store: 'mcached' users can specify a cache ID.

Users of 'mcached' can now specify a cache ID; furthermore, the cache
hit rate is automatically recorded for all the caches accessed with
'mcached'.

* guix/store.scm (%max-store-connection-caches)
(%store-connection-cache-names): New variables.
(recorder-for-cache): New procedure.
(record-cache-lookup!): Add 'cache-id' parameter and rewrite in terms of
'recorder-for-cache'.
(lookup-cached-object): Add 'cache-id' parameter and honor it.
(%mcached): Add #:cache parameter and honor it.
(mcached): Add '=>' keyword and corresponding clauses.

guix/store.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012-2022 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2018 Jan Nieuwenhuizen <janneke@gnu.org>
 ;;; Copyright © 2019, 2020 Mathieu Othacehe <m.othacehe@gmail.com>
 ;;; Copyright © 2020 Florian Pelz <pelzflorian@pelzflorian.de>
@@ -1793,6 +1793,14 @@ This makes sense only when the daemon was started with '--cache-failures'."
 ;; the 'caches' vector of <store-connection>.
 (define %store-connection-caches (make-atomic-box 0))
 
+(define %max-store-connection-caches
+  ;; Maximum number of caches returned by 'allocate-store-connection-cache'.
+  32)
+
+(define %store-connection-cache-names
+  ;; Mapping of cache ID to symbol.
+  (make-vector %max-store-connection-caches))
+
 (define (allocate-store-connection-cache name)
   "Allocate a new cache for store connections and return its identifier.  Said
 identifier can be passed as an argument to "
@@ -1800,7 +1808,9 @@ identifier can be passed as an argument to "
     (let ((previous (atomic-box-compare-and-swap! %store-connection-caches
                                                   current (+ current 1))))
       (if (= previous current)
-          current
+          (begin
+            (vector-set! %store-connection-cache-names current name)
+            current)
           (loop current)))))
 
 (define %object-cache-id
@@ -1926,16 +1936,37 @@ whether the cache lookup was a hit, and the actual cache (a vhash)."
       (lambda (x y)
         #t)))
 
-(define record-cache-lookup!
+(define recorder-for-cache
-  (cache-lookup-recorder "object-cache" "Store object cache"))
+  (let ((recorders (make-vector %max-store-connection-caches)))
+    (lambda (cache-id)
+      "Return a procedure to record lookup stats for CACHE-ID."
+      (match (vector-ref recorders cache-id)
+        ((? unspecified?)
+         (let* ((name (symbol->string
+                       (vector-ref %store-connection-cache-names cache-id)))
+                (description
+                 (string-titlecase
+                  (string-map (match-lambda
+                                (#\- #\space)
+                                (chr chr))
+                              name))))
+           (let ((proc (cache-lookup-recorder name description)))
+             (vector-set! recorders cache-id proc)
+             proc)))
+        (proc proc)))))
 
-(define-inlinable (lookup-cached-object object keys vhash-fold*)
+(define (record-cache-lookup! cache-id value cache)
-  "Return the cached object in the store connection corresponding to OBJECT
+  "Record the lookup of VALUE in CACHE-ID, whose current value is CACHE."
+  (let ((record! (recorder-for-cache cache-id)))
+    (record! value cache)))
+
+(define-inlinable (lookup-cached-object cache-id object keys vhash-fold*)
+  "Return the object in store cache CACHE-ID corresponding to OBJECT
 and KEYS; use VHASH-FOLD* to look for OBJECT in the cache.  KEYS is a list of
 additional keys to match against, and which are compared with 'equal?'.
 Return #f on failure and the cached result otherwise."
   (lambda (store)
-    (let* ((cache (store-connection-cache store %object-cache-id))
+    (let* ((cache (store-connection-cache store cache-id))
 
            ;; Escape as soon as we find the result.  This avoids traversing
            ;; the whole vlist chain and significantly reduces the number of
@@ -1949,40 +1980,50 @@ Return #f on failure and the cached result otherwise."
                                           result))))
                                  #f object
                                  cache))))
-      (record-cache-lookup! value cache)
+      (record-cache-lookup! cache-id value cache)
       (values value store))))
 
 (define* (%mcached mthunk object #:optional (keys '())
                    #:key
+                   (cache %object-cache-id)
                    (vhash-cons vhash-consq)
                    (vhash-fold* vhash-foldq*))
   "Bind the monadic value returned by MTHUNK, which supposedly corresponds to
 OBJECT/KEYS, or return its cached value.  Use VHASH-CONS to insert OBJECT into
 the cache, and VHASH-FOLD* to look it up."
-  (mlet %store-monad ((cached (lookup-cached-object object keys
+  (mlet %store-monad ((cached (lookup-cached-object cache object keys
                                                     vhash-fold*)))
     (if cached
         (return cached)
         (>>= (mthunk)
              (lambda (result)
                (cache-object-mapping object keys result
+                                     #:cache cache
                                      #:vhash-cons vhash-cons))))))
 
 (define-syntax mcached
-  (syntax-rules (eq? equal?)
+  (syntax-rules (eq? equal? =>)
     "Run MVALUE, which corresponds to OBJECT/KEYS, and cache it; or return the
 value associated with OBJECT/KEYS in the store's object cache if there is
 one."
-    ((_ eq? mvalue object keys ...)
+    ((_ eq? (=> cache) mvalue object keys ...)
      (%mcached (lambda () mvalue)
                object (list keys ...)
+               #:cache cache
                #:vhash-cons vhash-consq
                #:vhash-fold* vhash-foldq*))
-    ((_ equal? mvalue object keys ...)
+    ((_ equal? (=> cache) mvalue object keys ...)
      (%mcached (lambda () mvalue)
                object (list keys ...)
+               #:cache cache
                #:vhash-cons vhash-cons
                #:vhash-fold* vhash-fold*))
+    ((_ eq? mvalue object keys ...)
+     (mcached eq? (=> %object-cache-id)
+              mvalue object keys ...))
+    ((_ equal? mvalue object keys ...)
+     (mcached equal? (=> %object-cache-id)
+              mvalue object keys ...))
     ((_ mvalue object keys ...)
      (mcached eq? mvalue object keys ...))))