exists_non_stop_target: Avoid flushing frames

A following patch adds an exists_non_stop_target call in the
target_terminal routines, and that surprisingly caused a weird
regression / GDB crash:

 $ make check RUNTESTFLAGS="--target_board=native-extended-gdbserver" TESTS="gdb.base/signest.exp"
 ...
 configuring for gdbserver local testing (extended-remote)
 Using src/gdb/testsuite/config/extended-gdbserver.exp as tool-and-target-specific interface file.
 Running src/gdb/testsuite/gdb.base/signest.exp ...
 ERROR: GDB process no longer exists

Debugging the core, we see infinite recursion:

 (top-gdb) bt 20
 #0  0x0000561d6a1bfeff in frame_unwind_arch (next_frame=0x561d6b19f9c0) at src/gdb/frame.c:2950
 #1  0x0000561d6a1bfeb8 in get_frame_arch (this_frame=0x561d6b19f9c0) at src/gdb/frame.c:2939
 #2  0x0000561d6a1b989f in frame_unwind_find_by_frame (this_frame=0x561d6b19f9c0, this_cache=0x561d6b19f9d8) at src/gdb/frame-unwind.c:174
 #3  0x0000561d6a1bff04 in frame_unwind_arch (next_frame=0x561d6b19f9c0) at src/gdb/frame.c:2950
 #4  0x0000561d6a1bfeb8 in get_frame_arch (this_frame=0x561d6b19f9c0) at src/gdb/frame.c:2939
 #5  0x0000561d6a1b989f in frame_unwind_find_by_frame (this_frame=0x561d6b19f9c0, this_cache=0x561d6b19f9d8) at src/gdb/frame-unwind.c:174
 #6  0x0000561d6a1bff04 in frame_unwind_arch (next_frame=0x561d6b19f9c0) at src/gdb/frame.c:2950
 #7  0x0000561d6a1bfeb8 in get_frame_arch (this_frame=0x561d6b19f9c0) at src/gdb/frame.c:2939
 #8  0x0000561d6a1b989f in frame_unwind_find_by_frame (this_frame=0x561d6b19f9c0, this_cache=0x561d6b19f9d8) at src/gdb/frame-unwind.c:174
 #9  0x0000561d6a1bff04 in frame_unwind_arch (next_frame=0x561d6b19f9c0) at src/gdb/frame.c:2950
 #10 0x0000561d6a1bfeb8 in get_frame_arch (this_frame=0x561d6b19f9c0) at src/gdb/frame.c:2939
 #11 0x0000561d6a1b989f in frame_unwind_find_by_frame (this_frame=0x561d6b19f9c0, this_cache=0x561d6b19f9d8) at src/gdb/frame-unwind.c:174
 #12 0x0000561d6a1bff04 in frame_unwind_arch (next_frame=0x561d6b19f9c0) at src/gdb/frame.c:2950
 #13 0x0000561d6a1bfeb8 in get_frame_arch (this_frame=0x561d6b19f9c0) at src/gdb/frame.c:2939
 #14 0x0000561d6a1b989f in frame_unwind_find_by_frame (this_frame=0x561d6b19f9c0, this_cache=0x561d6b19f9d8) at src/gdb/frame-unwind.c:174
 #15 0x0000561d6a1bff04 in frame_unwind_arch (next_frame=0x561d6b19f9c0) at src/gdb/frame.c:2950
 #16 0x0000561d6a1bfeb8 in get_frame_arch (this_frame=0x561d6b19f9c0) at src/gdb/frame.c:2939
 #17 0x0000561d6a1b989f in frame_unwind_find_by_frame (this_frame=0x561d6b19f9c0, this_cache=0x561d6b19f9d8) at src/gdb/frame-unwind.c:174
 #18 0x0000561d6a1bff04 in frame_unwind_arch (next_frame=0x561d6b19f9c0) at src/gdb/frame.c:2950
 #19 0x0000561d6a1bfeb8 in get_frame_arch (this_frame=0x561d6b19f9c0) at src/gdb/frame.c:2939
 (More stack frames follow...)

