xds/server: fix RDS handling for non-inline route configs

[zasweq]

Continuation and squashed based on #6889.

Fix the xDS Server to align with spec in https://github.com/grpc/proposal/blob/master/A36-xds-for-servers.md.

xds/internal/server/listener_wrapper:

• Wait for all resources (LDS and RDS) to transition into serving, and if not serving accept() connections then close(). If received another LDS configuration, keep serving using old LDS until the new LDS has full RDS resources (i.e. each RDS resource has received an update or an error), then gracefully drain connections that were accepted and configured with old LDS, and switch to new LDS for new accepted connections.
• Build all routing configurations for Filter Chains when LDS goes READY.
• Build routing configuration dynamically using LDS + RDS on any RDS update.
• Handle mode switches inline, drain server transports on any transition into non serving, accept() + close() if not serving.

xds/internal/server/conn_wrapper: Read routing configuration dynamically through an atomic load of a pointer, also add L7 error representation.

xds/internal/server/rds_handler: Persist rds updates and error conditions for each specific route resource being watched. Add a helper which determines whether all dynamic rds resources needed have been received. RDS errors mean the RDS has received configuration, trigger failure at L7 routing layer against incoming RPCs.

xds/server: Don’t block on a “good update”, immediately start serving on lis passed into Serve once wrapped, if xDS resources aren’t ready or LDS returns resource not found Accept() and Close() in not serving mode. Log any L7 routing errors due to xDS Configuration problems.

server: Call a callback with the server transport once it’s created on the Conn. This gives access to the server transport to xDS layer, which will be gracefully closed on transitions into not serving and transitions to a new LDS configuration. It also guarantees at some point the server transport will be gracefully drained. This replaces the Drain() operation previously present.

All LDS + RDS callbacks from the client are synchronous. The events that can happen asynchronous are the Accepting of a Conn (protected by mutex) and also an RPC on the Conn (protected with an atomic pointer). Thus dropped spawned event goroutines handleServingModeChanges() in the xDS Server, and run() on the listener_wrapper. Handled all the xDS resources inline and synced data structures/behaviors using above methods. L7 error = fail RPC with status code UNAVAILABLE (don’t give back more information due to security risk).

Fixes #6788.

RELEASE NOTES:

• xds/server: fix RDS handling for non-inline route configs

[codecov]

Codecov Report

Merging #6915 (3db6c1b) into master (bb0d32f) will decrease coverage by 0.02%.
Report is 11 commits behind head on master.
The diff coverage is 82.95%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6915      +/-   ##
==========================================
- Coverage   83.65%   83.64%   -0.02%     
==========================================
  Files         286      287       +1     
  Lines       30756    30920     +164     
==========================================
+ Hits        25730    25862     +132     
- Misses       3963     3992      +29     
- Partials     1063     1066       +3     

Files	Coverage Δ
server.go	81.06% <100.00%> (-0.40%)	⬇️
xds/internal/xdsclient/client_new.go	84.72% <100.00%> (+1.64%)	⬆️
xds/xds.go	34.61% <ø> (ø)
xds/internal/server/conn_wrapper.go	73.49% <83.33%> (+1.85%)	⬆️
xds/server.go	82.73% <80.00%> (-3.77%)	⬇️
xds/internal/xdsclient/xdsresource/filter_chain.go	92.33% <91.22%> (+0.36%)	⬆️
xds/internal/xdsclient/clientimpl_watchers.go	76.36% <53.84%> (-6.97%)	⬇️
...ds/internal/xdsclient/xdsresource/resource_type.go	78.57% <71.42%> (-21.43%)	⬇️
xds/internal/server/listener_wrapper.go	73.61% <91.17%> (+5.72%)	⬆️
xds/internal/xdsclient/authority.go	89.92% <57.14%> (-1.89%)	⬇️
... and 1 more

... and 29 files with indirect coverage changes

[dfawley]

Looks really good overall! Mostly just some style and maintainability comments.

[sergiitk]

• grpc/go/master/branch/xds_k8s_lb
• grpc/go/master/branch/psm-security
• grpc/go/master/branch/xds_url_map

[dfawley]

I only glanced at the tests so far; will look a bit more closely next round.

Just a few minor things.

[sergiitk]

• grpc/go/master/branch/xds_k8s_lb
• grpc/go/master/branch/psm-security
• grpc/go/master/branch/xds_url_map

[zasweq]

All interop tests continue to pass.