GetLastError() (with and without debugger)

[Maurizio Cimadamore]

Hi Manuel,
thanks for submitting this issue.

I think your hunch is probably correct - something inside the JDK is 
resetting the value of LastError.

In Hotspot code I see some abstractions to preserve the LastError value 
(os_windows.cpp):

// A number of wrappers for more frequently used system calls, to add 
standard logging. struct PreserveLastError {
   const DWORD v;
   PreserveLastError() : v(::GetLastError()) {}
   ~PreserveLastError() { ::SetLastError(v); }
};


And this is used in a number of OS-specific function calls, to avoid 
polluting the last error value.

That said, when you are running with a debugger, especially inside an 
IDE (which might add its own hooks), I think most bets are off, as the 
debugger might indeed perform additional native calls which might not 
preserve lastError correctly.

One experiment would be to try and debugging using jdb - so that at 
least we'd rule the IDE out, and see if the issue is still there. If 
that's the case we'll try to reach out to somebody more intimate with 
architetcure of JPDA, to see if that's something that can be addressed 
(perhaps in a way similar to what hotspot code seems to be already doing).

Thanks
Maurizio




On 12/08/2022 10:21, Manuel Bleichenbacher wrote:
> Thanks for the work on project Panama. It's an exciting technology. 
> I'm using it to make native operating system services available to Java.
>
> On Windows I've run into an issue. This C/C++ Windows code:
>
>     BOOL res = WriteFile(INVALID_HANDLE_VALUE, NULL, 0, NULL, NULL);
>     DWORD err = GetLastError();
>     printf("WriteFile result: %d, GetLastError result: %d\n", res, err);
>
> prints (as expected):
>
>     WriteFile result: 0, GetLastError result: 6
>
> 6 is the value of the constant ERROR_INVALID_HANDLE.
>
> Using panama, I've translated the code to Java. The result is:
>
> Without debugger, the output is the same. Everything is ok.
>
> With the debugger, the result is incorrect:
>
>     WriteFile result: 0, GetLastError result: 0
>
> This is incorrect. WriteFile() indicates an error, but GetLastError() 
> returns 0 (= NO_ERROR). Could it be that the debugger calls another 
> Windows API function between those two functions, resetting the last 
> error?
>
> In my project that's a major issue. Since the software behaves 
> incorrectly with the debugger, the software can no longer be debugged. 
> This doesn't just affect it when debugging this particular piece of 
> code but anytime this code is run in a debugging session.
>
> Is there something I'm not doing incorrectly? Or is there a fix or 
> workaround?
>
> Here's the Java code:
>
> import java.lang.foreign.*; import java.lang.invoke.MethodHandle; import static java.lang.foreign.MemoryAddress.NULL; import static java.lang.foreign.ValueLayout.ADDRESS; import static java.lang.foreign.ValueLayout.JAVA_INT; public class WinApi {
>      static final MethodHandleWriteFile$Func; static final MethodHandleGetLastError$Func; static final MemoryAddressINVALID_HANDLE_VALUE = MemoryAddress.ofLong(-1); static {
>          var linker = Linker.nativeLinker(); var lookup = SymbolLookup.libraryLookup("Kernel32", MemorySession.global()); WriteFile$Func = linker.downcallHandle(
>                  lookup.lookup("WriteFile").get(), FunctionDescriptor.of(JAVA_INT, ADDRESS, ADDRESS, JAVA_INT, ADDRESS, ADDRESS)
>          ); GetLastError$Func = linker.downcallHandle(
>                  lookup.lookup("GetLastError").get(), FunctionDescriptor.of(JAVA_INT)
>          ); }
>
>      public static void main(String[] args) {
>          var res =WriteFile(INVALID_HANDLE_VALUE, NULL, 0, NULL, NULL); var err =GetLastError(); System.out.printf("WriteFile result: %d, GetLastError result: %d\n", res, err); }
>
>      static int WriteFile(MemoryAddress hFile, MemoryAddress lpBuffer, int nNumberOfBytesToWrite, MemoryAddress lpNumberOfBytesWritten, MemoryAddress lpOverlapped) {
>          try {
>              return (int)WriteFile$Func.invokeExact((Addressable)hFile, (Addressable)lpBuffer, nNumberOfBytesToWrite, (Addressable)lpNumberOfBytesWritten, (Addressable)lpOverlapped); }catch (Throwable e) {
>              throw new RuntimeException(e); }
>      }
>
>      static int GetLastError() {
>          try {
>              return (int)GetLastError$Func.invokeExact(); }catch (Throwable e) {
>              throw new RuntimeException(e); }
>      }
> }
>
> Cheers
> Manuel
>
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