 (top-gdb) bt -30
 #157054 0x0000561d6a1bfeb8 in get_frame_arch (this_frame=0x561d6b19f9c0) at src/gdb/frame.c:2939
 #157055 0x0000561d6a1b989f in frame_unwind_find_by_frame (this_frame=0x561d6b19f9c0, this_cache=0x561d6b19f9d8) at src/gdb/frame-unwind.c:174
 #157056 0x0000561d6a1bff04 in frame_unwind_arch (next_frame=0x561d6b19f9c0) at src/gdb/frame.c:2950
 #157057 0x0000561d6a1bfeb8 in get_frame_arch (this_frame=0x561d6b19f9c0) at src/gdb/frame.c:2939
 #157058 0x0000561d6a1b989f in frame_unwind_find_by_frame (this_frame=0x561d6b19f9c0, this_cache=0x561d6b19f9d8) at src/gdb/frame-unwind.c:174
 #157059 0x0000561d6a1bff04 in frame_unwind_arch (next_frame=0x561d6b19f9c0) at src/gdb/frame.c:2950
 #157060 0x0000561d6a1bbc65 in frame_unwind_pc (this_frame=0x561d6b19f9c0) at src/gdb/frame.c:970
 #157061 0x0000561d6a1bf54c in get_frame_pc (frame=0x561d6b19fa90) at src/gdb/frame.c:2625
 #157062 0x0000561d6a1bf63e in get_frame_address_in_block (this_frame=0x561d6b19fa90) at src/gdb/frame.c:2655
 #157063 0x0000561d6a0cae7f in dwarf2_frame_cache (this_frame=0x561d6b19fa90, this_cache=0x561d6b19faa8) at src/gdb/dwarf2/frame.c:1010
 #157064 0x0000561d6a0cb928 in dwarf2_frame_this_id (this_frame=0x561d6b19fa90, this_cache=0x561d6b19faa8, this_id=0x561d6b19faf0) at src/gdb/dwarf2/frame.c:1227
 #157065 0x0000561d6a1baf72 in compute_frame_id (fi=0x561d6b19fa90) at src/gdb/frame.c:588
 #157066 0x0000561d6a1bb16e in get_frame_id (fi=0x561d6b19fa90) at src/gdb/frame.c:636
 #157067 0x0000561d6a1bb224 in get_stack_frame_id (next_frame=0x561d6b19fa90) at src/gdb/frame.c:650
 #157068 0x0000561d6a26ecd0 in insert_hp_step_resume_breakpoint_at_frame (return_frame=0x561d6b19fa90) at src/gdb/infrun.c:7809
 #157069 0x0000561d6a26b88a in handle_signal_stop (ecs=0x7ffc67022830) at src/gdb/infrun.c:6428
 #157070 0x0000561d6a269d81 in handle_inferior_event (ecs=0x7ffc67022830) at src/gdb/infrun.c:5741
 #157071 0x0000561d6a265bd0 in fetch_inferior_event () at src/gdb/infrun.c:4120
 #157072 0x0000561d6a244c24 in inferior_event_handler (event_type=INF_REG_EVENT) at src/gdb/inf-loop.c:41
 #157073 0x0000561d6a435cc4 in remote_async_serial_handler (scb=0x561d6b4a8990, context=0x561d6b4a4c48) at src/gdb/remote.c:14403
 #157074 0x0000561d6a460bc5 in run_async_handler_and_reschedule (scb=0x561d6b4a8990) at src/gdb/ser-base.c:138
 #157075 0x0000561d6a460cae in fd_event (error=0, context=0x561d6b4a8990) at src/gdb/ser-base.c:189
 #157076 0x0000561d6a76a191 in handle_file_event (file_ptr=0x561d6b233ae0, ready_mask=1) at src/gdbsupport/event-loop.cc:575
 #157077 0x0000561d6a76a743 in gdb_wait_for_event (block=1) at src/gdbsupport/event-loop.cc:701
 #157078 0x0000561d6a7694ee in gdb_do_one_event () at src/gdbsupport/event-loop.cc:237
 #157079 0x0000561d6a2df16b in start_event_loop () at src/gdb/main.c:421
 #157080 0x0000561d6a2df2b6 in captured_command_loop () at src/gdb/main.c:481
 #157081 0x0000561d6a2e0d16 in captured_main (data=0x7ffc67022bd0) at src/gdb/main.c:1353
 #157082 0x0000561d6a2e0da8 in gdb_main (args=0x7ffc67022bd0) at src/gdb/main.c:1370
 #157083 0x0000561d69eb3d82 in main (argc=13, argv=0x7ffc67022cf8, envp=0x7ffc67022d68) at src/gdb/gdb.c:33

This was caused by exists_non_stop_target flushing the frame cache via
scoped_restore_current_thread/switch_to_thread, while we're in the
middle of unwinding.

