stacktrace

[AlexGuteniev]

Resolves #1447

Thanks @cbezault for suggestion to use <DbgEng.h>, @fsb4000 for making sure it works on Windows 7, @ben-craig for pointing out that <DbgEng.h> does not look in a path next to the module by default.

Before going forward with this, here are some open issues:

1. API family, thread safety, COM apartments safety

There are three API families, neither of which seems fully good:

DbgHelp

The classic one. Every function has the following remark:

All DbgHelp functions, such as this one, are single threaded. Therefore, calls from more than one thread to this function will likely result in unexpected behavior or memory corruption. To avoid this, you must synchronize all concurrent calls from more than one thread to this function.

This seem to exclude DbgHelp -- even though we can use locks internally, user may call the function bypassing the lock
On the good part, it does not have COM dependency.

DbgEng

<DbgEng.h> API is COM-like, but full COM. So still no COM dependency.

There's a problem with picking a .pdb next to image -- see below.

Also I've seen under a debugger that it uses <DbgHelp> underneath with locks. This means it is not thread safe too -- the client code may use DbgHelp directly.

There's a concert that the interfaces are documented as single-threaded and not usable via another thread. For now the problem is not seen, but if it is a problem, can be fixed by thread-local storage, also not caching these interfaces can be investigated.

DIA SDK

Uses COM. The usual usage of COM with CoInitialize[Ex] is problematic in STL - there are two incompatible COM modes, and if STL choices one, it will break programs which wants another (possibly in a subtle way).

There's information that NoRegCoCreate function can be used to avoid the need of CoInitialize[Ex]. I haven't tried yet. It is not a documented function, but it exists in DIA SDK headers (DIA SDK\include\diacreate.h) with a comment description.

DIA SDK is not a Windows component, but it is redistributable: https://docs.microsoft.com/en-us/visualstudio/releases/2022/redistribution#dia-sdk

4. Roll own .pdb parsing

Tedious, but full control over thread and COM safety.
If we are to go this route, probably should make a separate open source library that can be reused elsewhere (Boost.Stacktrace, libc++ std::stacktrace, LLVM ASan stack trace)

5. Use helper process

With this, DbgHelp will work fine. Though this won't be efficient

2. Looking up for .pdb next to image

As @ben-craig pointed out, seems like DbgEng does not work well when having .pdb in not exact path where it is expected. So having a .pdb next to EXE / DLL, but not in the same folder as where it was originally created makes symbol lookup failure.

Currently I have a workaround, module_symbols_load_from_module_dir function, but it is considered very harmful. The problems are:

• Added paths can accumulate infinitely
• Altering search path may cause loaded symbols to unload
  Storing previous search dir and restoring it later will eliminate the former problem by making the later problem worse.

Apparently, with DbgHelp looking up next to image is done by default, and its SymSetOptions has SYMOPT_IGNORE_IMAGEDIR option to explicitly disable this. But DbgEng symbol does not list it among its options.

Current resolution: use the workaround. Though I'd be more happy documenting the limitation and options to bypass it (like /PDBALTPATH:%_PDB% as @sylveon suggested).

3. Dll main safety

I cache pointers to <DbgEng.h> interfaces, removing them in atexit. for a DLL this is effectively in a DllMain. Though it is not safe to do most of thing in DllMain, I guess there are no other options. Not caching would be too inefficient, I think.
Need confirmation whether it is the way to go.

Current resolution: ignore the potential problem.

Information format

I made the following:

• description is like trace!f1+0x8a. This includes module name, function name, offset within function (hex). No params, as they are hard to get. The paper does not ask for module name and offset within function, but I think these are essential. Note that <DbgEng.h> also provides module name by default, so it actually takes some effort to strip it.
• source_file and source_line are obvious. Don't trim full path from source file if it is provided.
• the whole stacktrace_entry is file(line): description to be compatible with Visual Studio usual output. Don't report offset within line ( source_column) as the paper does not ask for it, and it is not very essential.
• finally stacktrace is entries separated by \n and prepended by n> to number entries starting from 0.

