api: make approvers required

I chose approvers for the KEP that needed modifcation by plumbing
through git history and looking at
kubernetes#1247 which introduced
the original KEP

api/proposal.go

@@ -110,7 +110,7 @@ type Proposal struct {
 	OwningSIG         string   `json:"owningSig" yaml:"owning-sig" validate:"required"`
 	ParticipatingSIGs []string `json:"participatingSigs" yaml:"participating-sigs,flow,omitempty"`
 	Reviewers         []string `json:"reviewers" yaml:",flow"`
-	Approvers         []string `json:"approvers" yaml:",flow"`
+	Approvers         []string `json:"approvers" yaml:",flow" validate:"required"`
 	PRRApprovers      []string `json:"prrApprovers" yaml:"prr-approvers,flow"`
 	Editor            string   `json:"editor" yaml:"editor,omitempty"`
 	CreationDate      string   `json:"creationDate" yaml:"creation-date"`

keps/sig-network/2449-move-externalDNS-out-of-kubernetes-incubator/kep.yaml

@@ -2,5 +2,8 @@ title: Move ExternalDNS out of Kubernetes incubator
 kep-number: 2449
 authors:
   - "@njuettner"
+approvers:
+  - bowei
+  - thockin
 owning-sig: sig-network
 status: implemented

pkg/repo/repo_test.go

@@ -70,6 +70,11 @@ func TestProposalValidate(t *testing.T) {
 		file string
 		errs []error
 	}{
+		{
+			name: "valid KEP: minimum fields",
+			file: "testdata/valid-kep-minimum.yaml",
+			errs: nil,
+		},
 		{
 			name: "valid KEP: all fields",
 			file: "testdata/valid-kep-full.yaml",

pkg/repo/testdata/repos/valid/keps/sig-owns-participates/1-the-bare-minimum/kep.yaml

@@ -9,13 +9,13 @@ status: a non-empty string
 # creation-date: yyyy-mm-dd
 # reviewers:
 #   - "@beth"
-# approvers:
-#   - "@carolyn"
+approvers:
+  - "@dorothy"
 # prr-approvers:
-#   - "@dorothy"
+#   - "@carolyn"
 # stage: stable
 # latest-milestone: v1.4
 # milestone:
 #   alpha: v1.1
 #   beta: v1.2
-#   stable: v1.3
+#   stable: v1.3

pkg/repo/testdata/valid-kep-minimum.yaml

@@ -0,0 +1,29 @@
+title: minimal
+kep-number: 1337
+authors:
+  - "@jpbetz"
+  - "@roycaihw"
+  - "@sttts"
+owning-sig: sig-api-machinery
+participating-sigs:
+  - sig-api-machinery
+  - sig-architecture
+reviewers:
+  - "@deads2k"
+  - "@lavalamp"
+  - "@liggitt"
+  - "@mbohlool"
+  - "@sttts"
+approvers:
+  - "@deads2k"
+  - "@lavalamp"
+creation-date: 2018-04-15
+last-updated: 2018-04-24
+status: provisional
+
+stage: beta
+latest-milestone: "v1.19"
+milestone:
+  alpha: "v1.19"
+  beta: "v1.20"
+  stable: "v1.22"