Apply TaskRun CR without TaskRef field will panic in webhook

[mathspanda]

Actual Behavior

Apply TaskRun CR without TaskRef field will panic in webhook.

Steps to Reproduce the Problem

kubectl apply a TaskRun CR without taskRef field like:

apiVersion: pipeline.knative.dev/v1alpha1
kind: TaskRun
metadata:
  name: build-push-task-run-2
spec:
  trigger:
    triggerRef:
      type: manual
  inputs:
    resources:
      - name: workspace
        resourceRef:
          name: go-example-git
  taskSpec:
    inputs:
      resources:
        - name: workspace
          type: git
    steps:
      - name: build-and-push
        image: gcr.io/kaniko-project/executor
        command:
          - /kaniko/executor
        args:
          - --destination=gcr.io/my-project/gohelloworld

The logs in webhook are:

{"level":"info","logger":"webhook","caller":"webhook/webhook.go:569","msg":"No changes in the spec, not bumping generation","knative.dev/controller":"webhook","knative.dev/kind":"pipeline.knative.dev/v1alpha1, Kind=TaskRun","knative.dev/namespace":"linpeiyu","knative.dev/name":"build-push-task-run-2","knative.dev/operation":"UPDATE","knative.dev/resource":"{pipeline.knative.dev v1alpha1 taskruns}","knative.dev/subresource":"","knative.dev/userinfo":"{system:serviceaccount:kube-system:generic-garbage-collector ebccf2e4-f148-11e8-b12b-00155d3e7f00 [system:serviceaccounts system:serviceaccounts:kube-system system:authenticated] map[]}"}
2018/12/26 11:11:48 http2: panic serving 10.100.123.192:1709: runtime error: invalid memory address or nil pointer dereference
goroutine 47 [running]:
net/http.(*http2serverConn).runHandler.func1(0xc0005fc0a0, 0xc0003b1faf, 0xc0004f0000)
	/usr/local/go/src/net/http/h2_bundle.go:5870 +0x17c
panic(0x103d6c0, 0x1cd3eb0)
	/usr/local/go/src/runtime/panic.go:513 +0x1b9
github.com/knative/build-pipeline/pkg/apis/pipeline/v1alpha1.(*TaskRunSpec).SetDefaults(...)
	/Users/linpeiyu/gopath/src/github.com/knative/build-pipeline/pkg/apis/pipeline/v1alpha1/taskrun_defaults.go:24
github.com/knative/build-pipeline/pkg/apis/pipeline/v1alpha1.(*TaskRun).SetDefaults(0xc000195440)
	/Users/linpeiyu/gopath/src/github.com/knative/build-pipeline/pkg/apis/pipeline/v1alpha1/taskrun_defaults.go:20 +0x30
github.com/knative/build-pipeline/vendor/github.com/knative/pkg/webhook.SetDefaults.func1(0xc0005a4520, 0x1301440, 0xc000195440, 0x1301440, 0xc000195200)
	/Users/linpeiyu/gopath/src/github.com/knative/build-pipeline/vendor/github.com/knative/pkg/webhook/webhook.go:228 +0x77
github.com/knative/build-pipeline/vendor/github.com/knative/pkg/webhook.(*AdmissionController).mutate(0xc000312000, 0x1301180, 0xc00061bb90, 0xc0005f8f80, 0x14, 0xc0005728c0, 0x8, 0xc0005728c8, 0x7, 0xc000689500, ...)
	/Users/linpeiyu/gopath/src/github.com/knative/build-pipeline/vendor/github.com/knative/pkg/webhook/webhook.go:531 +0xa72
github.com/knative/build-pipeline/vendor/github.com/knative/pkg/webhook.(*AdmissionController).admit(0xc000312000, 0x1301180, 0xc00061bb90, 0xc000532c80, 0xc00061bb90)
	/Users/linpeiyu/gopath/src/github.com/knative/build-pipeline/vendor/github.com/knative/pkg/webhook/webhook.go:468 +0x153
github.com/knative/build-pipeline/vendor/github.com/knative/pkg/webhook.(*AdmissionController).ServeHTTP(0xc000312000, 0x1300280, 0xc0005fc0a0, 0xc000486c00)
	/Users/linpeiyu/gopath/src/github.com/knative/build-pipeline/vendor/github.com/knative/pkg/webhook/webhook.go:435 +0xe40
net/http.serverHandler.ServeHTTP(0xc0003205b0, 0x1300280, 0xc0005fc0a0, 0xc000486c00)
	/usr/local/go/src/net/http/server.go:2741 +0xab
net/http.initNPNRequest.ServeHTTP(0xc00031ae00, 0xc0003205b0, 0x1300280, 0xc0005fc0a0, 0xc000486c00)
	/usr/local/go/src/net/http/server.go:3291 +0x8d
net/http.Handler.ServeHTTP-fm(0x1300280, 0xc0005fc0a0, 0xc000486c00)
	/usr/local/go/src/net/http/h2_bundle.go:5592 +0x4d
net/http.(*http2serverConn).runHandler(0xc0004f0000, 0xc0005fc0a0, 0xc000486c00, 0xc000594a00)
	/usr/local/go/src/net/http/h2_bundle.go:5877 +0x89
created by net/http.(*http2serverConn).processHeaders
	/usr/local/go/src/net/http/h2_bundle.go:5611 +0x4d3

Additional Info

/kind bug

[bobcatfish]

Thanks for the detailed report and for fixing this @mathspanda !! 🎉