Wallet birthday height for seed recovery on pruned nodes

[FreeOnlineUser]

When restoring an ldk-node wallet from seed, the node starts syncing from the current chain tip. Any on-chain deposits made before the restore are never seen by synchronize_listeners, so the wallet balance shows zero even though funds exist on-chain.

This affects all seed restores, but pruned nodes are hit hardest: historical blocks are gone, so there is no workaround via rescan.

There is an existing TODO acknowledging this:

// TODO: Use a proper wallet birthday once BDK supports it.

(src/builder.rs:1327)

Impact

• On-chain balance shows 0 after seed restore
• Cannot open Lightning channels (no visible funds)
• Funds are safe on-chain but inaccessible through ldk-node
• Mobile wallets (where reinstalls and data loss are common) are the primary use case

What we did

We hit this building Bitcoin Pocket Node, an Android app running bitcoind + ldk-node on GrapheneOS with pruned storage.

Our workaround overrides chain_tip_opt in the Builder when a birthday height is provided. Caller flow:

1. User restores seed words
2. App uses bitcoind scantxoutset to find UTXOs and their block heights (works on pruned nodes)
3. Sets birthday to min(heights) - 10 as safety margin
4. ldk-node syncs from there, balance appears with zero transaction fees

Working fork with the changes (~100 lines): FreeOnlineUser/ldk-node@upstream/wallet-birthday

Verified end-to-end on real hardware: seed restore, UTXO scan, birthday set, 110,628 sats recovered in seconds.

Upstream context

BDK is working on related pieces:

• bdk#2126: Adding start_height to FilterIter::new
• bdk#2050: Pruned node support via scantxoutset

Once BDK ships native birthday support, ldk-node can use that directly (which is what the TODO anticipates). Our fork is a pragmatic solution in the meantime.

We are also the folks working on the watchtower improvements discussed in #813.

[tnull]

Note this was addressed in #769 where we added a recovery_mode flag to the builder which restores the ability to resync from genesis when importing a pre-existing wallet. Otherwise the wallet birthday is set to the current block height at first run. I guess we could add the ability to specify a specific block height instead. Would you prefer that over a simple bool flag?

[FreeOnlineUser]

Thanks for the pointer to #769! The recovery_mode flag helps for full nodes, but for pruned nodes resyncing from genesis isn't possible since historical blocks are gone.

A specific block height would be ideal for our use case. The flow we use:

1. User restores seed
2. scantxoutset finds UTXOs and their block heights (works on pruned nodes)
3. Set birthday to min(heights) - 10
4. ldk-node syncs from that height forward

With just the bool flag, a pruned node would fail trying to resync blocks it doesn't have. A height parameter lets us skip straight to where the funds are.

So yes, the ability to specify a block height would be much preferred over the bool flag for this use case.