substitute: Cache and reuse connections while substituting.

That way, when fetching a series of substitutes from the same server(s),
the connection is reused instead of being closed/opened for each
substitutes, which saves on network round trips and TLS handshakes.

* guix/http-client.scm (http-fetch): Add #:keep-alive? and honor it.
* guix/progress.scm (progress-report-port): Add #:close? parameter and
honor it.
* guix/scripts/substitute.scm (at-most): Return the tail as a second
value.
(fetch): Add #:port and #:keep-alive? and honor them.
(%max-cached-connections): New variable.
(open-connection-for-uri/cached, call-with-cached-connection): New
procedures.
(with-cached-connection): New macro.
(process-substitution): Wrap 'fetch' call in 'with-cached-connection'.
Pass #:close? to 'progress-report-port'.

1 files changed, 88 insertions, 15 deletions

diff --git a/guix/scripts/substitute.scm b/guix/scripts/substitute.scm
index 73abd3f029..25075eedff 100755
--- a/guix/scripts/substitute.scm
+++ b/guix/scripts/substitute.scm
@@ -188,9 +188,14 @@ again."
         (sigaction SIGALRM SIG_DFL)
         (apply values result)))))
 
-(define* (fetch uri #:key (buffered? #t) (timeout? #t))
+(define* (fetch uri #:key (buffered? #t) (timeout? #t)
+                (keep-alive? #f) (port #f))
   "Return a binary input port to URI and the number of bytes it's expected to
-provide."
+provide.
+
+When PORT is true, use it as the underlying I/O port for HTTP transfers; when
+PORT is false, open a new connection for URI.  When KEEP-ALIVE? is true, the
+connection (typically PORT) is kept open once data has been fetched from URI."
   (case (uri-scheme uri)
     ((file)
      (let ((port (open-file (uri-path uri)
@@ -206,7 +211,7 @@ provide."
        ;;   sudo tc qdisc add dev eth0 root netem delay 1500ms
        ;; and then cancel with:
        ;;   sudo tc qdisc del dev eth0 root
-       (let ((port #f))
+       (let ((port port))
          (with-timeout (if timeout?
                            %fetch-timeout
                            0)
@@ -217,10 +222,11 @@ provide."
            (begin
              (when (or (not port) (port-closed? port))
                (set! port (guix:open-connection-for-uri
-                           uri #:verify-certificate? #f))
-               (unless (or buffered? (not (file-port? port)))
-                 (setvbuf port 'none)))
+                           uri #:verify-certificate? #f)))
+             (unless (or buffered? (not (file-port? port)))
+               (setvbuf port 'none))
              (http-fetch uri #:text? #f #:port port
+                         #:keep-alive? keep-alive?
                          #:verify-certificate? #f))))))
     (else
      (leave (G_ "unsupported substitute URI scheme: ~a~%")
@@ -478,17 +484,17 @@ indicates that PATH is unavailable at CACHE-URL."
     (build-request (string->uri url) #:method 'GET #:headers headers)))
 
 (define (at-most max-length lst)
-  "If LST is shorter than MAX-LENGTH, return it; otherwise return its
-MAX-LENGTH first elements."
+  "If LST is shorter than MAX-LENGTH, return it and the empty list; otherwise
+return its MAX-LENGTH first elements and its tail."
   (let loop ((len 0)
              (lst lst)
              (result '()))
     (match lst
       (()
-       (reverse result))
+       (values (reverse result) '()))
       ((head . tail)
        (if (>= len max-length)
-           (reverse result)
+           (values (reverse result) lst)
            (loop (+ 1 len) tail (cons head result)))))))
 
 (define* (http-multiple-get base-uri proc seed requests
@@ -962,6 +968,68 @@ the URI, its compression method (a string), and the compressed file size."
     (((uri compression file-size) _ ...)
      (values uri compression file-size))))
 
