rewrite fake-nmea to not need socat

pkgs/maps/fake-nmea.fnl

@@ -1,10 +1,72 @@
-(local { : sleep } (require :posix.unistd))
+(local { : sleep : close } (require :posix.unistd))
+(local sock (require :posix.sys.socket))
+(local poll (require :posix.poll))
+(local { : view }  (require :fennel))
+
+(fn handle-client [fd event]
+  (case event
+    {:HUP true}
+    (do
+      (print fd " disconnected")
+      (close fd)
+      (values fd nil))
+    {:IN true}
+    (do
+      (sock.recv fd 1024)               ; discard input and carry on
+      nil)))
+
+(fn accept-new-connection [server-sock event]
+  (case event
+    {:IN true}
+    (do
+      (print "new connection")
+      (let [connected (sock.accept server-sock)]
+        (print connected)
+        (values connected handle-client)))
+    {:HUP true}
+    (do
+      (print "HUP on server socket")
+      (os.exit 1))
+    _
+    (values nil nil)))
+
+(fn server-socket [path]
+  (let [s (assert (sock.socket sock.AF_UNIX sock.SOCK_STREAM 0))
+        sa { :family sock.AF_UNIX
+             :path path
+             }]
+    (assert (sock.bind s sa))
+    (assert (sock.listen s 10))
+    s))
+
+(fn read-with-rewind [handle]
+  (let [(s err) (handle:read "L")]
+    (if s
+        s
+        (if (and (not err) (handle:seek "set" 0)) ; eof
+            (read-with-rewind handle)
+            (error err)))))
+
+(fn write-sentences [handle outputs]
+  (var done false)
+  (while (not done)
+    (let [l (read-with-rewind handle)]
+      (if (string.match l "GNS") (set done true))
+      (each [fd action (pairs outputs)]
+        (when (sock.getpeername fd) ; cheeky
+          (sock.send fd l))))))
 
 (let [[filename socketname]  arg
-      socket (io.popen (.. "socat - unix-listen:" socketname) :w)]
+      server (server-socket socketname)
+      fd-actions { server accept-new-connection }]
   (with-open [handle (io.open filename :r)]
-    (each [l (handle:lines "L")]
-      (if (string.match l "GNS") (sleep 1))
-      (socket:write l)
-      (socket:flush)
-      )))
+    (while true
+      (let [poll-fds (collect [fd _ (pairs fd-actions)]
+                       (values fd {:events { :IN true } }))]
+        (if (> (poll.poll poll-fds (* 1 1000)) 0)
+            (each [fd v (pairs poll-fds)]
+              (when (. v :revents)
+                (print :polled fd (view v.revents))
+                (let [(k v) ((. fd-actions fd) fd v.revents)]
+                  (and k (tset fd-actions k v)))))
+            (write-sentences handle fd-actions))))))