fix call-with-atomic-output-file for pathless filename

Closes #1156

pkgs/racket-doc/scribblings/reference/filesystem.scrbl

@@ -296,7 +296,7 @@ exists---to the path @racket[new]. If the file or directory is not
 renamed successfully, the @exnraise[exn:fail:filesystem].
 
 This procedure can be used to move a file/directory to a different
-directory (on the same disk) as well as rename a file/directory within
+directory (on the same filesystem) as well as rename a file/directory within
 a directory. Unless @racket[exists-ok?]  is provided as a true value,
 @racket[new] cannot refer to an existing file or directory. Even if
 @racket[exists-ok?] is true, @racket[new] cannot refer to an existing

pkgs/racket-test-core/tests/racket/filelib.rktl

@@ -181,6 +181,44 @@
 (delete-file tempfile)
 (delete-file (make-lock-file-name tempfile))
 
+;;----------------------------------------------------------------------
+;; Atomic output
+
+(define (try-atomic-output fn)
+  (call-with-output-file*
+   fn
+   #:exists 'truncate
+   (lambda (o) (display "()" o)))
+  (define ts
+    (append
+     ;; Writers
+     (for/list ([i 10])
+       (thread (lambda ()
+                 (for ([j 100])
+                   (call-with-atomic-output-file
+                    fn
+                    (lambda (o tmp-path)
+                      (test (or (path-only fn) (current-directory))
+                            path-only tmp-path)
+                      (display "(" o)
+                      (flush-output o)
+                      (sync (system-idle-evt))
+                      (display ")" o)))))))
+     ;; Readers
+     (for/list ([i 10])
+       (thread (lambda ()
+                 (for ([j 100])
+                   (sync (system-idle-evt))
+                   (test '() call-with-input-file fn read)))))))
+  (for-each sync ts)
+  (delete-file fn))
+
+(try-atomic-output (make-temporary-file))
+;; The user's add-on directory should be writable and might be a
+;; different filesystem, so try that:
+(parameterize ([current-directory (find-system-path 'addon-dir)])
+  (try-atomic-output (format "atomic-output-~a" (current-inexact-milliseconds))))
+
 ;; ----------------------------------------
 
 (report-errs)

racket/collects/racket/file.rkt

@@ -213,7 +213,7 @@
       (delete-file path)))
   (let ([bp (current-break-parameterization)]
         [tmp-path (parameterize ([current-security-guard (or guard (current-security-guard))])
-                    (make-temporary-file "tmp~a" #f (path-only path)))]
+                    (make-temporary-file "tmp~a" #f (or (path-only path) (current-directory))))]
         [ok? #f])
     (dynamic-wind
      void