[bug] [HTTPPipeline] loadBalance is required but causes panic if missing

[bigangryrobot]

If you create an http pipeline without a loadBalance definition, like is given here, you will receive a panic

payload

name: something
kind: HTTPPipeline
flow:
  - filter: proxyToBackend
filters:
  - name: proxyToBackend
    kind: Proxy
    mainPool:
      servers:
      - url: some backend

client error

Error: 400: validate spec failed:
generalErrs:
- |
  filters: jsonschemaErrs:
  - 'mainPool: loadBalance is required'
  systemErr: 'BUG: call Validate for proxy.PoolSpec panic: runtime error: invalid memory
    address or nil pointer dereference'

exit status 1

server error

2021-06-23T19:09:03.093Z	ERROR	v/validaterecorder.go:145	BUG: call Validate for proxy.PoolSpec panic: runtime error: invalid memory address or nil pointer dereference: goroutine 305224 [running]:
runtime/debug.Stack(0x17c4de1, 0x23, 0xc000f56250)
	/usr/local/go/src/runtime/debug/stack.go:24 +0x9f
github.com/megaease/easegress/pkg/v.(*ValidateRecorder).recordGeneral.func1(0x7f8b8c218db8, 0xc00131a600, 0xc0015ebb00)
	/go/pkg/mod/github.com/megaease/easegress@v1.0.0/pkg/v/validaterecorder.go:145 +0xf5
panic(0x1584bc0, 0x23145b0)
	/usr/local/go/src/runtime/panic.go:969 +0x1b9
github.com/megaease/easegress/pkg/filter/proxy.PoolSpec.Validate(0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0xc000548928, 0x1, 0x1, 0x0, ...)
	/go/pkg/mod/github.com/megaease/easegress@v1.0.0/pkg/filter/proxy/pool.go:89 +0xbf
github.com/megaease/easegress/pkg/v.(*ValidateRecorder).recordGeneral(0xc0015ebb00, 0xc0022ac0e0, 0x0)
	/go/pkg/mod/github.com/megaease/easegress@v1.0.0/pkg/v/validaterecorder.go:150 +0x14b
github.com/megaease/easegress/pkg/v.traverseGo(0xc0022ac0e0, 0x0, 0xc000f56d58)
	/go/pkg/mod/github.com/megaease/easegress@v1.0.0/pkg/v/v.go:185 +0xf2
github.com/megaease/easegress/pkg/v.traverseGo(0xc0022ac0a0, 0xc00103db20, 0xc000f56d58)
	/go/pkg/mod/github.com/megaease/easegress@v1.0.0/pkg/v/v.go:214 +0x7a5
github.com/megaease/easegress/pkg/v.traverseGo(0xc001fb1fc0, 0x0, 0xc000f56d58)
	/go/pkg/mod/github.com/megaease/easegress@v1.0.0/pkg/v/v.go:198 +0x657
github.com/megaease/easegress/pkg/v.traverseGo(0xc001fb1be0, 0x0, 0xc000f56d58)
	/go/pkg/mod/github.com/megaease/easegress@v1.0.0/pkg/v/v.go:214 +0x7a5
github.com/megaease/easegress/pkg/v.Validate(0x158f700, 0xc000fc6850, 0xc0004bf030, 0x66, 0x70, 0x0)
	/go/pkg/mod/github.com/megaease/easegress@v1.0.0/pkg/v/v.go:118 +0x77d
github.com/megaease/easegress/pkg/object/httppipeline.newFilterSpecInternal(0xc00229f230, 0xc001fab2c0, 0xc0021c6ab0, 0x7)
	/go/pkg/mod/github.com/megaease/easegress@v1.0.0/pkg/object/httppipeline/spec.go:91 +0x457
github.com/megaease/easegress/pkg/object/httppipeline.Spec.Validate(0xc0019e26e0, 0x1, 0x1, 0xc0005486c0, 0x1, 0x1, 0xc000ed4c00, 0xafb, 0xc00, 0x0, ...)
	/go/pkg/mod/github.com/megaease/easegress@v1.0.0/pkg/object/httppipeline/httppipeline.go:251 +0x2b3
github.com/megaease/easegress/pkg/v.Validate(0x15538a0, 0xc00229e750, 0xc000ed4c00, 0xafb, 0xc00, 0xc00)
	/go/pkg/mod/github.com/megaease/easegress@v1.0.0/pkg/v/v.go:107 +0x836
github.com/megaease/easegress/pkg/supervisor.NewSpec(0xc0005b1000, 0xafb, 0xafb, 0xc0005b1000, 0xafb)

Some notes on this

• parser should never panic
• errors should be clearer (look at k8s for examples)

[bigangryrobot]

it should be notes as well that the docs seem to indicate that loadBalance is not required

https://github.com/megaease/easegress/blob/main/doc/filters.md#proxy