Expose min_funding_satoshis via Config

[vincenzopalazzo]

Summary

• Add min_funding_sats field to Config that maps to LDK's ChannelHandshakeLimits::min_funding_satoshis
• Server operators can now enforce a higher minimum channel funding amount for inbound channels (default remains 1000 sats)
• Updated UniFFI bindings

Fixes #842

Test plan

• cargo check passes
• cargo test --lib config passes
• CI passes

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com
AI-assisted-by: OpenAI Codex

cc @tankyleo

[ldk-reviews-bot]

👋 Thanks for assigning @tankyleo as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[tankyleo]

I would have cleaned it up like this; I don't think it's worth adding the Option there, the config setting logically is just a single number.

diff --git a/src/config.rs b/src/config.rs
index ffd9d134..d6402257 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -28,6 +28,7 @@ const DEFAULT_LDK_WALLET_SYNC_INTERVAL_SECS: u64 = 30;
 const DEFAULT_FEE_RATE_CACHE_UPDATE_INTERVAL_SECS: u64 = 60 * 10;
 const DEFAULT_PROBING_LIQUIDITY_LIMIT_MULTIPLIER: u64 = 3;
 const DEFAULT_ANCHOR_PER_CHANNEL_RESERVE_SATS: u64 = 25_000;
+const DEFAULT_MIN_FUNDING_SATS: u64 = 1000;

 // The default timeout after which we abort a wallet syncing operation.
 const DEFAULT_BDK_WALLET_SYNC_TIMEOUT_SECS: u64 = 60;
@@ -125,7 +126,7 @@ pub(crate) const LNURL_AUTH_TIMEOUT_SECS: u64 = 15;
 /// | `node_alias`                           | None               |
 /// | `trusted_peers_0conf`                  | []                 |
 /// | `probing_liquidity_limit_multiplier`   | 3                  |
-/// | `min_funding_sats`                     | None               |
+/// | `min_funding_sats`                     | 1000               |
 /// | `anchor_channels_config`               | Some(..)           |
 /// | `route_parameters`                     | None               |
 /// | `tor_config`                           | None               |
@@ -172,13 +173,7 @@ pub struct Config {
        /// they open inbound channels to us.
        ///
        /// Channels with funding below this value will be rejected.
-       ///
-       /// If unset, the `rust-lightning` default of `1000` sats will be used.
-       ///
-       /// Please refer to [`ChannelHandshakeLimits::min_funding_satoshis`] for further details.
-       ///
-       /// [`ChannelHandshakeLimits::min_funding_satoshis`]: lightning::util::config::ChannelHandshakeLimits::min_funding_satoshis
-       pub min_funding_sats: Option<u64>,
+       pub min_funding_sats: u64,
        /// Configuration options pertaining to Anchor channels, i.e., channels for which the
        /// `option_anchors_zero_fee_htlc_tx` channel type is negotiated.
        ///
@@ -222,7 +217,7 @@ impl Default for Config {
                        announcement_addresses: None,
                        trusted_peers_0conf: Vec::new(),
                        probing_liquidity_limit_multiplier: DEFAULT_PROBING_LIQUIDITY_LIMIT_MULTIPLIER,
-                       min_funding_sats: None,
+                       min_funding_sats: DEFAULT_MIN_FUNDING_SATS,
                        anchor_channels_config: Some(AnchorChannelsConfig::default()),
                        tor_config: None,
                        route_parameters: None,
@@ -352,9 +347,7 @@ pub(crate) fn default_user_config(config: &Config) -> UserConfig {
        // will mostly be relevant for inbound channels.
        let mut user_config = UserConfig::default();
        user_config.channel_handshake_limits.force_announced_channel_preference = false;
-       if let Some(min_funding_sats) = config.min_funding_sats {
-               user_config.channel_handshake_limits.min_funding_satoshis = min_funding_sats;
-       }
+       user_config.channel_handshake_limits.min_funding_satoshis = config.min_funding_sats;
        user_config.channel_handshake_config.negotiate_anchors_zero_fee_htlc_tx =
                config.anchor_channels_config.is_some();
        user_config.reject_inbound_splices = false;

[vincenzopalazzo]

@tankyleo Fixed in e2113b3. Switched Config::min_funding_sats to a plain u64 with an explicit 1000-sat default, removed the conditional assignment into the LDK user config, and added a unit test covering both the default and override path.