xdsclient: correct logic used to suppress empty ADS requests on new streams

[dfawley]

Fixes #7013

RELEASE NOTES:

• xds: fix an issue that would cause the client to send an empty list of resources for LDS/CDS upon reconnecting with the management server

[codecov]

Codecov Report

Merging #7026 (19f938a) into master (f7c5e6a) will increase coverage by 0.04%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7026      +/-   ##
==========================================
+ Coverage   82.48%   82.52%   +0.04%     
==========================================
  Files         296      296              
  Lines       31468    31463       -5     
==========================================
+ Hits        25955    25964       +9     
+ Misses       4457     4447      -10     
+ Partials     1056     1052       -4     

Files	Coverage Δ
xds/internal/xdsclient/transport/transport.go	96.49% <100.00%> (+4.96%)	⬆️

... and 16 files with indirect coverage changes

[atollena]

I'm not familiar with this code, but I'm wondering whether beside never sending empty subscriptions on new ADS streams, as done in this PR, why the xDS resolver itself is not closed (and hence the management server connection, too), when all channels using it are idle or shutdown.

[dfawley]

I'm not familiar with this code, but I'm wondering whether beside never sending empty subscriptions on new ADS streams, as done in this PR, why the xDS resolver itself is not closed (and hence the management server connection, too), when all channels using it are idle or shutdown.

I think that would be a separate issue. The xdsClient is a global singleton shared between all channels. In theory when the last reference to it goes away, it should shut down, according to:

grpc-go/xds/internal/xdsclient/singleton.go

Line 59 in f7c5e6a

singletonClient.clientImpl.close()

Are you seeing evidence that the xdsClient and/or its connection to the management server are not being closed when the last consumer goes away?

This change is needed regardless of that to prevent empty resources from being requested for any type.

[atollena]

Are you seeing evidence that the xdsClient and/or its connection to the management server are not being closed when the last consumer goes away?

I thought so, but let me check again. I'll open another issue if needed.