bug#26316: Enable threaded compression of source tarballs

[Leo Famulari <leo@famulari.name>]

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This patch enables multi-threaded xz compression in the repacking stage
of building source tarballs.

With it applied, I get a ~2.2x speedup building the source of
linux-libre with 4 threads, compared to 1 thread.

Specifically, with the upstream linux-libre tarball in the page cache,
`guix build -S linux-libre` took ~366 seconds using a single thread for
xz, and ~164 seconds using 4 threads.

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From 78547a09e50440c649c3d28d7691f32fdd47cc25 Mon Sep 17 00:00:00 2001
From: Leo Famulari <leo@famulari.name>
Date: Thu, 30 Mar 2017 03:10:48 -0400
Subject: [PATCH] packages: Enable threaded compression of source tarballs.

This provides a ~2x speedup when using 4 threads.

* guix/packages.scm (patch-and-repack)[build]: Invoke xz with
'--threads=0' when re-packing tarballs.
---
 guix/packages.scm | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/guix/packages.scm b/guix/packages.scm
index 4bc4b017f..30be5bf38 100644
--- a/guix/packages.scm
+++ b/guix/packages.scm
@@ -573,7 +573,12 @@ specifies modules in scope when evaluating SNIPPET."
                                                     #:fail-on-error? #t)))))
                         (zero? (apply system*
                                       (string-append #+tar "/bin/tar")
-                                      "cvfa" #$output
+                                      "cvf" #$output
+                                      ;; The bootstrap xz does not support
+                                      ;; threaded compression (introduced in
+                                      ;; 5.2.0), but it ignores the extra flag.
+                                      (string-append "--use-compress-program="
+                                                     #+xz "/bin/xz --threads=0")
                                       ;; avoid non-determinism in the archive
                                       "--mtime=@0"
                                       "--owner=root:0"
-- 
2.12.2


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