logtap: add WIP backfill command

pkgs/logtap/Makefile

@@ -1,6 +1,9 @@
-TARGETS=logtap
+TARGETS=logtap backfill
 CFLAGS=-Wall
 default: $(TARGETS)
 
+backfill: backfill.fnl
+	fennelc $<
+
 install: $(TARGETS)
-	install -Dt $(PREFIX)/bin/ logtap
+	install -Dt $(PREFIX)/bin/ $(TARGETS)

pkgs/logtap/backfill.fnl

@@ -0,0 +1,57 @@
+(local { : file-exists? } (require :anoia))
+(local tai64 (require :anoia.tai64))
+(local { : write &as ll } (require :lualinux))
+
+(local O_WRONLY 1)
+(local O_RDWR 2)
+(local O_CREAT 64)
+
+(fn send-message [fd message]
+  (case (write fd message)
+    n (if (> (# message) n) (send-message (string.sub message n)) true)
+    (nil code) nil))
+
+(fn parse-timestamp [message]
+  (let [{ : s : n } (tai64.from-timestamp message)]
+    (+ (* s 1000000000) n)))
+
+(fn spit [filename body]
+  (with-open [f (assert (io.open filename :w))]
+    (f:write  body)))
+
+(fn write-timestamp [file ts]
+  (spit file (string.format "%d" ts))
+  ts)
+
+(fn slurp [filename]
+  (with-open [f (assert (io.open filename :r))]
+    (f:read "a")))
+
+(let [[fifo-name stampfile] arg
+      fifo (ll.open fifo-name O_WRONLY)]
+  (var backfill? true)
+  (var next-ts
+       (if (file-exists? stampfile)
+           (tonumber (slurp stampfile))
+           0))
+
+  (each [l (: (io.input) :lines  "L")]
+    (case l
+      "START SHIPPING"
+      (set backfill? false)
+      "STOP SHIPPING"
+      (set backfill? true)
+      message
+      (let [message-ts (parse-timestamp message)]
+        (when (> message-ts next-ts)
+          (set next-ts (write-timestamp stampfile message-ts)))
+        (when (>= message-ts next-ts)
+          (io.stderr:write (.. "writing " message backfill?))
+          (if backfill?
+              (when (not (send-message fifo message))
+                ;; If the fifo write fails, it could be a glitch or it
+                ;; could be some kind of wider outage.  Give up,
+                ;; and rely on the supervisor to restart us when
+                ;; conditions are again auspicious
+                (io.stderr:write "write failed\n")
+                (os.exit 1))))))))

pkgs/logtap/default.nix

@@ -1,8 +1,14 @@
 {
   stdenv,
+  fennelc,
+  lualinux,
+  lua,
+  anoia
 }:
 stdenv.mkDerivation {
   name = "logtap";
+  nativeBuildInputs = [ fennelc ];
+  buildInputs = [ lua lualinux anoia ];
   makeFlags = [ "PREFIX=${placeholder "out"}" ];
   src = ./.;
 }