import cran: Fetch DESCRIPTION files from Github mirror.

* guix/import/cran.scm (%bioconductor-svn-url): Remove variable.
(bioconductor-mirror-url): New procedure.
(fetch-description): Take a REPOSITORY symbol instead of a BASE-URL string.
(cran->guix-package): Pass REPOSITORY symbol to "fetch-description".
(latest-cran-release, latest-bioconductor-release): Adjust accordingly.
(bioconductor-package?): Update comment about SVN.

guix/import/cran.scm

@@ -125,17 +125,19 @@ package definition."
 
 ;; The latest Bioconductor release is 3.5.  Bioconductor packages should be
 ;; updated together.
-(define %bioconductor-svn-url
-  (string-append "https://readonly:readonly@"
-                 "hedgehog.fhcrc.org/bioconductor/branches/RELEASE_3_5/"
-                 "madman/Rpacks/"))
+(define (bioconductor-mirror-url name)
+  (string-append "https://raw.githubusercontent.com/Bioconductor-mirror/"
+                 name "/release-3.5"))
 
-
-(define (fetch-description base-url name)
+(define (fetch-description repository name)
   "Return an alist of the contents of the DESCRIPTION file for the R package
-NAME, or #f in case of failure.  NAME is case-sensitive."
+NAME in the given REPOSITORY, or #f in case of failure.  NAME is
+case-sensitive."
   ;; This API always returns the latest release of the module.
-  (let ((url (string-append base-url name "/DESCRIPTION")))
+  (let ((url (string-append (case repository
+                              ((cran)         (string-append %cran-url name))
+                              ((bioconductor) (bioconductor-mirror-url name)))
+                            "/DESCRIPTION")))
     (guard (c ((http-get-error? c)
                (format (current-error-port)
                        "error: failed to retrieve package information \
@@ -291,11 +293,8 @@ from the alist META, which was derived from the R package's DESCRIPTION file."
    (lambda* (package-name #:optional (repo 'cran))
      "Fetch the metadata for PACKAGE-NAME from REPO and return the `package'
 s-expression corresponding to that package, or #f on failure."
-     (let* ((url (case repo
-                   ((cran)         %cran-url)
-                   ((bioconductor) %bioconductor-svn-url)))
-            (module-meta (fetch-description url package-name)))
-       (and=> module-meta (cut description->package repo <>))))))
+     (and=> (fetch-description repo package-name)
+            (cut description->package repo <>)))))
 
 (define* (recursive-import package-name #:optional (repo 'cran))
   "Generate a stream of package expressions for PACKAGE-NAME and all its
@@ -386,7 +385,7 @@ dependencies."
     (package->upstream-name package))
 
   (define meta
-    (fetch-description %cran-url upstream-name))
+    (fetch-description 'cran upstream-name))
 
   (and meta
        (let ((version (assoc-ref meta "Version")))
@@ -403,7 +402,7 @@ dependencies."
     (package->upstream-name package))
 
   (define meta
-    (fetch-description %bioconductor-svn-url upstream-name))
+    (fetch-description 'bioconductor upstream-name))
 
   (and meta
        (let ((version (assoc-ref meta "Version")))
@@ -430,7 +429,10 @@ dependencies."
   "Return true if PACKAGE is an R package from Bioconductor."
   (let ((predicate (lambda (uri)
                      (and (string-prefix? "http://bioconductor.org" uri)
-                          ;; Data packages are not listed in SVN
+                          ;; Data packages are neither listed in SVN nor on
+                          ;; the Github mirror, so we have to exclude them
+                          ;; from the set of bioconductor packages that can be
+                          ;; updated automatically.
                           (not (string-contains uri "/data/annotation/"))))))
     (and (string-prefix? "r-" (package-name package))
          (match (and=> (package-source package) origin-uri)