libpqxx: thread_local storage in strconv.cxx crashes under MSys.

Use of dumb_stringstream in conjunction with have_thread_local=true in a multi-threaded program compiled using MinGW/GCC under MSys causes a segfault on exit:

(gdb) bt
#0  0x6f4e39ee in ?? () from C:\msys64\mingw32\bin\libstdc++-6.dll
#1  0x6f4f3b16 in ?? () from C:\msys64\mingw32\bin\libstdc++-6.dll
#2  0x00e81775 in (anonymous namespace)::dumb_stringstream<double>::~dumb_stringstream (this=0x31b17ac, __in_chrg=<optimized out>,
    __vtt_parm=<optimized out>)
    at C:/Work/BUILDS/apollo/PostgreSQL_XX/PostgreSQL_XX-src/src/strconv.cxx:480
#3  0x00e8abe5 in run_dtor_list (ptr=<synthetic pointer>)
    at C:/_/M/mingw-w64-crt-git/src/mingw-w64/mingw-w64-crt/crt/tls_atexit.c:59
#4  tls_callback (hDllHandle=0xe60000, dwReason=3, lpReserved=0x0)
    at C:/_/M/mingw-w64-crt-git/src/mingw-w64/mingw-w64-crt/crt/tls_atexit.c:155
#5  0x77072976 in ntdll!RtlIpv6AddressToStringA ()
   from C:\Windows\SysWOW64\ntdll.dll
#6  0x7704dd22 in ntdll!RtlActivateActivationContextUnsafeFast ()
   from C:\Windows\SysWOW64\ntdll.dll
#7  0x7704f141 in ntdll!RtlRbInsertNodeEx ()
   from C:\Windows\SysWOW64\ntdll.dll
#8  0x7704f7be in ntdll!LdrShutdownThread ()
   from C:\Windows\SysWOW64\ntdll.dll
#9  0x28600952 in ?? ()
#10 0x00000000 in ?? ()

Overriding “have_thread_local” to false prevents the crash.

The program I reproduce this with is a test program where the code under test is writing a double to a database in one thread, and the test is reading the database in another thread.

Sorry to just dump this on you but I don’t really have time for further investigation. I suspect thread_local is just broken with MinGW/GCC/MSys and that the fix is to detect this build environment in the CMake/configure scripts. E.g. something like:

if (not MSYS)
try_compile(
	PQXX_HAVE_THREAD_LOCAL
	${PROJECT_BINARY_DIR}
	SOURCES ${PROJECT_SOURCE_DIR}/config-tests/thread_local.cxx)
endif()