proxy: Remove the kernel container directory upon termination.

* guix/jupyter/proxy.scm (remove-kernel-directory): New procedure.
(terminate-proxied-kernel, terminate-proxied-kernels): Call it.
* guix/jupyter/kernel.scm (spawn-kernel/container): Remove FIXME
comment.

guix/jupyter/kernel.scm

@@ -166,7 +166,6 @@ monadic value, a <kernel> connected to that process."
     ;; network namespace.  TODO: Arrange to use Unix-domain sockets instead.
     (delq 'net %namespaces))
 
-  ;; FIXME: Remove ROOT once the kernel has terminated.
   (mlet* %store-monad ((root ((lift0 make-container-directory %store-monad)))
                        (home -> (string-append root "/home"))
                        (pid  (eval/container* (spawn profile)

guix/jupyter/proxy.scm

@@ -21,6 +21,7 @@
   #:use-module (jupyter servers)
   #:use-module (json)
   #:use-module (gcrypt base16)
+  #:use-module ((guix build utils) #:select (delete-file-recursively))
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-9 gnu)
   #:use-module (ice-9 vlist)
@@ -103,6 +104,18 @@ found."
   (let ((count (proxy-state-execution-count state)))
     (set-proxy-state-execution-count state (+ 1 count))))
 
+(define (remove-kernel-directory kernel)
+  "Return the directory used to store KERNEL's root and home directories, if
+any."
+  ;; Proxied kernels running in a container have their 'directory' property
+  ;; set in 'spawn-kernel/container'.
+  (match (assq-ref (kernel-properties kernel) 'directory)
+    (#f #t)
+    ((? string? directory)
+     (format/log "deleting '~a' of kernel ~a~%"
+                 directory (kernel-pid kernel))
+     (delete-file-recursively directory))))
+
 (define (terminate-proxied-kernel kernel)
   "Send KERNEL a \"shutdown_request\" and terminate its process."
   (define shutdown
@@ -118,7 +131,8 @@ found."
   (catch 'system-error
     (lambda ()
       (kill (kernel-pid kernel) SIGTERM))
-    (const #t)))
+    (const #t))
+  (remove-kernel-directory kernel))
 
 (define (terminate-proxied-kernels message state)
   "Terminate all the proxies listed in the CONTAINERS vhash, sending them
@@ -132,7 +146,8 @@ MESSAGE.  Return the new state."
                                    #:kernel-socket kernel-control)
                      (unmonitor-client proxy)
                      (close-kernel proxy)
-                     (false-if-exception (kill (kernel-pid proxy) SIGTERM))))
+                     (false-if-exception (kill (kernel-pid proxy) SIGTERM))
+                     (remove-kernel-directory proxy)))
                   (proxy-state-proxied state))
   (set-proxy-state-proxied state vlist-null))