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[debugger] Work around a debugger issue when using the interpreter #15451

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Merged
rolfbjarne merged 9 commits into from
Jul 15, 2022
Merged

[debugger] Work around a debugger issue when using the interpreter #15451

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d05066a
fix debugger
thaystg Jul 11, 2022
359ec53
Merge branch 'main' into thays_debug_workaround
thaystg Jul 11, 2022
610c6f2
Merge branch 'main' into thays_debug_workaround
mandel-macaque Jul 11, 2022
86ceac3
Hide output, add comment
rolfbjarne Jul 12, 2022
88a834b
We only need the workaround for .NET.
rolfbjarne Jul 12, 2022
7bd541b
Keep calling mono_debug_init if workaround is not in effect.
rolfbjarne Jul 12, 2022
2f8e7c6
Add an escape hatch.
rolfbjarne Jul 13, 2022
9b78d77
NULL-terminate the argc array.
rolfbjarne Jul 13, 2022
718494f
Add a dummy argument.
rolfbjarne Jul 14, 2022
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37 changes: 34 additions & 3 deletions runtime/monovm-bridge.m
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Original file line number Diff line number Diff line change
Expand Up @@ -60,6 +60,12 @@
void
xamarin_bridge_initialize ()
{
#if DOTNET
bool use_mono_workaround = xamarin_init_mono_debug && getenv ("XAMARIN_SKIP_INTERPRETER_DEBUGGING_WORKAROUND") == 0;
#else
bool use_mono_workaround = false;
#endif

if (xamarin_register_modules != NULL)
xamarin_register_modules ();
DEBUG_LAUNCH_TIME_PRINT ("\tAOT register time");
Expand All @@ -69,9 +75,9 @@
DEBUG_LAUNCH_TIME_PRINT ("\tDebug init time");
#endif

if (xamarin_init_mono_debug)
if (xamarin_init_mono_debug && !use_mono_workaround)
mono_debug_init (MONO_DEBUG_FORMAT_MONO);

mono_install_assembly_preload_hook (xamarin_assembly_preload_hook, NULL);
mono_install_load_aot_data_hook (xamarin_load_aot_data, xamarin_free_aot_data, NULL);

Expand All @@ -85,7 +91,32 @@
mono_install_unhandled_exception_hook (xamarin_unhandled_exception_handler, NULL);
mono_install_ftnptr_eh_callback (xamarin_ftnptr_exception_handler);

mono_jit_init_version ("MonoTouch", "mobile");
const char *argc[] = {
"",
"--interp=-all",
"",
NULL,
};

if (use_mono_workaround) {
// This is a workaround for a runtime bug that prevents debugging from working properly.
// We call mono_main to disable interpreter optimizations when debugging, because
// mono's own interpreter initialization doesn't take into account that the debugger
// might be attached, and the subsequent optimizations can break the debugger.
// The stdout dance is to hide mono's "helpful" output telling us the command line
// arguments are incorrect (which they would be normally, but we're abusing mono_main
// to get the side effects).
fflush (stdout);
int originalStdout = dup (STDOUT_FILENO);
int devnull = open ("/dev/null", O_RDWR);
dup2 (devnull, STDOUT_FILENO);
mono_main (2, (char**)argc);
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@rolfbjarne rolfbjarne Jul 11, 2022

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Could you explain runtime-wise why this works around the problem?

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@thaystg thaystg Jul 11, 2022

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Yes, sure.
The issue on runtime side was that we are initializing interpreter before setting the debugger attributes. So when we execute this code: https://github.com/dotnet/runtime/blob/5529dc79e9f35d70e9d01318d31844984b860dd4/src/mono/mono/mini/interp/interp.c#L8121
mdb_optimizations was false, then we keep the default optimizations of the interpreter which was breaking the debugger experience.
In this workaround I'm passing --interp=-all in the parameters, so I'm disabling the interpreter optimizations and the debugger works well.

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@rolfbjarne rolfbjarne Jul 11, 2022

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Is this only a problem in .NET 6? Or legacy mono as well? (i.e. mono/mono's 2020-02 branch)

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@thaystg thaystg Jul 11, 2022 •
edited
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I'm almost sure that it's a problem only on .net 6, but I would ask someone to test to double check. It's really easy to reproduce:

public static void MethodMyMethod()
{
      var e2 = new Exception("hi thays");
      var e1 = e2;
      if (e1.Equals(e2))
	      Console.WriteLine("equals");
}

Adding a breakpoint on if statement and looking at locals, with the optimizations turned on we only see the content of variable e2, e1 is null.

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@samhouts samhouts Jul 12, 2022

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Is this going to have implications for hot reload on .NET MAUI?

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@thaystg thaystg Jul 12, 2022

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I don't think so. @lambdageek do you have any thoughts about this workaround?

fflush (stdout);
dup2 (originalStdout, STDOUT_FILENO);
} else {
mono_jit_init_version ("MonoTouch", "mobile");
}

/*
As part of mono initialization a preload hook is added that overrides ours, so we need to re-instate it here.
This is wasteful, but there's no way to manipulate the preload hook list except by adding to it.
Expand Down