xdsclient: update watcher API as per gRFC A88

[purnesh42H]

This is the first part of implementing gRFC A88 (grpc/proposal#466).

This introduces the new watcher API but does not change any of the existing behavior. This table summarizes the API changes and behavior for each case:

Case	Old API	New API	Behavior
Resource timer fires	OnResourceDoesNotExist()	OnResourceChanged(NOT_FOUND)	Fail data plane RPCs
LDS or CDS resource deletion	OnResourceDoesNotExist()	OnResourceChanged(NOT_FOUND)	Drop resource and fail data plane RPCs
xDS channel reports TRANSIENT_FAILURE	OnError()	OnResourceChanged(status) if resource NOT already cached; OnAmbientError(status) otherwise	Continue using cached resource, if any; otherwise, fail data plane RPCs
ADS stream terminates without receiving a response	OnError()	OnResourceChanged(status) if resource NOT already cached; OnAmbientError(status) otherwise	Continue using cached resource, if any; otherwise, fail data plane RPCs
Invalid resource update (client NACK)	OnError()	OnResourceChanged(status) if resource NOT already cached; OnAmbientError(status) otherwise	Continue using cached resource, if any; otherwise, fail data plane RPCs
Valid resource update	OnUpdate(resource)	OnResourceChanged(resource)	use the new resource

RELEASE NOTES:

• xds: TBD

[codecov]

Codecov Report

Attention: Patch coverage is 81.36646% with 30 lines in your changes missing coverage. Please review.

Project coverage is 82.45%. Comparing base (724f450) to head (b9d2a92).
Report is 44 commits behind head on master.

Files with missing lines	Patch %	Lines
xds/internal/testutils/resource_watcher.go	20.00%	11 Missing and 1 partial ⚠️
xds/internal/server/listener_wrapper.go	42.10%	9 Missing and 2 partials ⚠️
.../balancer/clusterresolver/resource_resolver_eds.go	78.57%	2 Missing and 1 partial ⚠️
xds/internal/resolver/xds_resolver.go	66.66%	2 Missing ⚠️
xds/internal/xdsclient/clientimpl_watchers.go	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7977      +/-   ##
==========================================
+ Coverage   82.28%   82.45%   +0.17%     
==========================================
  Files         381      388       +7     
  Lines       38539    39048     +509     
==========================================
+ Hits        31712    32198     +486     
+ Misses       5535     5530       -5     
- Partials     1292     1320      +28     

Files with missing lines	Coverage Δ
xds/internal/balancer/cdsbalancer/cdsbalancer.go	81.69% <100.00%> (-0.07%)	⬇️
...s/internal/balancer/cdsbalancer/cluster_watcher.go	100.00% <100.00%> (ø)
xds/internal/resolver/watch_service.go	100.00% <100.00%> (+8.57%)	⬆️
xds/internal/server/rds_handler.go	90.69% <100.00%> (-0.61%)	⬇️
xds/internal/xdsclient/authority.go	77.21% <100.00%> (+0.34%)	⬆️
...nal/xdsclient/xdsresource/cluster_resource_type.go	78.37% <100.00%> (+1.23%)	⬆️
...l/xdsclient/xdsresource/endpoints_resource_type.go	76.47% <100.00%> (+1.47%)	⬆️
...al/xdsclient/xdsresource/listener_resource_type.go	86.44% <100.00%> (+0.47%)	⬆️
...ds/internal/xdsclient/xdsresource/resource_type.go	100.00% <ø> (ø)
...dsclient/xdsresource/route_config_resource_type.go	76.47% <100.00%> (+1.47%)	⬆️
... and 5 more

... and 58 files with indirect coverage changes

[purnesh42H]

@easwars ptal

[easwars]

Based on the PR description, for the following two cases:

• xDS channel reports TRANSIENT_FAILURE |
• ADS stream terminates without receiving a response
  the old behavior was to call OnError(), while the new behavior is to call OnResourceChanged(status) if resource NOT already cached, and OnAmbientError(status) otherwise.

But, we are only calling the latter here. Am I missing something?

[purnesh42H]

yeah so, since we have already checked for ErrTypeStreamFailedAfterRecv above, the only case left is un cached which means this should always be OnResourceChanged()? or we should look up the cache and call accordingly?

@easwars

[easwars]

ErrTypeStreamFailedAfterRecv just means that some message was received on the stream before it broke. It does not say anything specific to the resource under question here.

[purnesh42H]

oh i see. I thought ErrTypeStreamFailedAfterRecv meant to state that stream failure happened but we have received the resource once before.

[easwars]

This came up during some other discussion with @dfawley.

1. Callback methods named OnXxx are more of a C++ style. In Go, we would probably just name them Xxx.
2. Whether we should have two callback methods, one named ResourceChanged(ResourceData), and the other named ResourceError(error). This is to work around the fact that we don't have a StatusOr type in Go, and without that, having two APIs is better than one API (where we need to add a new struct to hold one of two possible values, with no guarantees that only one of the two values is set).

[purnesh42H]

This came up during some other discussion with @dfawley.

1. Callback methods named OnXxx are more of a C++ style. In Go, we would probably just name them Xxx.
2. Whether we should have two callback methods, one named ResourceChanged(ResourceData), and the other named ResourceError(error). This is to work around the fact that we don't have a StatusOr type in Go, and without that, having two APIs is better than one API (where we need to add a new struct to hold one of two possible values, with no guarantees that only one of the two values is set).

I will wait for #8042 before making further changes to this PR since that has the discussion on decision