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windows: check FormatMessage result in getErrorMessage #3719

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windows: check FormatMessage result in getErrorMessage #3719

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24 changes: 18 additions & 6 deletions src/backend/common/module_loading_windows.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -29,14 +29,26 @@ LibHandle loadLibrary(const char* library_name) {
void unloadLibrary(LibHandle handle) { FreeLibrary(handle); }

string getErrorMessage() {
const char* lpMsgBuf;
DWORD dw = GetLastError();
char* lpMsgBuf = nullptr;
DWORD dw = GetLastError();

// FormatMessage only writes lpMsgBuf when it succeeds. On failure it
// returns 0 and leaves the pointer untouched, so it must be initialized
// and the result checked before it is dereferenced.
DWORD chars = FormatMessage(
FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM |
FORMAT_MESSAGE_IGNORE_INSERTS,
NULL, dw, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
reinterpret_cast<LPTSTR>(&lpMsgBuf), 0, NULL);

if (chars == 0 || lpMsgBuf == nullptr) {
return "Unknown error (system error code " + std::to_string(dw) + ")";
}

FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM |
FORMAT_MESSAGE_IGNORE_INSERTS,
NULL, dw, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
(LPTSTR)&lpMsgBuf, 0, NULL);
string error_message(lpMsgBuf);
// FORMAT_MESSAGE_ALLOCATE_BUFFER allocates with LocalAlloc; the caller
// owns the buffer and must release it.
LocalFree(lpMsgBuf);
return error_message;
}

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