xds: keep ads flow control local to xdsclient/transport package

[easwars]

This PR ensures that the ADS stream level flow control functionality added recently is kept local to the xdsclient/transport package. It also simplifies the implementation in a number of ways.

Summary of changes:

• Simplify the public watch API by having a func parameter instead of an interface to notify that local processing of the update is complete
• Simplify the flow control implementation by not having to worry about the actual number of watchers that are yet to process the most recent update, and instead having a boolean that indicates whether or not the update has been processed by all watchers.
  • The actual work of tracking how many watchers are yet to process the most recent update is moved to the authority, which is the one that is actually invoking the watcher callbacks.
• The reason for defining xdsresource.OnDoneFunc and making the watcher callbacks use that instead of a plan func() is to ease documenting the interface.
  • Willing to change this if deemed to cause more harm than good.

One of the main motivations for this change is the ongoing refactor to move ADS and LRS functionality out of the xDS transport type into separate types. With the flow control functionality becoming local to the transport package, it can easily be moved into the eventual type that will provide the ADS functionality.

#ads-stream-flow-control

RELEASE NOTES: none

[codecov]

Codecov Report

Attention: Patch coverage is 91.86992% with 10 lines in your changes missing coverage. Please review.

Project coverage is 81.86%. Comparing base (845f62c) to head (d221151).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
.../balancer/clusterresolver/resource_resolver_eds.go	57.14%	3 Missing ⚠️
xds/internal/resolver/watch_service.go	83.33%	3 Missing ⚠️
xds/internal/testutils/resource_watcher.go	66.66%	2 Missing ⚠️
.../balancer/clusterresolver/resource_resolver_dns.go	75.00%	1 Missing ⚠️
xds/internal/xdsclient/clientimpl_watchers.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7578      +/-   ##
==========================================
+ Coverage   81.76%   81.86%   +0.09%     
==========================================
  Files         361      361              
  Lines       27825    27814      -11     
==========================================
+ Hits        22752    22769      +17     
+ Misses       3866     3850      -16     
+ Partials     1207     1195      -12     

Files with missing lines	Coverage Δ
...s/internal/balancer/cdsbalancer/cluster_watcher.go	100.00% <100.00%> (ø)
...ternal/balancer/clusterresolver/clusterresolver.go	72.12% <100.00%> (ø)
...rnal/balancer/clusterresolver/resource_resolver.go	93.63% <100.00%> (ø)
xds/internal/server/listener_wrapper.go	78.61% <100.00%> (ø)
xds/internal/server/rds_handler.go	86.76% <100.00%> (ø)
xds/internal/xdsclient/authority.go	89.20% <100.00%> (+0.20%)	⬆️
xds/internal/xdsclient/transport/transport.go	96.42% <100.00%> (+2.38%)	⬆️
...nal/xdsclient/xdsresource/cluster_resource_type.go	80.00% <100.00%> (ø)
...l/xdsclient/xdsresource/endpoints_resource_type.go	78.57% <100.00%> (ø)
...al/xdsclient/xdsresource/listener_resource_type.go	87.23% <100.00%> (ø)
... and 7 more

... and 29 files with indirect coverage changes