download: Enlarge your receive buffer.

* guix/build/download.scm (open-connection-for-uri): Remove call to
  'setsockopt'.
* guix/http-client.scm (open-socket-for-uri)[rmem-max, buffer-size]: New
  variables.  Add call to 'setsockopt'.

guix/build/download.scm

@@ -167,8 +167,6 @@ which is not available during bootstrap."
 
           ;; Buffer input and output on this port.
           (setvbuf s _IOFBF)
-          ;; Enlarge the receive buffer.
-          (setsockopt s SOL_SOCKET SO_RCVBUF (* 12 1024))
 
           (if (eq? 'https (uri-scheme uri))
               (tls-wrap s)

guix/http-client.scm

@@ -162,7 +162,19 @@ closed it will also close PORT, unless the KEEP-ALIVE? is true."
 (define* (open-socket-for-uri uri #:key (buffered? #t))
   "Return an open port for URI.  When BUFFERED? is false, the returned port is
 unbuffered."
+  (define rmem-max
+    ;; The maximum size for a receive buffer on Linux, see socket(7).
+    "/proc/sys/net/core/rmem_max")
+
+  (define buffer-size
+    (if (file-exists? rmem-max)
+        (call-with-input-file rmem-max read)
+        126976))                   ; the default for Linux, per 'rmem_default'
+
   (let ((s ((@ (web client) open-socket-for-uri) uri)))
+    ;; Work around <http://bugs.gnu.org/15368> by restoring a decent
+    ;; buffer size.
+    (setsockopt s SOL_SOCKET SO_RCVBUF buffer-size)
     (unless buffered?
       (setvbuf s _IONBF))
     s))