build for multiple platforms only in CI, add s390x

Developers should not be forced to build for all platforms by
default. We also don't want to copy-and-paste the go invocation for
each new platform.

To address both, the target platform(s) are now configurable via
BUILD_PLATFORMS and additional platforms are only enabled in the Prow
CI.

For now this serves as a test that the source actually compiles for
multiple platforms. Building images for different target platforms is a
different problem.

Author: pohly

Diff for: build.make

@@ -60,18 +60,25 @@ else
 TESTARGS =
 endif
 
-ARCH := $(if $(GOARCH),$(GOARCH),$(shell go env GOARCH))
-
 # Specific packages can be excluded from each of the tests below by setting the *_FILTER_CMD variables
 # to something like "| grep -v 'github.com/kubernetes-csi/project/pkg/foobar'". See usage below.
 
+# BUILD_PLATFORMS contains a set of <os> <arch> <suffix> triplets,
+# separated by semicolon. An empty variable or empty entry (= just a
+# semicolon) builds for the default platform of the current Go
+# toolchain.
+BUILD_PLATFORMS =
+
+# This builds each command (= the sub-directories of ./cmd) for the target platform(s)
+# defined by BUILD_PLATFORMS.
 build-%: check-go-version-go
 	mkdir -p bin
-	CGO_ENABLED=0 GOOS=linux go build $(GOFLAGS_VENDOR) -a -ldflags '-X main.version=$(REV) -extldflags "-static"' -o ./bin/$* ./cmd/$*
-	if [ "$$ARCH" = "amd64" ]; then \
-		CGO_ENABLED=0 GOOS=windows go build $(GOFLAGS_VENDOR) -a -ldflags '-X main.version=$(REV) -extldflags "-static"' -o ./bin/$*.exe ./cmd/$* ; \
-		CGO_ENABLED=0 GOOS=linux GOARCH=ppc64le go build $(GOFLAGS_VENDOR) -a -ldflags '-X main.version=$(REV) -extldflags "-static"' -o ./bin/$*-ppc64le ./cmd/$* ; \
-	fi
+	echo '$(BUILD_PLATFORMS)' | tr ';' '\n' | while read -r os arch suffix; do \
+		if ! (set -x; CGO_ENABLED=0 GOOS="$$os" GOARCH="$$arch" go build $(GOFLAGS_VENDOR) -a -ldflags '-X main.version=$(REV) -extldflags "-static"' -o "./bin/$*$$suffix" ./cmd/$*); then \
+			echo "Building $* for GOOS=$$os GOARCH=$$arch failed, see error(s) above."; \
+			exit 1; \
+		fi; \
+	done
 
 container-%: build-%
 	docker build -t $*:latest -f $(shell if [ -e ./cmd/$*/Dockerfile ]; then echo ./cmd/$*/Dockerfile; else echo Dockerfile; fi) --label revision=$(REV) .

Diff for: prow.sh

@@ -85,6 +85,8 @@ get_versioned_variable () {
     echo "$value"
 }
 
+configvar CSI_PROW_BUILD_PLATFORMS "linux amd64; windows amd64 .exe; linux ppc64le -ppc64le; linux s390x -s390x" "Go target platforms (= GOOS + GOARCH) and file suffix of the resulting binaries"
+
 # If we have a vendor directory, then use it. We must be careful to only
 # use this for "make" invocations inside the project's repo itself because
 # setting it globally can break other go usages (like "go get <some command>"
@@ -1026,7 +1028,7 @@ main () {
     images=
     if ${CSI_PROW_BUILD_JOB}; then
         # A successful build is required for testing.
-        run_with_go "${CSI_PROW_GO_VERSION_BUILD}" make all "GOFLAGS_VENDOR=${GOFLAGS_VENDOR}" || die "'make all' failed"
+        run_with_go "${CSI_PROW_GO_VERSION_BUILD}" make all "GOFLAGS_VENDOR=${GOFLAGS_VENDOR}" "BUILD_PLATFORMS=${CSI_PROW_BUILD_PLATFORMS}" || die "'make all' failed"
         # We don't want test failures to prevent E2E testing below, because the failure
         # might have been minor or unavoidable, for example when experimenting with
         # changes in "release-tools" in a PR (that fails the "is release-tools unmodified"