Merge #734: commands: add weights before converting to vbytes

176859e commands: add weights before converting to vbytes (jp1ac4)

Pull request description:

  The `sanity_check_psbt()` function checks, among other things, that the feerate in sats/vbytes is in the required range. When estimating the transaction size, converting each part to vbytes first can lead to a larger result due to rounding multiple times and therefore a lower feerate, which could fall below 1 and fail the sanity check.

  This issue has arisen in #560 when creating a transaction using coin selection.

ACKs for top commit:
  darosior:
    utACK 176859e -- great catch

Tree-SHA512: 5e4fc795c5977c71a0f02b21c09cd71d59c9412f94bcafd5a4adb429c4ae3a568a25c4174bd305e0be927f06ae4e9be9aef791ffbbee2a7ac1c40740f639ab10

src/commands/mod.rs

@@ -202,7 +202,14 @@ fn sanity_check_psbt(
     }
 
     // Check the feerate isn't insane.
-    let tx_vb = (tx.vsize() + spent_desc.max_sat_vbytes() * tx.input.len()) as u64;
+    // Add weights together before converting to vbytes to avoid rounding up multiple times
+    // and increasing the result, which could lead to the feerate in sats/vb falling below 1.
+    let tx_wu = tx.weight().to_wu() + (spent_desc.max_sat_weight() * tx.input.len()) as u64;
+    let tx_vb = tx_wu
+        .checked_add(descriptors::WITNESS_FACTOR as u64 - 1)
+        .unwrap()
+        .checked_div(descriptors::WITNESS_FACTOR as u64)
+        .unwrap();
     let feerate_sats_vb = abs_fee
         .checked_div(tx_vb)
         .ok_or(CommandError::InsaneFees(InsaneFeeInfo::InvalidFeerate))?;

src/descriptors/mod.rs

@@ -17,7 +17,7 @@ pub use keys::*;
 pub mod analysis;
 pub use analysis::*;
 
-const WITNESS_FACTOR: usize = 4;
+pub const WITNESS_FACTOR: usize = 4;
 
 #[derive(Debug)]
 pub enum LianaDescError {