Use a web cache on the file system

1 files changed, 281 insertions, 0 deletions

diff --git a/src/scm/webid-oidc/cache.scm b/src/scm/webid-oidc/cache.scm
new file mode 100644
index 0000000..9d9da0f
--- /dev/null
+++ b/src/scm/webid-oidc/cache.scm
@@ -0,0 +1,281 @@
+(define-module (webid-oidc cache)
+  #:use-module ((webid-oidc stubs) #:prefix stubs:)
+  #:use-module (web client)
+  #:use-module (web request)
+  #:use-module (web response)
+  #:use-module (web uri)
+  #:use-module (ice-9 ftw)
+  #:use-module (ice-9 receive)
+  #:use-module (ice-9 optargs)
+  #:use-module (srfi srfi-19)
+  #:use-module (rnrs bytevectors))
+
+;; The cache follows the recommendations of
+;; https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching
+
+;; In a directory, we dump files, indexed by the hash of the URI, and
+;; containing the request, response and response body.
+
+;; We keep only one version of the resource, even with varying
+;; headers. If the cache validation fails due to varying headers, we
+;; simply drop the old variant and replace it with the new. For things
+;; like user agents or accepted language or encoding, you’re not
+;; likely to alternate quickly between two values anyway.
+
+;; We’re not very concerned by the cache growing. We provide a cleanup
+;; function that will kill a few percents of the cached entries at
+;; random, and that you should call with a cron job regularly. It will
+;; sometimes drop valid cached entries, but life goes on. By default,
+;; each cache update has a 5% chance to trigger the elimination of 5%
+;; of the cache.
+
+;; There is a global lock file at the root of the cache, which serves
+;; for region locking. Do not remove it!
+
+(define (default-cache-dir)
+  (let ((xdg-cache-home
+         (or (getenv "XDG_CACHE_HOME")
+             (format #f "~a/.cache"))))
+    (format #f "~a/webid-oidc" xdg-cache-home)))
+
+(define (web-cache-dir dir)
+  (when (thunk? dir)
+    (set! dir (dir)))
+  (string-append dir
+                 "/web-cache/"))
+
+(define (file-name uri dir)
+  (when (string? uri)
+    (set! uri (string->uri uri)))
+  (string-append (web-cache-dir dir)
+                 (stubs:hash 'SHA-256 (uri->string uri))))
+
+(define (lock-file-name dir)
+  (string-append (web-cache-dir dir) ".lock"))
+
+(define (event? percents)
+  (<= (* (random:uniform) 100)
+      percents))
+
+(define*-public (clean-cache
+                 #:key
+                 (percents 5)
+                 (dir default-cache-dir))
+  (define lock-file (lock-file-name dir))
+  (define (survives?)
+    (not (event? percents)))
+  (define (enter? name stat result)
+    #t)
+  (define (leaf name stat result)
+    (unless (or (string-suffix? ".lock" name)
+                (survives?))
+      (unless
+          (false-if-exception
+           (begin
+             (format (current-error-port) "Dropping cache item ~a.~%" name)
+             (stubs:atomically-update-file
+              name
+              lock-file
+              (lambda (whatever)
+                #f))))
+        (format (current-error-port) "Could not clean file ~a.~%" name)))
+    result)
+  (define (down name stat result) result)
+  (define (up name stat result) result)
+  (define (skip name stat result) result)
+  (define (error name stat errno result)
+    (format (current-error-port) "While cleaning the cache: ~a: ~a~%"
+            name (strerror errno))
+    result)
+  (file-system-fold enter? leaf down up skip error 0
+                    (web-cache-dir dir)))
+
+(define (maybe-clean-cache
+         pc-happen
+         pc-cleaned
+         dir)
+  (when (event? pc-happen)
+    (clean-cache #:percents pc-cleaned #:dir dir)))
+
+(define (remove-uncacheable-headers response)
+  (let ((headers (response-headers response)))
+    (let ((filtered
+           (filter
+            (lambda (h)
+              (case (car h)
+                ((connection keep-alive transfer-encoding)
+                 #f)
+                (else #t)))
+            headers)))
+      (build-response
+       #:code (response-code response)
+       #:reason-phrase (response-reason-phrase response)
+       #:headers filtered
+       #:port #f))))
+
+(define*-public (add request response response-body
+                     #:key (dir default-cache-dir))
+  ;; Don’t store it if there’s a cache-control no-store
+  (unless
+      (let ((cc (response-cache-control response '())))
+        (assq-ref cc 'no-store))
+    (set! response (remove-uncacheable-headers response))
+    (let ((final-file-name (file-name (request-uri request) dir))
+          (lock-file (lock-file-name dir)))
+      (maybe-clean-cache 5 5 dir)
+      (stubs:atomically-update-file
+       final-file-name
+       lock-file
+       (lambda (port)
+         (write-request request port)
+         (let ((file-response
+                (write-response response port)))
+           (when (string? response-body)
+             (set! response-body (string->utf8 response-body)))
+           (when response-body
+             (write-response-body file-response response-body))
+           #t))))))
+
+(define (the-current-time)
+  (time-utc->date
+   (current-time)))
+
+(define*-public (read uri
+                      #:key
+                      (dir default-cache-dir))
+  (let ((final-file-name (file-name uri dir)))
+    (catch 'system-error
+      (lambda ()
+        (call-with-input-file final-file-name
+          (lambda (port)
