[Bug] a user encounters an error when trying to sync their wallet

[true-jared]

Slack report: https://zcash.slack.com/archives/C06S4T7F3J5/p1714555758041279

Logs containing given error:
scratch_110.txt

[str4d]

My hypothesis is that this is a zebrad bug.

The error in question is due to lightwalletd returning an error when asked for the (presumably Sapling) tree state at height 419200. At that height, the tree was empty (because no Sapling transactions were mined in the first Sapling block); the tree gains its first note at height 419201.

z_gettreestate is designed to support backing nodes that don't persist a tree state for every block. finalState is omitted if the backing node does not have it, and instead skipHash is provided pointing to the most recent prior block height for which the backing node has a tree state.

In zcashd's implementation, if there is no such prior state, then skipHash is also omitted. However, zcashd encodes the empty tree state for blocks in which the tree is empty, so there is always a base case (at least for height requests from Sapling activation).

My suspicion is that zebrad does not store the empty tree, and as such when queried for z_gettreestate 419200 it omits finalState, leading to lightwalletd reporting this error: https://github.com/zcash/lightwalletd/blob/6e3816b5834583c492c37ce05b0faaf9fe12c87f/frontend/service.go#L288-L290 (which itself is misleading as it implicates zcashd no matter the backend).