messenger: Add handler decorator that is resource aware

[lukedirtwalker]

If a resource is not healthy the handler should not be executed, instead an error should be returned.
This can be used to make handlers resource aware so that they don't execute when e.g. a database is not available.

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This change is 

[scrye]

Reviewed 3 of 3 files at r1.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @lukedirtwalker)

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go/lib/infra/common.go, line 251 at r1 (raw file):

// ResourceHealth indicates the health of a resource. A resource could for example be a database.
// The resource health can be registered on the messenger, if any resource is not healthy it will

Change the "resource health can be registered on the messenger" part to instead mention the handler decorator.

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go/lib/infra/common.go, line 332 at r1 (raw file):

	// RegisterResource registers the given resource with the messenger. If the resource is not
	// healthy the messenger replies to all requests with an error.
	RegisterResource(resource ResourceHealth)

This is no longer needed.

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go/lib/infra/common_test.go, line 54 at r1 (raw file):

		rHandler := infra.NewResourceAwareHandler(handler, &mockResource{name: "tstFail", healthy: false})
		rwMock := mock_infra.NewMockResponseWriter(ctrl)
		ctx := infra.NewContextWithResponseWriter(context.TODO(), rwMock)

Use context.Background() here instead, it will stay out of code greps that search for unfinished context implementations.

[lukedirtwalker]

Reviewable status: 1 of 3 files reviewed, 3 unresolved discussions (waiting on @scrye)

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go/lib/infra/common.go, line 251 at r1 (raw file):

Previously, scrye (Sergiu Costea) wrote…

Change the "resource health can be registered on the messenger" part to instead mention the handler decorator.

Done.

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go/lib/infra/common.go, line 332 at r1 (raw file):

Previously, scrye (Sergiu Costea) wrote…

This is no longer needed.

Done.

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go/lib/infra/common_test.go, line 54 at r1 (raw file):

Previously, scrye (Sergiu Costea) wrote…

Use context.Background() here instead, it will stay out of code greps that search for unfinished context implementations.

Done.

[scrye]

Reviewed 2 of 2 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved

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go/lib/infra/common.go, line 251 at r2 (raw file):

// ResourceHealth indicates the health of a resource. A resource could for example be a database.
// The resource health can be added to a handler, so that the handler only replies if all it's

since this needs a rebase anyway :P, it's -> its.

[scrye]

Reviewed 1 of 1 files at r3.
Reviewable status: complete! all files reviewed, all discussions resolved