Fix this by making exists_non_stop_target only switch the inferior,
like done in target_pass_ctrlc.

The annota1.exp change is necessary because we'd get a regression
otherwise:

  @@ -238,8 +238,6 @@ Continuing.

   \032\032breakpoints-invalid

  -\032\032frames-invalid
  -
   \032\032breakpoint 3

   Breakpoint 3,
  @@ -276,7 +274,7 @@ printf.c
   \032\032pre-prompt
   (gdb)
   \032\032prompt
  -PASS: gdb.base/annota1.exp: continue to printf
  +FAIL: gdb.base/annota1.exp: continue to printf

... because this patch avoids flushing the frame cache that lead to
that missing frames-invalid.  We still need to match frames-invalid
because against gdbserver + "maint set target non-stop on", some other
code path flushes the frame cache resulting in the annotation being
emitted anyway.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <pedro@palves.net>

	* target.c (exists_non_stop_target): Use
	scoped_restore_current_inferior and set_current_inferior instead
	of scoped_restore_current_thread / switch_to_inferior_no_thread.

Change-Id: I8402483ee755e64e54d8b7c4a67c177557f569bd

2 files changed, 11 insertions, 3 deletions

diff --git a/gdb/target.c b/gdb/target.c
index 6babfc5..63aa3e9 100644
--- a/gdb/target.c
+++ b/gdb/target.c
@@ -4390,11 +4390,18 @@ exists_non_stop_target ()
   if (target_is_non_stop_p ())
     return true;
 
-  scoped_restore_current_thread restore_thread;
+  /* We can get here quite deep in core code, e.g., from
+     target_terminal::inferior().  Avoid switching thread context or
+     anything that would communicate with the target (e.g., to fetch
+     registers), or flushing e.g., the frame cache, as we may end up
+     called from within the frame building code.  We just switch
+     inferior in order to be able to call through the
+     target_stack.  */
+  scoped_restore_current_inferior restore_inferior;
 
   for (inferior *inf : all_inferiors ())
     {
-      switch_to_inferior_no_thread (inf);
+      set_current_inferior (inf);
       if (target_is_non_stop_p ())
 	return true;
     }
diff --git a/gdb/testsuite/gdb.base/annota1.exp b/gdb/testsuite/gdb.base/annota1.exp
index 3689f49..0e28dc8 100644
--- a/gdb/testsuite/gdb.base/annota1.exp
+++ b/gdb/testsuite/gdb.base/annota1.exp
@@ -24,6 +24,7 @@ if ![target_can_use_run_cmd] {
 }
 
 set breakpoints_invalid "\r\n\032\032breakpoints-invalid\r\n"
+set frames_invalid "\r\n\032\032frames-invalid\r\n"
 
 #
 # test running programs
@@ -226,7 +227,7 @@ gdb_test_multiple "break printf" "break printf" {
 #
 # get to printf
 #
-set pat_begin "\r\n\032\032post-prompt\r\nContinuing.\r\n\r\n\032\032starting\r\n\r\n\032\032frames-invalid\r\n${breakpoints_invalid}\r\n\032\032frames-invalid\r\n"
+set pat_begin "\r\n\032\032post-prompt\r\nContinuing.\r\n\r\n\032\032starting\r\n${frames_invalid}${breakpoints_invalid}(${frames_invalid})?"
 set pat_adjust "warning: Breakpoint 3 address previously adjusted from $hex to $hex.\r\n"
 set pat_end "\r\n\032\032breakpoint 3\r\n\r\nBreakpoint 3, \r\n\032\032frame-begin 0 $hex\r\n\r\n(\032\032frame-address\r\n$hex\r\n\032\032frame-address-end\r\n in \r\n)*.*\032\032frame-function-name\r\n.*printf(@.*)?\r\n\032\032frame-args\r\n.*\032\032frame-end\r\n\r\n\032\032stopped\r\n$gdb_prompt$"