC_LO (inclusive),
 # FUNC_HI (exclusive).  Return true on success and false on failure.
 proc get_function_bounds {function func_lo func_hi} {
@@ -108,6 +130,7 @@ proc get_next_insn {} {
     return $next
 }
 
+set hw_step [probe_target_hardware_step]
 
 if ![get_function_bounds "main" main_lo main_hi] {
     # Can't do the following tests if main's bounds are unknown.
@@ -140,3 +163,84 @@ gdb_test "p /x *(char *) $main_lo = 1" \
 gdb_test "break *$main_lo" \
     "Cannot insert breakpoint .*Cannot set software breakpoint at read-only address $main_lo.*" \
     "inserting software breakpoint in read-only memory fails"
+
+delete_breakpoints
+
+set supports_hbreak 0
+set test "probe hbreak support"
+gdb_test_multiple "hbreak *$main_lo" $test {
+    -re "You may have requested too many.*$gdb_prompt $" {
+	pass "$test (no support)"
+    }
+    -re "No hardware breakpoint support.*$gdb_prompt $" {
+	pass "$test (no support)"
+    }
+    -re "$gdb_prompt $" {
+	pass "$test (support)"
+	set supports_hbreak 1
+    }
+}
+
+delete_breakpoints
+
+# Check that the "auto-hw on/off" setting affects single-step
+# breakpoints as expected, by stepping through the read-only region.
+# If the target does hardware stepping, we won't exercise that aspect,
+# but we should be able to step through the region without seeing the
+# hardware breakpoint or read-only address errors.
+proc test_single_step { always_inserted auto_hw } {
+    global gdb_prompt
+    global decimal
+    global supports_hbreak
+    global hw_step
+
+    gdb_test_no_output "set breakpoint always-inserted $always_inserted"
+    gdb_test_no_output "set breakpoint auto-hw $auto_hw"
+
+    # Get the address of the current instruction so we know where SI is
+    # starting from.
+    set curr_insn [get_curr_insn]
+
+    # Get the address of the next instruction so we know where SI should
+    # land.
+    set next_insn [get_next_insn]
+
+    set test "step in ro region"
+    gdb_test_multiple "si" $test {
+	-re "Could not insert hardware breakpoints.*$gdb_prompt $" {
+	    gdb_assert {!$hw_step && $auto_hw == "on" && !$supports_hbreak} \
+		"$test (cannot insert hw break)"
+	}
+	-re "Cannot set software breakpoint at read-only address $next_insn.*$gdb_prompt $" {
+	    gdb_assert {!$hw_step && $auto_hw == "off"} \
+		"$test (cannot insert sw break)"
+	}
+	-re "^si\r\nNote: automatically using hardware breakpoints for read-only addresses\.\r\n${decimal}\[ \t\]+i = 0;\r\n$gdb_prompt $" {
+	    gdb_assert {!$hw_step && $auto_hw == "on" && $supports_hbreak} \
+		"$test (auto-hw)"
+	}
+	-re "^si\r\n${decimal}\[ \t\]+i = 0;\r\n$gdb_prompt $" {
+	    gdb_assert {$hw_step || ($auto_hw == "on" && $supports_hbreak)} \
+		"$test (no error)"
+	}
+    }
+
+    gdb_test "maint info breakpoints 0" \
+	"No breakpoint or watchpoint matching '0'\." \
+	"single-step breakpoint is not left behind"
+
+    # Confirm the thread really advanced.
+    if {$hw_step || ($auto_hw == "on" && $supports_hbreak)} {
+	gdb_test "p /x \$pc" " = $next_insn" "thread advanced"
+    } else {
+	gdb_test "p /x \$pc" " = $curr_insn" "thread did not advance"
+    }
+}
+
+foreach always_inserted {"off" "on"} {
+    foreach auto_hw {"off" "on"} {
+	with_test_prefix "always-inserted $always_inserted: auto-hw $auto_hw" {
+	    test_single_step $always_inserted $auto_hw
+	}
+    }
+}