Improve process launch comments for Windows

[amccarth]

Summary

The existing comment about over-allocating the command line was incorrect. The contents of the command line may be changed, but it's not necessary to over allocate. The changes will be limited to the existing contents of the string (e.g., by replacing spaces with L'\0' to tokenize the command line).

Also added a comment explaining a possible cause of failure to save the next programmer some time when they try to debug a 64-bit process from a 32-bit LLDB.

[amccarth]

Forgot to include context in original diff.

[labath]

Looks fine to me. If this "The request is not supported" error is something that makes it's way to the user, and the error code is reasonably unambiguous (it +/- only happens when debugging 64 bit process from 32bit lldb), you may want to consider changing the error message to something more understandable.

[amccarth]

Thanks. I really don't know what other types of conditions may trigger that "request is not supported" message, so I'm going to shy away from trying to make it more specific.

[amccarth]

Closed with r355121.

lldb/source/Host/windows/ProcessLauncherWindows.cpp

ProcessLauncherWindows::LaunchProcess(const ProcessLaunchInfo &launch_info,
   executable = launch_info.GetExecutableFile().GetPath();
   GetFlattenedWindowsCommandString(launch_info.GetArguments(), commandLine);
 
   std::wstring wexecutable, wcommandLine, wworkingDirectory;
   llvm::ConvertUTF8toWide(executable, wexecutable);
   llvm::ConvertUTF8toWide(commandLine, wcommandLine);
   llvm::ConvertUTF8toWide(launch_info.GetWorkingDirectory().GetCString(),
                           wworkingDirectory);
+  // If the command line is empty, it's best to pass a null pointer to tell
+  // CreateProcessW to use the executable name as the command line.  If the
+  // command line is not empty, its contents may be modified by CreateProcessW.
+  WCHAR *pwcommandLine = wcommandLine.empty() ? nullptr : &wcommandLine[0];
 
-  wcommandLine.resize(PATH_MAX); // Needs to be over-allocated because
-                                 // CreateProcessW can modify it
   BOOL result = ::CreateProcessW(
-      wexecutable.c_str(), &wcommandLine[0], NULL, NULL, TRUE, flags, env_block,
+      wexecutable.c_str(), pwcommandLine, NULL, NULL, TRUE, flags, env_block,
       wworkingDirectory.size() == 0 ? NULL : wworkingDirectory.c_str(),
       &startupinfo, &pi);
 
   if (!result) {
     // Call GetLastError before we make any other system calls.
     error.SetError(::GetLastError(), eErrorTypeWin32);
+    // Note that error 50 ("The request is not supported") will occur if you
+    // try debug a 64-bit inferior from a 32-bit LLDB.
   }
 
   if (result) {
     // Do not call CloseHandle on pi.hProcess, since we want to pass that back
     // through the HostProcess.
     ::CloseHandle(pi.hThread);
   }