substitute-binary: Remove expired cache entries once in a while.

* guix/scripts/substitute-binary.scm (%narinfo-expired-cache-entry-removal-delay):
  New variable.
  (obsolete?): New procedure, formerly in `lookup-narinfo'.
  (lookup-narinfo): Adjust accordingly.
  (remove-expired-cached-narinfos, maybe-remove-expired-cached-narinfo):
  New procedures.
  (guix-substitute-binary): Call `maybe-remove-expired-cached-narinfo'.

guix/scripts/substitute-binary.scm

@@ -28,6 +28,7 @@
   #:use-module (ice-9 match)
   #:use-module (ice-9 threads)
   #:use-module (ice-9 format)
+  #:use-module (ice-9 ftw)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-9)
   #:use-module (srfi srfi-11)
@@ -64,6 +65,10 @@
   ;; Likewise, but for negative lookups---i.e., cached lookup failures.
   (* 3 3600))
 
+(define %narinfo-expired-cache-entry-removal-delay
+  ;; How often we want to remove files corresponding to expired cache entries.
+  (* 7 24 3600))
+
 (define (with-atomic-file-output file proc)
   "Call PROC with an output port for the file that is going to replace FILE.
 Upon success, FILE is atomically replaced by what has been written to the
@@ -263,19 +268,17 @@ reading PORT."
                                        ".narinfo"))
               (cute read-narinfo <> (cache-url cache)))))
 
+(define (obsolete? date now ttl)
+  "Return #t if DATE is obsolete compared to NOW + TTL seconds."
+  (time>? (subtract-duration now (make-time time-duration 0 ttl))
+          (make-time time-monotonic 0 date)))
+
 (define (lookup-narinfo cache path)
   "Check locally if we have valid info about PATH, otherwise go to CACHE and
 check what it has."
   (define now
     (current-time time-monotonic))
 
-  (define (->time seconds)
-    (make-time time-monotonic 0 seconds))
-
-  (define (obsolete? date ttl)
-    (time>? (subtract-duration now (make-time time-duration 0 ttl))
-            (->time date)))
-
   (define cache-file
     (string-append %narinfo-cache-directory "/"
                    (store-path-hash-part path)))
@@ -294,13 +297,13 @@ check what it has."
                            (('narinfo ('version 0) ('date date)
                                       ('value #f))
                             ;; A cached negative lookup.
-                            (if (obsolete? date %narinfo-negative-ttl)
+                            (if (obsolete? date now %narinfo-negative-ttl)
                                 (values #f #f)
                                 (values #t #f)))
                            (('narinfo ('version 0) ('date date)
                                       ('value value))
                             ;; A cached positive lookup
-                            (if (obsolete? date %narinfo-ttl)
+                            (if (obsolete? date now %narinfo-ttl)
                                 (values #f #f)
                                 (values #t (string->narinfo value))))))))
                    (lambda _
@@ -314,6 +317,59 @@ check what it has."
               (write (cache-entry narinfo) out)))
           narinfo))))
 
+(define (remove-expired-cached-narinfos)
+  "Remove expired narinfo entries from the cache.  The sole purpose of this
+function is to make sure `%narinfo-cache-directory' doesn't grow
+indefinitely."
+  (define now
+    (current-time time-monotonic))
+
+  (define (expired? file)
+    (catch 'system-error
+      (lambda ()
+        (call-with-input-file file
+          (lambda (port)
+            (match (read port)
+              (('narinfo ('version 0) ('date date)
+                         ('value #f))
+               (obsolete? date now %narinfo-negative-ttl))
+              (('narinfo ('version 0) ('date date)
+                         ('value _))
+               (obsolete? date now %narinfo-ttl))
+              (_ #t)))))
+      (lambda args
+        ;; FILE may have been deleted.
+        #t)))
+
+  (for-each (lambda (file)
+              (let ((file (string-append %narinfo-cache-directory
+                                         "/" file)))
+                (when (expired? file)
+                  ;; Wrap in `false-if-exception' because FILE might have been
+                  ;; deleted in the meantime (TOCTTOU).
+                  (false-if-exception (delete-file file)))))
+            (scandir %narinfo-cache-directory
+                     (lambda (file)
+                       (= (string-length file) 32)))))
+
+(define (maybe-remove-expired-cached-narinfo)
+  "Remove expired narinfo entries from the cache if deemed necessary."
+  (define now
+    (current-time time-monotonic))
+
+  (define expiry-file
+    (string-append %narinfo-cache-directory "/last-expiry-cleanup"))
+
+  (define last-expiry-date
+    (or (false-if-exception
+         (call-with-input-file expiry-file read))
+        0))
+
+  (when (obsolete? last-expiry-date now %narinfo-expired-cache-entry-removal-delay)
+    (remove-expired-cached-narinfos)
+    (call-with-output-file expiry-file
+      (cute write (time-second now) <>))))
+
 (define (filtered-port command input)
   "Return an input port (and PID) where data drained from INPUT is filtered
 through COMMAND.  INPUT must be a file input port."
@@ -351,6 +407,7 @@ through COMMAND.  INPUT must be a file input port."
 (define (guix-substitute-binary . args)
   "Implement the build daemon's substituter protocol."
   (mkdir-p %narinfo-cache-directory)
+  (maybe-remove-expired-cached-narinfo)
   (match args
     (("--query")
      (let ((cache (delay (open-cache %cache-url))))