Validate the minimum supported k8s version

Fixes #103

cmd/flagger/main.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 	"time"
 
+	semver "github.com/Masterminds/semver"
 	clientset "github.com/weaveworks/flagger/pkg/client/clientset/versioned"
 	informers "github.com/weaveworks/flagger/pkg/client/informers/externalversions"
 	"github.com/weaveworks/flagger/pkg/controller"
@@ -115,6 +116,26 @@ func main() {
 		logger.Fatalf("Error calling Kubernetes API: %v", err)
 	}
 
+	k8sVersionConstraint := "^1.11.0"
+
+	// We append -alpha.1 to the end of our version constraint so that prebuilds of later versions
+	// are considered valid for our purposes, as well as some managed solutions like EKS where they provide
+	// a version like `v1.12.6-eks-d69f1b`. It doesn't matter what the prelease value is here, just that it
+	// exists in our constraint.
+	semverConstraint, err := semver.NewConstraint(k8sVersionConstraint + "-alpha.1")
+	if err != nil {
+		logger.Fatalf("Error parsing kubernetes version constraint: %v", err)
+	}
+
+	k8sSemver, err := semver.NewVersion(ver.GitVersion)
+	if err != nil {
+		logger.Fatalf("Error parsing kubernetes version as a semantic version: %v", err)
+	}
+
+	if !semverConstraint.Check(k8sSemver) {
+		logger.Fatalf("Unsupported version of kubernetes detected.  Expected %s, got %v", k8sVersionConstraint, ver)
+	}
+
 	labels := strings.Split(selectorLabels, ",")
 	if len(labels) < 1 {
 		logger.Fatalf("At least one selector label is required")

go.mod

@@ -4,6 +4,7 @@ go 1.12
 
 require (
 	cloud.google.com/go v0.37.4 // indirect
+	github.com/Masterminds/semver v1.4.2
 	github.com/beorn7/perks v1.0.0 // indirect
 	github.com/bxcodec/faker v2.0.1+incompatible // indirect
 	github.com/envoyproxy/go-control-plane v0.8.0 // indirect

go.sum

@@ -8,6 +8,7 @@ github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03
 github.com/BurntSushi/xgb v0.0.0-20160522181843-27f122750802/go.mod h1:IVnqGOEym/WlBOVXweHU+Q+/VP0lqqI8lqeDx9IjBqo=
 github.com/MakeNowJust/heredoc v0.0.0-20171113091838-e9091a26100e/go.mod h1:64YHyfSL2R96J44Nlwm39UHepQbyR5q10x7iYa1ks2E=
 github.com/Masterminds/goutils v1.1.0/go.mod h1:8cTjp+g8YejhMuvIA5y2vz3BpJxksy863GQaJW2MFNU=
+github.com/Masterminds/semver v1.4.2 h1:WBLTQ37jOCzSLtXNdoo8bNM8876KhNqOKvrlGITgsTc=
 github.com/Masterminds/semver v1.4.2/go.mod h1:MB6lktGJrhw8PrUyiEoblNEGEQ+RzHPF078ddwwvV3Y=
 github.com/Masterminds/sprig v2.18.0+incompatible/go.mod h1:y6hNFY5UBTIWBxnzTeuNhlNS5hqE0NB0E6fgfo2Br3o=
 github.com/NYTimes/gziphandler v0.0.0-20170623195520-56545f4a5d46/go.mod h1:3wb06e3pkSAbeQ52E9H9iFoQsEEwGN64994WTCIhntQ=