makepkg: respect $SOURCE_DATE_EPOCH to activate reproducible builds

If SOURCE_DATE_EPOCH is set, `touch` all source files between the (optional)
prepare() and build() functions to unify the modification times. This works
around build systems and compilers that embed the file modification times
into the file contents of release artifacts.

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Allan McRae <allan@archlinux.org>

doc/makepkg.8.txt

@@ -206,6 +206,7 @@ Options
 *\--printsrcinfo*::
 	Generate and print the SRCINFO file to stdout.
 
+
 Additional Features
 -------------------
 makepkg supports building development versions of packages without having to
@@ -214,6 +215,19 @@ separate utility 'versionpkg'. See linkman:PKGBUILD[5] for details on how to
 set up a development PKGBUILD.
 
 
+Reproducibility
+---------------
+makepkg is designed to be compatible with
+link:https://reproducible-builds.org/docs/[Reproducible Builds]. If the
+**SOURCE_DATE_EPOCH** environment variable is set, it will be exported to
+subprocesses, and source and package file modification times and package
+metadata will be unified based on the timestamp specified.
+
+If the **SOURCE_DATE_EPOCH** environment variable is not set, makepkg will use
+its own start date for internal use, but will not unify source file timestamps
+before building.
+
+
 Environment Variables
 ---------------------
 **PACMAN**::
@@ -265,6 +279,8 @@ Environment Variables
 	Specify a key to use when signing packages, overriding the GPGKEY setting
 	in linkman:makepkg.conf[5]
 
+**SOURCE_DATE_EPOCH=**"<date>"::
+	Used for link:https://reproducible-builds.org/docs/[Reproducible Builds].
 
 Configuration
 -------------

scripts/makepkg.sh.in

@@ -79,6 +79,7 @@ PKGFUNC=0
 PKGVERFUNC=0
 PREPAREFUNC=0
 REPKG=0
+REPRODUCIBLE=0
 RMDEPS=0
 SKIPCHECKSUMS=0
 SKIPPGPCHECK=0
@@ -87,7 +88,12 @@ SPLITPKG=0
 SOURCEONLY=0
 VERIFYSOURCE=0
 
-export SOURCE_DATE_EPOCH=${SOURCE_DATE_EPOCH:-$(date +%s)}
+if [[ -n $SOURCE_DATE_EPOCH ]]; then
+	REPRODUCIBLE=1
+else
+	SOURCE_DATE_EPOCH=$(date +%s)
+fi
+export SOURCE_DATE_EPOCH
 
 PACMAN_OPTS=()
 
@@ -1686,6 +1692,11 @@ if (( !REPKG )); then
 		if (( PREPAREFUNC )); then
 			run_prepare
 		fi
+		if (( REPRODUCIBLE )); then
+			# We have activated reproducible builds, so unify source times before
+			# building
+			find "$srcdir" -exec touch -h -d @$SOURCE_DATE_EPOCH {} +
+		fi
 	fi
 
 	if (( PKGVERFUNC )); then