Let debugger handle process spawn errors on w32 (Bug#33016)

Since child_setup() is called between block_input()...unblock_input(),
when an error is signaled the Lisp debugger is prevented from
starting.  Therefore, let the callers signal the error instead (which
they already do for non-w32 platforms, just the error message needs an
update).
* src/callproc.c (child_setup) [WINDOWSNT]: Don't call
report_file_error here.
(call_process) [WINDOWNT]:
* src/process.c (create_process) [WINDOWSNT]: Call report_file_errno
here instead, after the unblock_input() call, same as for !WINDOWSNT.
* src/lisp.h (CHILD_SETUP_ERROR_DESC): New preprocessor define.  Flip
the containing ifndef DOS_NT branches so that it's ifdef DOS_NT.
* src/eval.c (when_entered_debugger): Remove.
(syms_of_eval) <internal-when-entered-debugger>: Define it as a Lisp
integer variable instead.
(maybe_call_debugger): Update comment.
* test/src/process-tests.el (make-process-w32-debug-spawn-error):
* test/src/callproc-tests.el (call-process-w32-debug-spawn-error): New
tests.

1 files changed, 2 insertions, 5 deletions

diff --git a/src/callproc.c b/src/callproc.c
index a3d09609d7b..2cdf84d9a80 100644
--- a/src/callproc.c
+++ b/src/callproc.c
@@ -681,7 +681,7 @@ call_process (ptrdiff_t nargs, Lisp_Object *args, int filefd,
   unblock_input ();
 
   if (pid < 0)
-    report_file_errno ("Doing vfork", Qnil, child_errno);
+    report_file_errno (CHILD_SETUP_ERROR_DESC, Qnil, child_errno);
 
   /* Close our file descriptors, except for callproc_fd[CALLPROC_PIPEREAD]
      since we will use that to read input from.  */
@@ -1174,7 +1174,7 @@ exec_failed (char const *name, int err)
    executable directory by the parent.
 
    On GNUish hosts, either exec or return an error number.
-   On MS-Windows, either return a pid or signal an error.
+   On MS-Windows, either return a pid or return -1 and set errno.
    On MS-DOS, either return an exit status or signal an error.  */
 
 CHILD_SETUP_TYPE
@@ -1319,9 +1319,6 @@ child_setup (int in, int out, int err, char **new_argv, bool set_pgrp,
   /* Spawn the child.  (See w32proc.c:sys_spawnve).  */
   cpid = spawnve (_P_NOWAIT, new_argv[0], new_argv, env);
   reset_standard_handles (in, out, err, handles);
-  if (cpid == -1)
-    /* An error occurred while trying to spawn the process.  */
-    report_file_error ("Spawning child process", Qnil);
   return cpid;
 
 #else  /* not WINDOWSNT */