bug: `get_tree_state` fails at sapling activation height + 0 and +1

[AArnott]

What happened?

I called the get_tree_state gRPC function to my zebra-backed litewalletd server.

I expected to see this happen:

419200 is the sapling activation height, so I would expect that block to work. It's odd that 419202 works, but not 419201.

Instead, this happened:

The litewalletd eturns this error when I use 419200 or 419201 as the birthday height:

status: Unknown, message: "zcashd did not return treestate", details: [], metadata: MetadataMap { headers: {"content-type": "application/grpc"} }

What were you doing when the issue happened?

Exploratory testing of a wallet.

Zebra logs

No response

Zebra Version

1.6.1

Which operating systems does the issue happen on?

• [X] Linux
• [ ] macOS
• [ ] Windows
• [ ] Other OS

OS details

Ubuntu 22.04.1

Additional information

@str4d diagnosed this:

Those first two blocks contained no Sapling notes IIRC, so the tree is empty. This is a bug in Zebra, because it should be returning the entry TreeState in that case