prow.sh: take Go version from Kubernetes source

Using the same (recent) Go version for all Kubernetes versions can
break for older versions when there are incompatible changes in Go. To
avoid that, we use exactly the minimum version of Go required for each
Kubernetes version. This is based on the assumption that this
combination was tested successfully.

When building the E2E suite from Kubernetes (the default) we do the
same, but still allow building it from elsewhere.

We allow the Go version to be empty when it doesn't matter.

prow.sh

@@ -60,8 +60,7 @@ go_from_travis_yml () {
     grep "^ *- go:" "${RELEASE_TOOLS_ROOT}/travis.yml" | sed -e 's/.*go: *//'
 }
 configvar CSI_PROW_GO_VERSION_BUILD "$(go_from_travis_yml)" "Go version for building the component" # depends on component's source code
-configvar CSI_PROW_GO_VERSION_K8S 1.12.1 "Go version for building Kubernetes for the test cluster" # depends on Kubernetes version
-configvar CSI_PROW_GO_VERSION_E2E 1.12.1 "Go version for building the Kubernetes E2E test suite" # depends on CSI_PROW_E2E settings below
+configvar CSI_PROW_GO_VERSION_E2E "" "override Go version for building the Kubernetes E2E test suite" # normally doesn't need to be set, see install_e2e
 configvar CSI_PROW_GO_VERSION_SANITY "${CSI_PROW_GO_VERSION_BUILD}" "Go version for building the csi-sanity test suite" # depends on CSI_PROW_SANITY settings below
 configvar CSI_PROW_GO_VERSION_KIND "${CSI_PROW_GO_VERSION_BUILD}" "Go version for building 'kind'" # depends on CSI_PROW_KIND_VERSION below
 configvar CSI_PROW_GO_VERSION_GINKGO "${CSI_PROW_GO_VERSION_BUILD}" "Go version for building ginkgo" # depends on CSI_PROW_GINKGO_VERSION below
@@ -292,12 +291,14 @@ mkdir -p "${CSI_PROW_BIN}"
 PATH="${CSI_PROW_BIN}:$PATH"
 
 # Ensure that PATH has the desired version of the Go tools, then run command given as argument.
+# Empty parameter uses the already installed Go. In Prow, that version is kept up-to-date by
+# bumping the container image regularly.
 run_with_go () {
     local version
     version="$1"
     shift
 
-    if go version 2>/dev/null | grep -q "go$version"; then
+    if ! [ "$version" ] || go version 2>/dev/null | grep -q "go$version"; then
         run "$@"
     else
         if ! [ -d "${CSI_PROW_WORK}/go-$version" ];  then
@@ -374,6 +375,20 @@ list_gates () (
     done
 )
 
+go_version_for_kubernetes () (
+    local path="$1"
+    local version="$2"
+    local go_version
+
+    # We use the minimal Go version specified for each K8S release (= minimum_go_version in hack/lib/golang.sh).
+    # More recent versions might also work, but we don't want to count on that.
+    go_version="$(grep minimum_go_version= "$path/hack/lib/golang.sh" | sed -e 's/.*=go//')"
+    if ! [ "$go_version" ]; then
+        die "Unable to determine Go version for Kubernetes $version from hack/lib/golang.sh."
+    fi
+    echo "$go_version"
+)
+
 csi_prow_kind_have_kubernetes=false
 # Brings up a Kubernetes cluster and sets KUBECONFIG.
 # Accepts additional feature gates in the form gate1=true|false,gate2=...
@@ -399,7 +414,8 @@ start_cluster () {
             # while v1.14.0-fake.1 did.
             (cd "$GOPATH/src/k8s.io/kubernetes" && run git tag -f v1.14.0-fake.1) || die "git tag failed"
 
-            run_with_go "${CSI_PROW_GO_VERSION_K8S}" kind build node-image --type bazel --image csiprow/node:latest --kube-root "$GOPATH/src/k8s.io/kubernetes" || die "'kind build node-image' failed"
+            go_version="$(go_version_for_kubernetes "$GOPATH/src/k8s.io/kubernetes" "$version")" || die "cannot proceed without knowing Go version for Kubernetes"
+            run_with_go "$go_version" kind build node-image --type bazel --image csiprow/node:latest --kube-root "$GOPATH/src/k8s.io/kubernetes" || die "'kind build node-image' failed"
             csi_prow_kind_have_kubernetes=true
         fi
         image="csiprow/node:latest"
@@ -563,7 +579,8 @@ install_e2e () {
 
     git_checkout "${CSI_PROW_E2E_REPO}" "${GOPATH}/src/${CSI_PROW_E2E_IMPORT_PATH}" "${CSI_PROW_E2E_VERSION}" --depth=1 &&
     if [ "${CSI_PROW_E2E_IMPORT_PATH}" = "k8s.io/kubernetes" ]; then
-        run_with_go "${CSI_PROW_GO_VERSION_E2E}" make WHAT=test/e2e/e2e.test "-C${GOPATH}/src/${CSI_PROW_E2E_IMPORT_PATH}" &&
+        go_version="${CSI_PROW_GO_VERSION_E2E:-$(go_version_for_kubernetes "${GOPATH}/src/${CSI_PROW_E2E_IMPORT_PATH}" "${CSI_PROW_E2E_VERSION}")}" &&
+        run_with_go "$go_version" make WHAT=test/e2e/e2e.test "-C${GOPATH}/src/${CSI_PROW_E2E_IMPORT_PATH}" &&
         ln -s "${GOPATH}/src/${CSI_PROW_E2E_IMPORT_PATH}/_output/bin/e2e.test" "${CSI_PROW_WORK}"
     else
         run_with_go "${CSI_PROW_GO_VERSION_E2E}" go test -c -o "${CSI_PROW_WORK}/e2e.test" "${CSI_PROW_E2E_IMPORT_PATH}/test/e2e"