[WIN] Enhance threadsafety of DbgHelp - #754
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Sep 6, 2026
DbgHelp is single-threaded and the caller must serialize it, but TVMFFIBacktrace called it unguarded. An exception raised while another thread was formatting a backtrace crashed the process inside the reporter, so the original error was never printed. Guard every DbgHelp call with one mutex. Key the session on a duplicated process handle rather than GetCurrentProcess(), whose value is shared by every component, so another DbgHelp user cannot close our symbols. Initialize once and keep the session instead of doing it per call, which also removes module enumeration from the locked region. A failed initialization now yields a backtrace carrying only the caller's own frame instead of walking against a session that was never established.
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src/ffi/backtrace_win.cccalled DbgHelp unguarded, though DbgHelp issingle-threaded and requires the caller to serialize it. The process died while
formatting a backtrace for another thread's exception, so the original error was
never printed.
GetCurrentProcess()hasthe same value in every component, so another DbgHelp user in the process could
close our symbols. A fresh key also makes
SymInitialize's failureunambiguous, so its result can be checked.
SymInitialize/SymCleanupand with it module enumeration inside the lock.There is deliberately no destructor:
SymCleanupat static-destruction timewould reintroduce teardown at an unpredictable point relative to DLL unload.
A failed initialization now returns a backtrace with only the caller's own frame.
StackWalk64resolves through the session, so without one there is no unwindingat all.