+            (let ((request (read-request port)))
+              (let ((response (read-response port)))
+                (values request response (read-response-body response)))))))
+      (lambda error
+        (format (current-error-port) "Cache miss for ~a: ~s~%"
+                (uri->string uri)
+                error)
+        (values #f #f #f)))))
+
+(define (varies-header? request-a request-b header)
+  (let ((a (assq-ref (request-headers request-a) header))
+        (b (assq-ref (request-headers request-b) header)))
+    (not (equal? a b))))
+
+(define (varies-any-header? request-a request-b headers)
+  (and (not (null? headers))
+       (or (varies-header? request-a request-b (car headers))
+           (varies-any-header? request-a request-b (cdr headers)))))
+
+(define-public (varies? request-a request-b response)
+  (let ((vary (response-vary response)))
+    (or (eq? vary '*)
+        (varies-any-header? request-a request-b vary))))
+
+(define*-public (valid? response
+                        #:key
+                        (current-time the-current-time))
+  (when (thunk? current-time)
+    (set! current-time (current-time)))
+  (when (integer? current-time)
+    (set! current-time
+          (make-time time-utc 0 current-time)))
+  (when (time? current-time)
+    (set! current-time (time-utc->date current-time)))
+  (set! current-time
+        (date->time-utc current-time))
+  (set! current-time
+        (time-second current-time))
+  (let ((cc (response-cache-control response #f))
+        (date (response-date response
+                             (time-utc->date
+                              (make-time time-utc 0 current-time))))
+        (last-modified (response-last-modified response)))
+    (set! date (date->time-utc date))
+    (set! date (time-second date))
+    (when last-modified
+      (set! last-modified (date->time-utc last-modified))
+      (set! last-modified (time-second last-modified)))
+    (if cc
+        ;; Use cache-control
+        (let ((cc-no-cache (assq-ref cc 'no-cache))
+              (cc-no-store (assq-ref cc 'no-store))
+              (cc-max-age
+               (or (assq-ref cc 'max-age)
+                   ;; Heuristic freshness
+                   (and last-modified
+                        (/ (- date last-modified) 10)))))
+          (and (not cc-no-cache)
+               (not cc-no-store)
+               cc-max-age
+               (>= (+ date cc-max-age) current-time)))
+        ;; Use expires
+        (let ((exp (response-expires response)))
+          (when exp
+            (set! exp (date->time-utc exp))
+            (set! exp (time-second exp)))
+          (and exp
+               (>= exp current-time))))))
+
+(define*-public (revalidate uri response body
+                            #:key
+                            (headers '())
+                            (http-get http-get))
+  (define (keep-header? h)
+    (case (car h)
+      ((if-none-match if-unmodified-since) #f)
+      (else #t)))
+  (let ((etag (response-etag response)))
+    (if etag
+        (receive (new-response new-response-body)
+            (http-get uri
+                      #:headers
+                      (acons 'if-none-match (list etag)
+                             (filter keep-header? headers)))
+          (if (eqv? (response-code new-response) 304)
+              (values
+               (build-response
+                #:headers
+                (append
+                 (response-headers new-response)
+                 (filter
+                  (lambda (h)
+                    (case (car h)
+                      ((cache-control content-location date etag expires vary)
+                       ;; These are overriden by new-response
+                       #f)
+                      (else #t)))
+                  (response-headers response))))
+               body)
+              (values new-response new-response-body)))
+        (http-get uri #:headers headers))))
+
+(define*-public (with-cache
+                 #:key
+                 (current-time the-current-time)
+                 (http-get http-get)
+                 (dir default-cache-dir))
+  (lambda* (uri #:key (headers '()))
+    (when (string? uri)
+      (set! uri (string->uri uri)))
+    (let ((dir (if (thunk? dir) (dir) dir))
+          (request (build-request uri #:headers headers)))
+      (receive (stored-request stored-response body)
+          (read uri #:dir dir)
+        (if stored-response
+            (let ((valid (valid? stored-response #:current-time the-current-time))
+                  (invariant (not (varies? request stored-request stored-response))))
+              (unless invariant
+                (format (current-error-port) "Cache entry for ~a varies.\n" (uri->string uri)))
+              (if (and valid invariant)
+                  (values stored-response body)
+                  (receive (final-response final-body)
+                      (revalidate uri stored-response body
+                                  #:headers headers
+                                  #:http-get http-get)
+                    (add request final-response final-body #:dir dir)
+                    (values final-response final-body))))
+            (receive (final-response final-body)
+                (http-get uri #:headers headers)
+              (add request final-response final-body #:dir dir)
+              (values final-response final-body)))))))