fix: make async iteration work for gRPC-fallback; refactor the code

[alexander-fenster]

TL;DR: this is a small refactor of the client functionality that was not released yet. Pretty safe to merge since async iteration in clients is not enabled yet.

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So this turned to be a refactor that made the async iteration code a little bit simpler.

Background: it turns out async iteration, as implemented, did not work for gRPC-fallback clients. The main reason for that is that createApiCall is implemented differently in fallback.ts, and fallback clients expect that exact implementation. Long story short, it was not possible to call a stub of pagedExpand in pagedExpandAsync; fallback clients needed the result of createApiCall.

So... since the result of createApiCall is a function and not a promise, we can remove all the complicated asynchronous stuff (uninitialized promises, etc.) from the async iterator implementation, and make it pretty much straightforward by reusing the paging logic already implemented elsewhere (so I'm just disabling auto-pagination with autoPaginate: false and reusing the regular paginator).

The corresponding client change is coming as well (see the test client change to get an idea how the client is changed).

[codecov]

Codecov Report

Merging #765 into master will decrease coverage by 0.03%.
The diff coverage is 94.87%.

@@            Coverage Diff             @@
##           master     #765      +/-   ##
==========================================
- Coverage   89.47%   89.43%   -0.04%     
==========================================
  Files          45       45              
  Lines        7476     7433      -43     
  Branches      481      457      -24     
==========================================
- Hits         6689     6648      -41     
+ Misses        784      782       -2     
  Partials        3        3              

Impacted Files	Coverage Δ
src/apitypes.ts	0.00% <0.00%> (ø)
src/paginationCalls/pageDescriptor.ts	90.24% <95.45%> (+8.13%)	⬆️
samples/pagination.js	100.00% <100.00%> (ø)
...ixtures/google-gax-packaging-test-app/src/index.js	94.88% <100.00%> (+0.02%)	⬆️
...-gax-packaging-test-app/src/v1beta1/echo_client.js	98.16% <100.00%> (ø)
src/normalCalls/retries.ts	93.58% <0.00%> (-5.13%)	⬇️
src/fallback.ts	74.21% <0.00%> (-2.90%)	⬇️
src/streamingCalls/streamingApiCaller.ts	89.24% <0.00%> (-1.08%)	⬇️
src/grpc.ts	89.14% <0.00%> (-0.58%)	⬇️
src/gax.ts	96.39% <0.00%> (+0.30%)	⬆️

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[xiaozhenliu-gg5]

Thanks for debugging and fixing this!

One question though: what does that mean by grpc-fallback expects the "exact same" implementation of the apiCall? I noticed the apiCall in unit test is changed to apiCall instead of apiCallPromise, but the way how to call async iterator stays the same? Thank you!

[xiaozhenliu-gg5]

Curious that if we remove this block, does setting includes all pagetoken/pageSize information?

[xiaozhenliu-gg5]

So here we don't have to wait for params to resolve, when asyncIterate is called, we have the apiCall and needed request, options, is that correct?

[alexander-fenster]

Yes, because apiCall is already a function and everything is synchronous.

[alexander-fenster]

The problem with fallback code is that it uses a different way to communicate with the server. gRPC parses proto files and provides its own stubs that we access by createStub and decorate by calling createApiCall, but fallback uses Protobuf.js rpcImpl described here. Protobuf.js expects rpcImpl to have callback interface, and callback interface is provided by createApiCall. So in the original code, when innerCallPromise[method] got called, no callback was passed to rpcImpl and it failed with a weird error:

  1) Showcase tests
       grpc-fallback works
         pagedExpandAsync:
     TypeError: undefined is not a function
      at Service.newServiceStub.<computed> [as pagedExpand] (node_modules/google-gax/build/src/fallback.js:187:64)
      at /Users/fenster/gapic-generator-typescript/.test-application-js/node_modules/showcase/build/src/v1beta1/echo_client.js:197:29
      at OngoingCallPromise.call (node_modules/google-gax/build/src/call.js:67:27)
      at PageDescriptor.getNextPageRequest (node_modules/google-gax/build/src/paginationCalls/pageDescriptor.js:141:21)
      at Object.next (node_modules/google-gax/build/src/paginationCalls/pageDescriptor.js:127:54)
      at async Context.<anonymous> (index.js:113:22)

TypeError: undefined is not a function occurs because a callback is not passed to rpcImpl and it tries to call an undefined function. The easiest fix was to switch to using apiCall instead of innerCallPromise, and it turns out it allows easier (synchronous) implementation.

^^^ This might be complicated, I spent about an hour trying to understand what's going on :)