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, 20 insertions, 0 deletions

diff --git a/test/src/process-tests.el b/test/src/process-tests.el
index 5dbf441e8c2..0bb7ebe50a8 100644
--- a/test/src/process-tests.el
+++ b/test/src/process-tests.el
@@ -215,6 +215,26 @@
                                       (string-to-list "stdout\n")
                                       (string-to-list "stderr\n"))))))
 
+(ert-deftest make-process-w32-debug-spawn-error ()
+  "Check that debugger runs on `make-process' failure (Bug#33016)."
+  (skip-unless (eq system-type 'windows-nt))
+  (let* ((debug-on-error t)
+         (have-called-debugger nil)
+         (debugger (lambda (&rest _)
+                     (setq have-called-debugger t)
+                     ;; Allow entering the debugger later in the same
+                     ;; test run, before going back to the command
+                     ;; loop.
+                     (setq internal-when-entered-debugger -1))))
+    (should (eq :got-error ;; NOTE: `should-error' would inhibit debugger.
+                (condition-case-unless-debug ()
+                    ;; Emacs doesn't search for absolute filenames, so
+                    ;; the error will be hit in the w32 process spawn
+                    ;; code.
+                    (make-process :name "test" :command '("c:/No-Such-Command"))
+                  (error :got-error))))
+    (should have-called-debugger)))
+
 (ert-deftest make-process/file-handler/found ()
   "Check that the ‘:file-handler’ argument of ‘make-process’
 works as expected if a file name handler is found."