Bug 28988

Created attachment 14031 [details]
A transcript of commands that produced the crash

After updating my every-day-use GDB to commit

commit c9178f285acf19e066be8367185d52837161b0a2 (HEAD -> master, origin/master)
Author: Alan Modra <amodra@gmail.com>
Date:   Thu Mar 17 20:05:39 2022 +1030

I'm experiencing GDB to crash on assertion failure in value_copy (value.c:1731).
This is triggered when I single-step through code while having (my, custom) GDB/MI
frontent displaying local variables with python pretty printers enabled. 

For example: doing this in frontend triggers the assertion failure:

    file gdb/testsuite/outputs/gdb.python/py-prettyprint/py-prettyprint-cxx
    source gdb/testsuite/outputs/gdb.python/py-prettyprint/py-prettyprint.py
    b add_item(container*, int)
    r
    dis 1
    fin
    n
    n (few more times until it crashes)

This is the backtrace when it crashes:

    /home/jv/Proj...atches/gdb/gdb [stopped]
      Thread  1 [stopped] "gdb"
         0 0x0000556077ADABD0 internal_error (errors.cc:51)
         1 0x00005560776148A7 value_copy (value.c:1731)
         2 0x0000556077360028 gdbpy_get_varobj_pretty_printer (py-prettyprint.c:655)
         3 0x0000556077620E60 install_default_visualizer (varobj.c:1056)
         4 0x00005560776210E9 install_new_value_visualizer (varobj.c:1127)
         5 0x00005560776218B2 install_new_value (varobj.c:1339)
         6 0x000055607761F6FB varobj_create (varobj.c:378)
         7 0x0000556077245C05 mi_cmd_var_create (mi-cmd-var.c:132)
         8 0x0000556077249F2B mi_command_mi::invoke (mi-cmds.c:58)
         9 0x0000556077264F15 mi_cmd_execute (mi-main.c:2091)
        10 0x000055607726450C captured_mi_execute_command (mi-main.c:1823)
        11 0x000055607726498A mi_execute_command (mi-main.c:1947)
        12 0x000055607724D21B mi_execute_command_wrapper (mi-interp.c:285)
        13 0x000055607724D2A4 mi_execute_command_input_handler (mi-interp.c:314)
        14 0x00005560770D7149 gdb_readline_no_editing_callback (event-top.c:878)
        15 0x00005560770D6948 stdin_event_handler (event-top.c:524)
        16 0x0000556077ADB57E gdb_wait_for_event (event-loop.cc:700)
        17 0x0000556077ADB7FB gdb_wait_for_event (event-loop.cc:596)
        18 0x0000556077ADB7FB gdb_do_one_event (event-loop.cc:237)
        19 0x000055607721BA1B start_event_loop (main.c:421)
        20 0x000055607721BB3F captured_command_loop (main.c:481)
        21 0x000055607721D535 captured_main (main.c:1351)
        22 0x000055607721D59B gdb_main (main.c:1366)
        23 0x0000556076DA7E93 main (gdb.c:32)
      Thread  2 [stopped] "gdb worker"
      Thread  3 [stopped] "gdb worker"
      Thread  4 [stopped] "gdb worker"
      Thread  5 [stopped] "gdb worker"


Unfortunately, so far I was unable to reproduce this outside my frontend. I tried
to simulate what the frontend does (see attached file), but running GDB like


   gdb -i mi < crash-in-value_copy-reproducer.txt


does not trigger it (arguably, the frontend issues silly / unnecessary MI commands, but still
it should not crash GDB - I would think :-).

If I revert commit


commit 5f8ab46bc6918efb678deb5956c033e466afe301
Author: Simon Marchi <simon.marchi@polymtl.ca>
Date:   Mon Jan 31 15:57:58 2022 -0500

    gdb: constify parameter of value_copy

Everything seems to work just fine for me. I'm not at all familiar with this
part of the GDB code so I do not know whether this change is the real culprit 
or not, let alone to explain why.