Helper function to create event from HTTP Request or Response

[grayside]

Fixes #766

This PR provides two new functions: NewEventFromHttpRequest and NewEventFromHttpResponse. Both are added to v2/alias.go, and NewEventFromHttpRequest is also demonstrated in the README and docs.

I included HttpResponse because I saw the Message methods covered it. I included docs updates because I've visited the README and docs multiple times looking for this function. Happy to remove either if they are considered unnecessary or out of scope.

Creating this PR as a draft because the test coverage I've created routinely segfaults when it begins executing the transformation logic at:

sdk-go/v2/binding/to_event.go

Line 61 in c623f8b

return &e, Transformers(transformers).Transform((*EventMessage)(&e), encoder)

Here is an example of how this looks on my machine:

go test -v -run TestNewEventFromHttpRequest
=== RUN   TestNewEventFromHttpRequest
=== RUN   TestNewEventFromHttpRequest/Event{"specversion":"1.0","id":"full-event-0","source":"http://example.com/source","type":"com.example.FullEvent","subject":"topic","datacontenttype":"text/json","dataschema":"http://example.com/schema","time":"2020-03-21T12:34:56.78Z","data":"hello","exurl":"http://example.com/source","extime":"2020-03-21T12:34:56.78Z","exbool":true,"exint":42,"exstring":"exstring","exbinary":"AAECAw=="}
=== RUN   TestNewEventFromHttpRequest/Event{"specversion":"1.0","id":"full-event-0","source":"http://example.com/source","type":"com.example.FullEvent","subject":"topic","datacontenttype":"text/json","dataschema":"http://example.com/schema","time":"2020-03-21T12:34:56.78Z","data":"hello","exurl":"http://example.com/source","extime":"2020-03-21T12:34:56.78Z","exbool":true,"exint":42,"exstring":"exstring","exbinary":"AAECAw=="}/Structured_encoding
--- FAIL: TestNewEventFromHttpRequest (0.00s)
    --- FAIL: TestNewEventFromHttpRequest/Event{"specversion":"1.0","id":"full-event-0","source":"http://example.com/source","type":"com.example.FullEvent","subject":"topic","datacontenttype":"text/json","dataschema":"http://example.com/schema","time":"2020-03-21T12:34:56.78Z","data":"hello","exurl":"http://example.com/source","extime":"2020-03-21T12:34:56.78Z","exbool":true,"exint":42,"exstring":"exstring","exbinary":"AAECAw=="} (0.00s)
        --- FAIL: TestNewEventFromHttpRequest/Event{"specversion":"1.0","id":"full-event-0","source":"http://example.com/source","type":"com.example.FullEvent","subject":"topic","datacontenttype":"text/json","dataschema":"http://example.com/schema","time":"2020-03-21T12:34:56.78Z","data":"hello","exurl":"http://example.com/source","extime":"2020-03-21T12:34:56.78Z","exbool":true,"exint":42,"exstring":"exstring","exbinary":"AAECAw=="}/Structured_encoding (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x141d695]

goroutine 6 [running]:
testing.tRunner.func1.2({0x151cfe0, 0x1982990})
	/Users/clipped/opt/go/libexec/src/testing/testing.go:1389 +0x24e
testing.tRunner.func1()
	/Users/clipped/opt/go/libexec/src/testing/testing.go:1392 +0x39f
panic({0x151cfe0, 0x1982990})
	/Users/clipped/opt/go/libexec/src/runtime/panic.go:838 +0x207
github.com/cloudevents/sdk-go/v2/binding.Transformers.Transform(...)
	/Users/clipped/prototypes/snowpea-golang/sdk-go/v2/binding/transformer.go:34
github.com/cloudevents/sdk-go/v2/binding.ToEvent({0xc0000ec100?, 0x1466716?}, {0x1671030, 0xc0000207d0?}, {0xc00006ae78, 0x1, 0x0?})
	/Users/clipped/prototypes/snowpea-golang/sdk-go/v2/binding/to_event.go:65 +0x215
github.com/cloudevents/sdk-go/v2/protocol/http.NewEventFromHttpRequest(0x16707a0?)
	/Users/clipped/prototypes/snowpea-golang/sdk-go/v2/protocol/http/utility.go:19 +0x50
github.com/cloudevents/sdk-go/v2/protocol/http.TestNewEventFromHttpRequest.func1.1(0x0?)
	/Users/clipped/prototypes/snowpea-golang/sdk-go/v2/protocol/http/utility_test.go:42 +0xc6
testing.tRunner(0xc0002b81a0, 0xc0002ae1d0)
	/Users/clipped/opt/go/libexec/src/testing/testing.go:1439 +0x102
created by testing.(*T).Run
	/Users/clipped/opt/go/libexec/src/testing/testing.go:1486 +0x35f
exit status 2

[grayside]

A bit more detail: The line linked above is reached for events in binary encoding. When it happens, the transform statement is creating a transformer. Non-binary events are successfully created and returned earlier, and the Transformers slice is empty.

[grayside]

Thanks for the help troubleshooting. I'm going to clean this up to make it a non-draft.

Do you think there should be a passthru for transformers?

[n3wscott]

Looks like you just need to fix the signing and you will be good to go

[grayside]

I've signed off many of the commits, not sure how to fix it for all of them. Looking at the DCO doc, it looks like someone with write access can override. Can you point me to what my next step should be?

[n3wscott]

I've signed off many of the commits, not sure how to fix it for all of them. Looking at the DCO doc, it looks like someone with write access can override. Can you point me to what my next step should be?

Thanks! I got it!