[gdb/tui] Fix segfault in tui_find_disassembly_address

PR29040 describes a FAIL for test-case gdb.threads/next-fork-other-thread.exp
and target board unix/-m32.

The FAIL happens due to the test executable running into an assert, which is
caused by a forked child segfaulting, like so:
...
 Program terminated with signal SIGSEGV, Segmentation fault.
 #0  0x00000000 in ?? ()
...

I tried to reproduce the segfault with exec next-fork-other-thread-fork, using
TUI layout asm.

I set a breakpoint at fork and ran to the breakpoint, and somewhere during the
following session I ran into a gdb segfault here in
tui_find_disassembly_address:
...
	  /* Disassemble forward.  */
	  next_addr = tui_disassemble (gdbarch, asm_lines, new_low, max_lines);
	  last_addr = asm_lines.back ().addr;
...
due to asm_lines being empty after the call to tui_disassemble, while
asm_lines.back () assumes that it's not empty.

I have not been able to reproduce that segfault in that original setting, I'm
not sure of the exact scenario (though looking back it probably involved
"set detach-on-fork off").

What likely happened is that I managed to reproduce PR29040, and TUI (attempted
to) display the disassembly for address 0, which led to the gdb segfault.

When gdb_print_insn encounters an insn it cannot print because it can't read
the memory, it throws a MEMORY_ERROR that is caught by tui_disassemble.

The specific bit that causes the gdb segfault is that if gdb_print_insn throws
a MEMORY_ERROR for the first insn in tui_disassemble, it returns an empty
asm_lines.

FWIW, I did manage to reproduce the gdb segfault as follows:
...
$ gdb -q \
    -iex "set pagination off" \
    /usr/bin/rustc \
    -ex "set breakpoint pending on" \
    -ex "b dl_main" \
    -ex run \
    -ex "up 4" \
    -ex "layout asm" \
    -ex "print \$pc"
  ...
<TUI>
  ...
$1 = (void (*)()) 0x1
(gdb)
...
Now press <up>, and the segfault triggers.

Fix the segfault by handling asm_lines.empty () results of tui_disassemble in
tui_find_disassembly_address.

I've written a unit test that exercises this scenario.

Tested on x86_64-linux.

Reviewed-by: Kevin Buettner <kevinb@redhat.com>

PR tui/30823
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30823

gdb/tui/tui-disasm.c

@@ -41,6 +41,8 @@
 #include "objfiles.h"
 #include "cli/cli-style.h"
 #include "tui/tui-location.h"
+#include "gdbsupport/selftest.h"
+#include "inferior.h"
 
 #include "gdb_curses.h"
 
@@ -203,6 +205,8 @@ tui_find_disassembly_address (struct gdbarch *gdbarch, CORE_ADDR pc, int from)
 	 instruction fails to disassemble we will take the address of the
 	 previous instruction that did disassemble as the result.  */
       tui_disassemble (gdbarch, asm_lines, pc, max_lines + 1);
+      if (asm_lines.empty ())
+	return pc;
       new_low = asm_lines.back ().addr;
     }
   else
@@ -244,6 +248,8 @@ tui_find_disassembly_address (struct gdbarch *gdbarch, CORE_ADDR pc, int from)
 
 	  /* Disassemble forward.  */
 	  next_addr = tui_disassemble (gdbarch, asm_lines, new_low, max_lines);
+	  if (asm_lines.empty ())
+	    break;
 	  last_addr = asm_lines.back ().addr;
 
 	  /* If disassembling from the current value of NEW_LOW reached PC
@@ -522,3 +528,36 @@ tui_disasm_window::display_start_addr (struct gdbarch **gdbarch_p,
   *gdbarch_p = m_gdbarch;
   *addr_p = m_start_line_or_addr.u.addr;
 }
+
+#if GDB_SELF_TEST
+namespace selftests {
+namespace tui {
+namespace disasm {
+
+static void
+run_tests ()
+{
+  if (current_inferior () != nullptr)
+    {
+      struct gdbarch *gdbarch = current_inferior ()->gdbarch;
+
+      /* Check that tui_find_disassembly_address robustly handles the case of
+	 being passed a PC for which gdb_print_insn throws a MEMORY_ERROR.  */
+      SELF_CHECK (tui_find_disassembly_address (gdbarch, 0, 1) == 0);
+      SELF_CHECK (tui_find_disassembly_address (gdbarch, 0, -1) == 0);
+    }
+}
+
+} /* namespace disasm */
+} /* namespace tui */
+} /* namespace selftests */
+#endif /* GDB_SELF_TEST */
+
+void _initialize_tui_disasm ();
+void
+_initialize_tui_disasm ()
+{
+#if GDB_SELF_TEST
+  selftests::register_test ("tui-disasm", selftests::tui::disasm::run_tests);
+#endif
+}