dispatcher should be readonly

[vaikas]

Addresses #3686

Proposed Changes

• Make the InMemoryChannel dispatcher reconciler read only.
• Our Dispatcher looks at the IMC spec to construct the dispatching logic, but there's no reason it should ever update status.

Release Note

- 🐛 Fix bug
Dispatcher was incorrectly behaving like a normal reconciler instead of skipping status updates. I wonder if this was causing grief battling the normal reconciler.

Docs

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vaikas

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• OWNERS [vaikas]

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[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-eventing-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/inmemorychannel/dispatcher/controller.go	83.3%	83.9%	0.5

[slinkydeveloper]

/lgtm

[vaikas]

=== CONT  TestChannelTracingWithReply/Channel-messaging.knative.dev/v1
    tracing_test_helper.go:74: Timeout waiting for at least 1 matches.
        Error: FAIL MATCHING: saw 0/1 matching events.
        Recent events: 
        0 events seen, last 0 events:
        Match errors: 
        
        
        knative.dev/eventing/test/lib/recordevents.(*EventInfoStore).AssertAtLeast
        	/home/prow/go/src/knative.dev/eventing/test/lib/recordevents/event_info_store.go:183
        knative.dev/eventing/test/conformance/helpers.tracingTest
        	/home/prow/go/src/knative.dev/eventing/test/conformance/helpers/tracing_test_helper.go:74
        knative.dev/eventing/test/conformance/helpers.ChannelTracingTestHelperWithChannelTestRunner.func1
        	/home/prow/go/src/knative.dev/eventing/test/conformance/helpers/channel_tracing_test_helper.go:46
        knative.dev/eventing/test/lib.(*ComponentsTestRunner).RunTests.func1
        	/home/prow/go/src/knative.dev/eventing/test/lib/test_runner.go:75
        testing.tRunner
        	/root/.gvm/gos/go1.14.6/src/testing/testing.go:1039
        runtime.goexit
        	/root/.gvm/gos/go1.14.6/src/runtime/asm_amd64.s:1373
    k8s_events.go:62: EventListener stopped

/test pull-knative-eventing-conformance-tests

[grantr]

Nice find @vaikas! I wonder if this could affect other dispatchers that use controllers?

[lionelvillard]

PingSource was affected and fixed. Kafka Channel looks like it hasn't been updated (@matzew), as well as eventing-github mt source