ci: Spread builds across multiple nodes.

When a .tarball-version file containing the version string is present,
it is taken for granted that the source tarball is present.  This
allows to lift the requirement on a Git checkout to use the source
tarball.  This makes it easy and cheap to sync this tarball and
accompanying .tarball-version file across different Jenkins agents to
distribute the packaging jobs, bring the total build time from about 1
h 40 to 40 minutes.

* Makefile (TARBALL_VERSION): New variable.  When defined, do not do
not invoke Git to derive the version string, simply use it.
[RELEASE_TARBALL_FILENAME]: Do not compute prerequisites when
TARBALL_VERSION is defined; assume the tarball is present.
* .gitignore: Ignore .tarball-version.
* Jenkinsfile: (Generate release tarball): Generate a .tarball-version
file, and stash it along the release tarball.  Explicit this should
run on a 'guix' agent.
(Build packages): Forward to any 'linux-builder' agent, clean the
workspace, unstash and extract the tarball and build.

GitLab: jami-packaging#98
Change-Id: I13088b75c9ccd5166662a84b7c5f8d4e8b7b65da

.gitignore

@@ -11,6 +11,7 @@ repositories
 tarballs.manifest
 manual-download
 .docker-image-*
+.tarball-version
 
 # QtCreator
 /build-*

Jenkinsfile

@@ -52,8 +52,10 @@ See https://wiki.savoirfairelinux.com/wiki/Jenkins.jami.net#Configuration"
                 // environment variables used by Guix.
                 sh '''#!/usr/bin/env bash
                    test -f $HOME/.bashrc && . $HOME/.bashrc
-                   make portable-release-tarball
+                   make portable-release-tarball .tarball-version
                    '''
+                stash(includes: '*.tar.gz, .tarball-version',
+                      name: 'release-tarball')
             }
         }
 
@@ -81,8 +83,16 @@ See https://wiki.savoirfairelinux.com/wiki/Jenkins.jami.net#Configuration"
                         // Note: The stage calls are wrapped in closures, to
                         // delay their execution.
                         stages["${target}"] =  {
-                            stage("stage: ${target}") {
-                                sh "make ${target}"
+                            stage("${target}") {
+                                // Offload builds to different agents.
+                                node('linux-builder') {
+                                    cleanWs()
+                                    unstash 'release-tarball'
+                                    sh """
+                                       tar xf *.tar.gz --strip-components=1
+                                       make ${target}
+                                       """
+                                }
                             }
                         }
                     }

Makefile

@@ -19,6 +19,9 @@
 ##############################
 ## Version number variables ##
 ##############################
+TARBALL_VERSION := $(shell cat $(CURDIR)/.tarball-version 2> /dev/null)
+
+ifeq ($(TARBALL_VERSION),)
 # YYYY-MM-DD
 LAST_COMMIT_DATE:=$(shell git log -1 --format=%cd --date=short)
 
@@ -32,6 +35,10 @@ LAST_COMMIT_DATE_SHORT:=$(shell echo $(LAST_COMMIT_DATE) | sed -s 's/-//g')
 COMMIT_ID:=$(shell git rev-parse --short HEAD)
 
 RELEASE_VERSION:=$(LAST_COMMIT_DATE_SHORT).$(NUMBER_OF_COMMITS).$(COMMIT_ID)
+else
+$(warning Using version from the .tarball-version file: $(TARBALL_VERSION))
+RELEASE_VERSION:=$(TARBALL_VERSION)
+endif
 RELEASE_TARBALL_FILENAME:=jami_$(RELEASE_VERSION).tar.gz
 
 # Debian versions
@@ -59,6 +66,12 @@ CURRENT_GID:=$(shell id -g)
 #############################
 .PHONY: release-tarball purge-release-tarballs portable-release-tarball
 
+# This file can be used when not wanting to invoke the tarball
+# producing machinery (which depends on the Git checkout), nor its
+# prerequisites.  It is used to set the TARBALL_VERSION Make variable.
+.tarball-version:
+	echo $(RELEASE_VERSION) > $@
+
 purge-release-tarballs:
 	rm -f jami_*.tar.* tarballs.manifest
 
@@ -96,6 +109,7 @@ tarballs.manifest: daemon/contrib/native/Makefile
 	$(MAKE) fetch -j && \
 	$(MAKE) --no-print-directory --silent list-tarballs > "$(CURDIR)/$@"
 
+ifeq ($(TARBALL_VERSION),)
 # Generate the release tarball.  To regenerate a fresh tarball
 # manually clear the tarballs.manifest file.
 $(RELEASE_TARBALL_FILENAME): tarballs.manifest
@@ -117,6 +131,11 @@ $(RELEASE_TARBALL_FILENAME): tarballs.manifest
 	gzip $(TMPDIR)/ring-project.tar
 	mv $(TMPDIR)/ring-project.tar.gz "$@"
 	rm -rf $(TMPDIR)
+else
+# If TARBALL_VERSION is defined, assume it's already been generated,
+# without doing any checks, which would require Git.
+$(RELEASE_TARBALL_FILENAME):
+endif
 
 #######################
 ## Packaging targets ##