svn-download: Reduce svn-fetch builder duplication.

Rather than creating a different builder in the store for every different
download (by hash), remove the hash from the builder and pass it in via an
environment variable.  This means that when svn-fetch is used by two different
package sources, the derivations will still differ but the builder will be
shared.

It used to be this way, but changed with
0e73f933b291c7e154c7e019b6de1e2f3a97e4c1.  I noticed the hash in the builder
script when wondering why the build coordinator on bayfront was substituting
svn-multi-download files over and over again.

To try and make the effects of introducing variance in to the builder script
more obvious, separate it out in to it's own procedure, so that it's clearer
when there's new data going in that could cause variance.

* guix/svn-download.scm (svn-fetch): Extract out builder script, include hash
in the derivation as an environment variable and update the comment to be more
directive.
(svn-fetch-builder): New procedure.

Change-Id: I256b94666296ad747f494f0b497ca209b77fbfb4

1 files changed, 63 insertions, 50 deletions

diff --git a/guix/svn-download.scm b/guix/svn-download.scm
index 812a46c9d4..62649e4374 100644
--- a/guix/svn-download.scm
+++ b/guix/svn-download.scm
@@ -74,14 +74,7 @@
   (let ((distro (resolve-interface '(gnu packages version-control))))
     (module-ref distro 'subversion)))
 
-(define* (svn-fetch ref hash-algo hash
-                    #:optional name
-                    #:key (system (%current-system)) (guile (default-guile))
-                    (svn (subversion-package)))
-  "Return a fixed-output derivation that fetches REF, a <svn-reference>
-object.  The output is expected to have recursive hash HASH of type
-HASH-ALGO (a symbol).  Use NAME as the file name, or a generic name if #f."
-
+(define (svn-fetch-builder svn hash-algo)
   (define guile-json
     (module-ref (resolve-interface '(gnu packages guile)) 'guile-json-4))
 
@@ -97,51 +90,64 @@ HASH-ALGO (a symbol).  Use NAME as the file name, or a generic name if #f."
           (module-ref (resolve-interface '(gnu packages base))
                       'tar)))
 
-  (define build
-    (with-imported-modules
-        (source-module-closure '((guix build svn)
-                                 (guix build download)
-                                 (guix build download-nar)
-                                 (guix build utils)
-                                 (guix swh)))
-      (with-extensions (list guile-json guile-gnutls   ;for (guix swh)
-                             guile-lzlib)
-        #~(begin
-            (use-modules (guix build svn)
-                         ((guix build download)
-                          #:select (download-method-enabled?))
-                         (guix build download-nar)
-                         (guix build utils)
-                         (guix swh)
-                         (ice-9 match))
+  (with-imported-modules
+      (source-module-closure '((guix build svn)
+                               (guix build download)
+                               (guix build download-nar)
+                               (guix build utils)
+                               (guix swh)))
+    (with-extensions (list guile-json guile-gnutls   ;for (guix swh)
+                           guile-lzlib)
+      #~(begin
+          (use-modules (guix build svn)
+                       ((guix build download)
+                        #:select (download-method-enabled?))
+                       (guix build download-nar)
+                       (guix build utils)
+                       (guix swh)
+                       (ice-9 match))
 
-            ;; Add tar and gzip to $PATH so
-            ;; 'swh-download-directory-by-nar-hash' can invoke them.
-            (set-path-environment-variable "PATH" '("bin") '(#+@tar+gzip))
+          ;; Add tar and gzip to $PATH so
+          ;; 'swh-download-directory-by-nar-hash' can invoke them.
+          (set-path-environment-variable "PATH" '("bin") '(#+@tar+gzip))
 
-            (or (and (download-method-enabled? 'upstream)
-                     (svn-fetch (getenv "svn url")
-                                (string->number (getenv "svn revision"))
-                                #$output
-                                #:svn-command #+(file-append svn "/bin/svn")
-                                #:recursive? (match (getenv "svn recursive?")
-                                               ("yes" #t)
-                                               (_ #f))
-                                #:user-name (getenv "svn user name")
-                                #:password (getenv "svn password")))
-                (and (download-method-enabled? 'nar)
-                     (download-nar #$output))
-                (and (download-method-enabled? 'swh)
-                     (parameterize ((%verify-swh-certificate? #f))
-                       (swh-download-directory-by-nar-hash #$hash '#$hash-algo
-                                                           #$output))))))))
+          (or (and (download-method-enabled? 'upstream)
+                   (svn-fetch (getenv "svn url")
+                              (string->number (getenv "svn revision"))
+                              #$output
+                              #:svn-command #+(file-append svn "/bin/svn")
+                              #:recursive? (match (getenv "svn recursive?")
+                                             ("yes" #t)
+                                             (_ #f))
+                              #:user-name (getenv "svn user name")
+                              #:password (getenv "svn password")))
+              (and (download-method-enabled? 'nar)
+                   (download-nar #$output))
+              (and (download-method-enabled? 'swh)
+                   (parameterize ((%verify-swh-certificate? #f))
+                     (swh-download-directory-by-nar-hash
+                      (u8-list->bytevector
+                       (map string->number
+                            (string-split (getenv "hash") #\,)))
+                      '#$hash-algo
+                      #$output))))))))
 
+(define* (svn-fetch ref hash-algo hash
+                    #:optional name
+                    #:key (system (%current-system)) (guile (default-guile))
+                    (svn (subversion-package)))
+  "Return a fixed-output derivation that fetches REF, a <svn-reference>
+object.  The output is expected to have recursive hash HASH of type
+HASH-ALGO (a symbol).  Use NAME as the file name, or a generic name if #f."
   (mlet %store-monad ((guile (package->derivation guile system)))
-    (gexp->derivation (or name "svn-checkout") build
-
-                      ;; Use environment variables and a fixed script name so
-                      ;; there's only one script in store for all the
-                      ;; downloads.
+    (gexp->derivation (or name "svn-checkout")
+                      ;; Avoid the builder differing for every single use as
+                      ;; having less builder is more efficient for computing
+                      ;; derivations.
+                      ;;
+                      ;; Don't pass package specific data in to the following
+                      ;; procedure, use #:env-vars below instead.
+                      (svn-fetch-builder svn hash-algo)
                       #:script-name "svn-download"
                       #:env-vars
                       `(("svn url" . ,(svn-reference-url ref))
@@ -161,7 +167,14 @@ HASH-ALGO (a symbol).  Use NAME as the file name, or a generic name if #f."
                         ,@(match (getenv "GUIX_DOWNLOAD_METHODS")
                             (#f '())
                             (value
-                             `(("GUIX_DOWNLOAD_METHODS" . ,value)))))
+                             `(("GUIX_DOWNLOAD_METHODS" . ,value))))
+                        ;; To avoid pulling in (guix base32) in the builder
+                        ;; script, use bytevector->u8-list from (rnrs
+                        ;; bytevectors)
+                        ("hash" . ,(string-join
+                                    (map number->string
+                                         (bytevector->u8-list hash))
+                                    ",")))
 
                       #:system system
                       #:hash-algo hash-algo