Make the current inferior a <proxy-state> property.

* guix-jupyter-kernel.scm (%inferior-property): New variable.
(proxy-state-inferior, set-proxy-state-inferior)
(ensure-proxy-state-inferior): New procedures.
(%inferior): Remove.
(create-environment): Call 'proxy-state-inferior' instead of refering to
%INFERIOR.
(reply-execute-request): Call 'ensure-proxy-state-inferior' before
calling 'create-environment'.
(reply-complete-request): Call 'ensure-proxy-state-inferior' before
calling 'inferior-available-packages'.  Return the new state.

guix-jupyter-kernel.scm

@@ -80,6 +80,29 @@
   (set-proxy-state-property state %default-environment-property
                             name))
 
+(define %inferior-property
+  ;; Property to access the inferior associated with this proxy.
+  (list 'inferior 'property))
+
+(define (proxy-state-inferior state)
+  "Return the inferior associated with STATE, or #f if there is none."
+  (proxy-state-property state %inferior-property))
+
+(define (set-proxy-state-inferior state inferior)
+  "Associate STATE with INFERIOR and return the new proxy state.  If there
+was already an inferior associated with STATE, close it."
+  (let ((previous (proxy-state-inferior state)))
+    (when previous
+      (close-inferior previous))
+    (set-proxy-state-property state %inferior-property
+                              inferior)))
+
+(define (ensure-proxy-state-inferior state)
+  "Return a new state based on STATE that has an associated inferior."
+  (if (proxy-state-inferior state)
+      state
+      (set-proxy-state-inferior state (open-default-inferior))))
+
 ;;
 ;; HTML.
 ;;
@@ -106,10 +129,6 @@
                         (kernel-info-reply->json %kernel-info-reply))))
   state)
 
-(define %inferior
-  ;; FIXME: This could be expensive!
-  (delay (open-default-inferior)))
-
 (define (manifest->shtml manifest name)
   "Return SHTML representing the contents of MANIFEST."
   `(div
@@ -151,7 +170,8 @@ NAME with MANIFEST."
 of package specifications such as \"guile@2.2\".  Send appropriate messages
 to KERNEL as a reply to MESSAGE, and return STATE suitably adjusted."
   (define manifest
-    (specifications->manifest (force %inferior) specs))
+    (specifications->manifest (proxy-state-inferior state)
+                              specs))
 
   (define store
     (proxy-state-store state))
@@ -236,20 +256,21 @@ stripped."
     (pub-busy kernel message)
     (match (string-tokenize line)
       ((";;guix" "environment" name "<-" specs ...)
-       (match (lookup-proxied name state)
-         (#f
-          (create-environment name specs state
-                              #:kernel kernel
-                              #:message message))
-         ((? kernel? proxy)
-          (format/log "terminating existing '~a' environment~%"
-                      name)
-          (unmonitor-client proxy)
-          (terminate-proxied-kernel proxy)
-          (create-environment name specs
-                              (unregister-proxied name state)
-                              #:kernel kernel
-                              #:message message))))
+       (let ((state (ensure-proxy-state-inferior state)))
+         (match (lookup-proxied name state)
+           (#f
+            (create-environment name specs state
+                                #:kernel kernel
+                                #:message message))
+           ((? kernel? proxy)
+            (format/log "terminating existing '~a' environment~%"
+                        name)
+            (unmonitor-client proxy)
+            (terminate-proxied-kernel proxy)
+            (create-environment name specs
+                                (unregister-proxied name state)
+                                #:kernel kernel
+                                #:message message)))))
       ((";;guix" "environment" name)
        (match (lookup-proxied name state)
          (#f
@@ -330,24 +351,29 @@ Return STATE."
           ((";;guix" command)
            (send-completion-reply '("environment")
                                   (- cursor (string-length command))
-                                  cursor))
+                                  cursor)
+           state)
           ((";;guix" "environment" prefix)
            (match (proxy-state-proxied state)
              (((names . _) ...)
               (send-completion-reply (filter (cut string-prefix? prefix <>)
                                              names)
                                      (- cursor (string-length prefix))
-                                     cursor))))
+                                     cursor)
+              state)))
           ((";;guix" "environment" _ "<-" _ ... prefix)
-           (match (inferior-available-packages (force %inferior))
-             (((names . _) ...)
-              (send-completion-reply (filter (cut string-prefix? prefix <>)
-                                             names)
-                                     (- cursor (string-length prefix))
-                                     cursor))))
+           (let* ((state    (ensure-proxy-state-inferior state))
+                  (inferior (proxy-state-inferior state)))
+             (match (inferior-available-packages inferior)
+               (((names . _) ...)
+                (send-completion-reply (filter (cut string-prefix? prefix <>)
+                                               names)
+                                       (- cursor (string-length prefix))
+                                       cursor)))
+             state))
           (_
            (format/log "ignoring completion request~%")
-           #f))
+           state))
 
         ;; Pass the completion request to one of the proxied kernels.
         (match (or (environment-from-magic first)
@@ -357,10 +383,11 @@ Return STATE."
              (format/log "forwarding completion request to \
 environment ~s (PID ~s)~%"
                          environment (kernel-pid proxied))
-             (send-message proxied message #:kernel-socket kind)))
+             (send-message proxied message #:kernel-socket kind)
+             state))
           (#f
-           (format/log "unknown target kernel for completion request~%")))))
-  state)
+           (format/log "unknown target kernel for completion request~%")
+           state)))))
 
 (define (reply-inspect-request kernel kind message state)
   "Handle MESSAGE, an \"inspect_request\" message, possibly by forwarding it