substitute: Rework connection error handling.

This is part of trying to reduce the interdependency of code within the
substitute module.

This commit addresses some of the error handling that was performed through
open-connection-for-uri/maybe. The new approach is to use
call-with-connection-error-handling, and wrap calls to http-multiple-get and
http-fetch with that procedure, which takes care of handling connection
errors.

I think this is even slightly more rigerous than the previous setup, because
this approach handles connection errors that occur when http-multiple-get
reconnects to a host.

* guix/scripts/substitute.scm (open-connection-for-uri/maybe): Transform in to
call-with-connection-error-handling.
(fetch-narinfos): Use call-with-connection-error-handling.
(process-query): Replace open-connection-for-uri/maybe with
open-connection-for-uri/cached.
(open-connection-for-uri/cached): Set a default timeout, matching the
behaviour in open-connection-for-uri/maybe.
(process-substitution): Use call-with-connection-error-handling.

guix/scripts/substitute.scm

@@ -281,22 +281,13 @@ if file doesn't exist, and the narinfo otherwise."
   ;; Set of names of unreachable hosts.
   (make-hash-table))
 
-(define* (open-connection-for-uri/maybe uri
-                                        #:key
-                                        fresh?
-                                        (time %fetch-timeout)
-                                        verify-certificate?)
-  "Open a connection to URI via 'open-connection-for-uri/cached' and return a
-port to it, or, if connection failed, print a warning and return #f.  Pass
-#:fresh? to 'open-connection-for-uri/cached'."
+(define* (call-with-connection-error-handling uri proc)
+  "Call PROC, and catch if a connection fails, print a warning and return #f."
   (define host
     (uri-host uri))
 
   (catch #t
-    (lambda ()
-      (open-connection-for-uri/cached uri #:timeout time
-                                      #:fresh? fresh?
-                                      #:verify-certificate? verify-certificate?))
+    proc
     (match-lambda*
       (('getaddrinfo-error error)
        (unless (hash-ref %unreachable-hosts host)
@@ -377,11 +368,14 @@ port to it, or, if connection failed, print a warning and return #f.  Pass
        (let* ((requests (map (cut narinfo-request url <>) paths))
               (result   (begin
                           (update-progress!)
+                          (call-with-connection-error-handling
+                           uri
+                           (lambda ()
                              (http-multiple-get uri
                                                 handle-narinfo-response '()
                                                 requests
                                                 #:open-connection open-connection
-                                             #:verify-certificate? #f))))
+                                                #:verify-certificate? #f))))))
          (newline (current-error-port))
          result))
       ((file #f)
@@ -595,7 +589,7 @@ authorized substitutes."
      ;; Return the subset of PATHS available in CACHE-URLS.
      (let ((substitutable (lookup-narinfos/diverse
                            cache-urls paths valid?
-                           #:open-connection open-connection-for-uri/maybe)))
+                           #:open-connection open-connection-for-uri/cached)))
        (for-each (lambda (narinfo)
                    (format #t "~a~%" (narinfo-path narinfo)))
                  substitutable)
@@ -604,7 +598,7 @@ authorized substitutes."
      ;; Reply info about PATHS if it's in CACHE-URLS.
      (let ((substitutable (lookup-narinfos/diverse
                            cache-urls paths valid?
-                           #:open-connection open-connection-for-uri/maybe)))
+                           #:open-connection open-connection-for-uri/cached)))
        (for-each display-narinfo-data substitutable)
        (newline)))
     (wtf
@@ -617,7 +611,7 @@ authorized substitutes."
 
 (define open-connection-for-uri/cached
   (let ((cache '()))
-    (lambda* (uri #:key fresh? timeout verify-certificate?)
+    (lambda* (uri #:key fresh? (timeout %fetch-timeout) verify-certificate?)
       "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.
@@ -704,11 +698,14 @@ the current output port."
              (warning (G_ "while fetching ~a: server is somewhat slow~%")
                       (uri->string uri))
              (warning (G_ "try `--no-substitutes' if the problem persists~%")))
+           (call-with-connection-error-handling
+            uri
+            (lambda ()
               (http-fetch uri #:text? #f
-                       #:open-connection open-connection-for-uri/maybe
+                          #:open-connection open-connection-for-uri/cached
                           #:keep-alive? #t
                           #:buffered? #f
-                       #:verify-certificate? #f))))
+                          #:verify-certificate? #f))))))
       (else
        (leave (G_ "unsupported substitute URI scheme: ~a~%")
               (uri->string uri)))))