vtexplain2

[ajm188]

This PR enables vtexplain to use non-evenly sharded keyspaces, by consuming a new piece of user input, namely a json blob in the form of { "ks1": $(vtctl FindAllShardsInKeyspace ks1), "ks2": $(vtctl FindAllShardsInKeyspace ks2) }.

A few notable changes were required for this to work:

• Add two new flags ks-shard-map and ks-shard-map-file to get the new json blob, in the same style as the sql/sqlfile and vschema/vschema-file args. For this, I made getFileParam take an additional arg to dictate whether a failure to pass exactly one of the two arguments should result in an error. This makes it so that ks-shard-map/ks-shard-map-file are optional. Failure to pass exactly one of them will cause vtexplain to fall back to the old NumShards behavior.
• ExplainTopo now tracks a nested map of topodatapb.ShardReference pointers, (so, keyspace -> shard_name -> shard_ref). ExplainTopo then uses this to construct a topodatapb.SrvKeyspace in GetSrvKeyspace() rather than doing it based on just NumShards + an assumption of evenly-sharded keyranges. This is the piece that gives us the flexibility to allow arbitrarily-sharded keyspaces.
• buildTopology constructs a set of ShardReferences and TabletConns based on the keyspaceShardMap. If the user didn't provide a keyspaceShardMap, go back to the NumShards+even sharding behavior. The significant change here is that this function must update the ExplainTopo.KeyspaceShards map, or it will return incorrect data when GetSrvKeyspace() is called on it.

I've manually tested that both the new and old behaviors work on sharded keyspaces so far, but I wanted to get input at this point.

I'll note that this code can probably simplified if we decide we don't want to support the NumShards case, and instead require users to give a json map of keyspace -> shard name -> shard ref. But, I also understand if we need to keep supporting that, if even for a little while.

[sougou]

Cool. Could you write some tests that demonstrate the new functionality? Also, we'll need to update the documentation https://vitess.io/docs/reference/programs/vtexplain/

[ajm188]

Tests added, and preliminary documentation change is here!