fix: make scripts more viable under Cygwin

Don't let the perfect be the enemy of slightly better.

Swish scripts have been broken under Cygwin, but we can at least try to
make make them work in some typical cases under Cygwin.

- don't rely on OSTYPE; bash doesn't export it by default
- be careful since installing Git Bash changes Cygwin's /tmp
- assume a Cygwin usertemp mount for /tmp
- use registry to get root directory

src/swish/app.ss

@@ -132,6 +132,36 @@
                      (software-version 'swish))])])
       (printf "~a~@[ Version ~a~]~@[ (~a)~]\n" name version revision)))
 
+  (meta-cond
+   [windows?
+    (define (split-path p n)
+      (let ([rest (path-rest p)])
+        (if (or (= n 0) (equal? p rest))
+            (list p)
+            (cons (path-first p) (split-path (path-rest p) (- n 1))))))
+    (define (rewrite-path script)
+      ;; Cygwin paths start with /.
+      ;; By default, MSYS2 and Git Bash automatically convert to Windows paths.
+      (match (split-path script 3)
+        [("/" "cygdrive" ,drive ,path)
+         (path-combine (string-append drive ":") path)]
+        [("/" "tmp" . ,more)
+         ;; assume Cygwin's fstab sets a usertemp mount for /tmp
+         (apply path-combine (osi_get_temp_directory) more)]
+        [("/" . ,_)
+         (let ([root (get-registry "HKEY_LOCAL_MACHINE\\Software\\Cygwin\\setup\\rootdir")])
+           (and root (path-combine root (path-rest script))))]
+        [,_ #f]))
+    (define (resolve script)
+      (if (file-exists? script)
+          script
+          (let ([new-path (rewrite-path script)])
+            (if (and new-path (file-exists? new-path))
+                new-path
+                script))))]
+   [else
+    (define (resolve script) script)])
+
   (define (run)
     (let* ([opt (parse-command-line-arguments cli)]
            [files (or (opt 'files) '())])
@@ -164,7 +194,7 @@
             (for-each load filenames)
             (new-cafe)))]
        [else                            ; script
-        (let ([script-file (car files)]
+        (let ([script-file (resolve (car files))]
               [cmdline (command-line-arguments)])
           (parameterize ([command-line cmdline]
                          [command-line-arguments (cdr cmdline)]