[Bug driver/107007] New: libiberty's win32_spawn error handling is poor

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107007

            Bug ID: 107007
           Summary: libiberty's win32_spawn error handling is poor
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: driver
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---
              Host: *-*-mingw*

As described in
https://gcc.gnu.org/pipermail/gcc-help/2022-September/141906.html the
pex_win32_exec_child function just assumes that all errors are ENOENT:

  /* Create the child process.  */  
  pid = win32_spawn (executable, (flags & PEX_SEARCH) != 0,
                     argv, env, dwCreationFlags, &si, &pi);
  if (pid == (pid_t) -1)
    pid = spawn_script (executable, argv, env, dwCreationFlags,
                        &si, &pi);
  if (pid == (pid_t) -1)
    {
      *err = ENOENT;
      *errmsg = "CreateProcess";
    }


This gives users incorrect information when something goes wrong for a reason
that isn't "No such file or directory".

Maybe win32_spawn should use the Win32 GetLastError() function to get a Windows
Error Code and then translate that to an errno value, and store it in errno. Or
ensure that GetLastError will still be valid for the caller of win32_spawn and
spawn_script to retrieve.

libstdc++-v3/src/c++11/system_error.cc already has code to do that translation
(so maybe it should move to libiberty, and then reuse that in libstdc++).