0> C:\Project\trace\st_try.cpp(5): P0881R7_stacktrace!innermost+0x35
1> C:\Project\trace\st_try.cpp(10): P0881R7_stacktrace!inner+0x17
2> C:\Project\trace\st_try.cpp(14): P0881R7_stacktrace!outer+0x17
3> C:\Project\trace\st_try.cpp(18): P0881R7_stacktrace!outermost+0x17
4> C:\Project\trace\st_try.cpp(22): P0881R7_stacktrace!main+0x17
5> d:\a01\_work\43\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl(79): P0881R7_stacktrace!invoke_main+0x39
6> d:\a01\_work\43\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl(288): P0881R7_stacktrace!__scrt_common_main_seh+0x12e
7> d:\a01\_work\43\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl(331): P0881R7_stacktrace!__scrt_common_main+0xe
8> d:\a01\_work\43\s\src\vctools\crt\vcstartup\src\startup\exe_main.cpp(17): P0881R7_stacktrace!mainCRTStartup+0xe
9> KERNEL32!BaseThreadInitThunk+0x14
10> ntdll!RtlUserThreadStart+0x21

@ben-craig points to Boost.Stacktrace has a different format, notably:

• module/offset information is first, this makes output more aligned, as this information is always available
• # symbol for entries umbering
• file.cpp:3 format of line numbering

This is similar to ASan output, like this:

    #0 0x7ff66557280f in innermost C:\Users\Aleksandr.Gutenev\source\repos\ConsoleApplication2\ConsoleApplication2\ConsoleApplication2.cpp:6
    #1 0x7ff665572844 in inner C:\Users\Aleksandr.Gutenev\source\repos\ConsoleApplication2\ConsoleApplication2\ConsoleApplication2.cpp:10
    #2 0x7ff665572864 in outer C:\Users\Aleksandr.Gutenev\source\repos\ConsoleApplication2\ConsoleApplication2\ConsoleApplication2.cpp:14
    #3 0x7ff665572884 in outermost C:\Users\Aleksandr.Gutenev\source\repos\ConsoleApplication2\ConsoleApplication2\ConsoleApplication2.cpp:19
    #4 0x7ff6655728a4 in main C:\Users\Aleksandr.Gutenev\source\repos\ConsoleApplication2\ConsoleApplication2\ConsoleApplication2.cpp:24
    #5 0x7ff6655736f8 in invoke_main d:\a01\_work\43\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:78
    #6 0x7ff6655735dd in __scrt_common_main_seh d:\a01\_work\43\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288
    #7 0x7ff66557349d in __scrt_common_main d:\a01\_work\43\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:330
    #8 0x7ff66557378d in mainCRTStartup d:\a01\_work\43\s\src\vctools\crt\vcstartup\src\startup\exe_main.cpp:16
    #9 0x7ffe4de97033 in BaseThreadInitThunk+0x13 (C:\WINDOWS\System32\KERNEL32.DLL+0x180017033)
    #10 0x7ffe4f8a2650 in RtlUserThreadStart+0x20 (C:\WINDOWS\SYSTEM32\ntdll.dll+0x180052650)

The drawback is that IDE does not understand this format when trying to click on entry. There's DevCom-1660805 for that. If it is taken, then probably this format is better.

Network symbols

Symbols can be loaded from network. Typically it is Microsoft Symbols Server, but could be any location where _NT_SYMBOL_PATH points to.

Should we load over network by default? Only when _NT_SYMBOL_PATH is like SRV*c:\temp\WinSymbols*http://msdl.microsoft.com/download/symbols; ? Or avoid network locations at all?

The primary concerns are:

• Long waits
• Security

[StephanTLavavej]

I pushed two commits:

• Call atexit() within if (!initialize_attempted).
  • lock_and_uninitialize() is highly robust to partial initialization, so we can register it immediately.
  • Note that if atexit() fails, we haven't initialized anything, so we don't need to call uninitialize() to clean up. Instead, we can immediately return false.
  • Finally, note that if atexit() fails, we leave initialize_attempted set to true (uninitialize() would unset it), so we will never attempt initialization again.
• Mark try_initialize() as [[nodiscard]]; its return value is important.
  • Also use [[nodiscard]] instead of _NODISCARD for dbg_eng_data, to be consistent with other occurrences in this source file.

[strega-nil-ms]

Mostly nits! Otherwise, this looks good :)

[strega-nil-ms]

Approved! If you want, you can also change to using resize if you like.

[strega-nil-ms]

@StephanTLavavej - I have not fully researched DbgEng.h; I feel comfortable with <stacktrace>, but don't necessarily fully know that stacktrace.cpp is completely correct. If you feel comfortable with stacktrace.cpp as it's written, please merge; else, I want someone to actually go fully through stacktrace.cpp.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

I have pushed an additional commit to fix the internal test harness.

[StephanTLavavej]

Thanks for implementing this major C++23 feature!

😻 🎉 ✨

[TylerMSFT]

@AlexGuteniev , @StephanTLavavej , it looked like there was a doc ask here. What are the details?

[AlexGuteniev]

#2779 documnent limits in my take on limits. Still want that to be reviewed by maintainers first