build: add alternate output format for bootstrap.bash, as used by builders

I've been doing these tweaks by hand. I was going to write a tool in
Go for it, but it's not much additional shell here.

Fixes #22912
Updates #9797 (already closed)

Change-Id: Ia15bd9b6876e6f6a76aa9ca86b10f113095e96a3
Reviewed-on: https://go-review.googlesource.com/80895Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/bootstrap.bash

@@ -14,6 +14,15 @@
 #
 # Only changes that have been committed to Git (at least locally,
 # not necessary reviewed and submitted to master) are included in the tree.
+#
+# As a special case for Go's internal use only, if the
+# BOOTSTRAP_FORMAT environment variable is set to "mintgz", the
+# resulting archive is intended for use by the Go build system and
+# differs in that the mintgz file:
+#   * is a tar.gz file instead of bz2
+#   * has many unnecessary files deleted to reduce its size
+#   * does not have a shared directory component for each tar entry
+# Do not depend on the mintgz format.
 
 set -e
 
@@ -28,6 +37,11 @@ if [ -e $targ ]; then
 	exit 2
 fi
 
+if [ "$BOOTSTRAP_FORMAT" != "mintgz" -a "$BOOTSTRAP_FORMAT" != "" ]; then
+	echo "unknown BOOTSTRAP_FORMAT format"
+	exit 2
+fi
+
 unset GOROOT
 src=$(cd .. && pwd)
 echo "#### Copying to $targ"
@@ -62,8 +76,36 @@ else
 	rmdir bin/*_*
 	rm -rf "pkg/${gohostos}_${gohostarch}" "pkg/tool/${gohostos}_${gohostarch}"
 fi
+
+GITREV=$(git rev-parse --short HEAD)
 rm -rf pkg/bootstrap pkg/obj .git
 
+# Support for building minimal tar.gz for the builders.
+# The build system doesn't support bzip2, and by deleting more stuff,
+# they start faster, especially on machines without fast filesystems
+# and things like tmpfs configures.
+# Do not depend on this format. It's for internal use only.
+if [ "$BOOTSTRAP_FORMAT" = "mintgz" ]; then
+	OUTGZ="gobootstrap-${GOOS}-${GOARCH}-${GITREV}.tar.gz"
+	echo "Preparing to generate build system's ${OUTGZ}; cleaning ..."
+	rm -rf bin/gofmt
+	rm -rf src/runtime/race/race_*.syso
+	rm -rf api test doc misc/cgo/test misc/trace
+	rm -rf pkg/tool/*_*/{addr2line,api,cgo,cover,doc,fix,nm,objdump,pack,pprof,test2json,trace,vet}
+	rm -rf pkg/*_*/{image,database,cmd}
+	rm -rf $(find . -type d -name testdata)
+	find . -type f -name '*_test.go' -exec rm {} \;
+	# git clean doesn't clean symlinks apparently, and the buildlet
+	# rejects them, so:
+	find . -type l -exec rm {} \;
+
+	echo "Writing ${OUTGZ} ..."
+	tar cf - . | gzip -9 > ../$OUTGZ
+	cd ..
+	ls -l "$(pwd)/$OUTGZ"
+	exit 0
+fi
+
 echo ----
 echo Bootstrap toolchain for "$GOOS/$GOARCH" installed in "$(pwd)".
 echo Building tbz.