+(define %max-cached-connections
+  ;; Maximum number of connections kept in cache by
+  ;; 'open-connection-for-uri/cached'.
+  16)
+
+(define open-connection-for-uri/cached
+  (let ((cache '()))
+    (lambda* (uri #:key fresh?)
+      "Return a connection for URI, possibly reusing a cached connection.
+When FRESH? is true, delete any cached connections for URI and open a new
+one.  Return #f if URI's scheme is 'file' or #f."
+      (define host (uri-host uri))
+      (define scheme (uri-scheme uri))
+      (define key (list host scheme (uri-port uri)))
+
+      (and (not (memq scheme '(file #f)))
+           (match (assoc-ref cache key)
+             (#f
+              ;; Open a new connection to URI and evict old entries from
+              ;; CACHE, if any.
+              (let-values (((socket)
+                            (guix:open-connection-for-uri
+                             uri #:verify-certificate? #f))
+                           ((new-cache evicted)
+                            (at-most (- %max-cached-connections 1) cache)))
+                (for-each (match-lambda
+                            ((_ . port)
+                             (false-if-exception (close-port port))))
+                          evicted)
+                (set! cache (alist-cons key socket new-cache))
+                socket))
+             (socket
+              (if (or fresh? (port-closed? socket))
+                  (begin
+                    (false-if-exception (close-port socket))
+                    (set! cache (alist-delete key cache))
+                    (open-connection-for-uri/cached uri))
+                  (begin
+                    ;; Drain input left from the previous use.
+                    (drain-input socket)
+                    socket))))))))
+
+(define (call-with-cached-connection uri proc)
+  (let ((port (open-connection-for-uri/cached uri)))
+    (catch #t
+      (lambda ()
+        (proc port))
+      (lambda (key . args)
+        ;; If PORT was cached and the server closed the connection in the
+        ;; meantime, we get EPIPE.  In that case, open a fresh connection and
+        ;; retry.  We might also get 'bad-response or a similar exception from
+        ;; (web response) later on, once we've sent the request.
+        (if (or (and (eq? key 'system-error)
+                     (= EPIPE (system-error-errno `(,key ,@args))))
+                (memq key '(bad-response bad-header bad-header-component)))
+            (proc (open-connection-for-uri/cached uri #:fresh? #t))
+            (apply throw key args))))))
+
+(define-syntax-rule (with-cached-connection uri port exp ...)
+  "Bind PORT with EXP... to a socket connected to URI."
+  (call-with-cached-connection uri (lambda (port) exp ...)))
+
 (define* (process-substitution store-item destination
                                #:key cache-urls acl print-build-trace?)
   "Substitute STORE-ITEM (a store file name) from CACHE-URLS, and write it to
@@ -984,10 +1052,12 @@ DESTINATION as a nar file.  Verify the substitute against ACL."
               (G_ "Downloading ~a...~%") (uri->string uri)))
 
     (let*-values (((raw download-size)
-                   ;; Note that Hydra currently generates Nars on the fly
-                   ;; and doesn't specify a Content-Length, so
-                   ;; DOWNLOAD-SIZE is #f in practice.
-                   (fetch uri #:buffered? #f #:timeout? #f))
+                   ;; 'guix publish' without '--cache' doesn't specify a
+                   ;; Content-Length, so DOWNLOAD-SIZE is #f in this case.
+                   (with-cached-connection uri port
+                     (fetch uri #:buffered? #f #:timeout? #f
+                            #:port port
+                            #:keep-alive? #t)))
                   ((progress)
                    (let* ((dl-size  (or download-size
                                         (and (equal? compression "none")
@@ -1001,7 +1071,9 @@ DESTINATION as a nar file.  Verify the substitute against ACL."
                                          (uri->string uri) dl-size
                                          (current-error-port)
                                          #:abbreviation nar-uri-abbreviation))))
-                     (progress-report-port reporter raw)))
+                     ;; Keep RAW open upon completion so we can later reuse
+                     ;; the underlying connection.
+                     (progress-report-port reporter raw #:close? #f)))
                   ((input pids)
                    ;; NOTE: This 'progress' port of current process will be
                    ;; closed here, while the child process doing the
@@ -1218,6 +1290,7 @@ default value."
 
 ;;; Local Variables:
 ;;; eval: (put 'with-timeout 'scheme-indent-function 1)
+;;; eval: (put 'with-cached-connection 'scheme-indent-function 2)
 ;;; End:
 
 ;;; substitute.scm ends here