Remove panic in dust value functions

Calculating the minimum non-dust fee currently panics if either the
script is really big or the dust fee rate is really big.

Harden the API by returning an `Option` instead of panicing.

bitcoin/src/blockdata/script/borrowed.rs

@@ -261,7 +261,7 @@ crate::internal_macros::define_extension_trait! {
         /// Returns the minimum value an output with this script should have in order to be
         /// broadcastable on today’s Bitcoin network.
         #[deprecated(since = "0.32.0", note = "use `minimal_non_dust` etc. instead")]
-        fn dust_value(&self) -> Amount { self.minimal_non_dust() }
+        fn dust_value(&self) -> Option<Amount> { self.minimal_non_dust() }
 
         /// Returns the minimum value an output with this script should have in order to be
         /// broadcastable on today's Bitcoin network.
@@ -272,7 +272,7 @@ crate::internal_macros::define_extension_trait! {
         /// To use a custom value, use [`minimal_non_dust_custom`].
         ///
         /// [`minimal_non_dust_custom`]: Script::minimal_non_dust_custom
-        fn minimal_non_dust(&self) -> Amount {
+        fn minimal_non_dust(&self) -> Option<Amount> {
             self.minimal_non_dust_internal(DUST_RELAY_TX_FEE.into())
         }
 
@@ -287,7 +287,7 @@ crate::internal_macros::define_extension_trait! {
         /// To use the default Bitcoin Core value, use [`minimal_non_dust`].
         ///
         /// [`minimal_non_dust`]: Script::minimal_non_dust
-        fn minimal_non_dust_custom(&self, dust_relay_fee: FeeRate) -> Amount {
+        fn minimal_non_dust_custom(&self, dust_relay_fee: FeeRate) -> Option<Amount> {
             self.minimal_non_dust_internal(dust_relay_fee.to_sat_per_kwu() * 4)
         }
 
@@ -394,7 +394,7 @@ mod sealed {
 
 crate::internal_macros::define_extension_trait! {
     pub(crate) trait ScriptExtPriv impl for Script {
-        fn minimal_non_dust_internal(&self, dust_relay_fee: u64) -> Amount {
+        fn minimal_non_dust_internal(&self, dust_relay_fee: u64) -> Option<Amount> {
             // This must never be lower than Bitcoin Core's GetDustThreshold() (as of v0.21) as it may
             // otherwise allow users to create transactions which likely can never be broadcast/confirmed.
             let sats = dust_relay_fee
@@ -408,13 +408,12 @@ crate::internal_macros::define_extension_trait! {
                     32 + 4 + 1 + 107 + 4 + // The spend cost copied from Core
                     8 + // The serialized size of the TxOut's amount field
                     self.consensus_encode(&mut sink()).expect("sinks don't error").to_u64() // The serialized size of this script_pubkey
-                })
-                .expect("dust_relay_fee or script length should not be absurdly large")
+                })?
                 / 1000; // divide by 1000 like in Core to get value as it cancels out DEFAULT_MIN_RELAY_TX_FEE
                         // Note: We ensure the division happens at the end, since Core performs the division at the end.
                         //       This will make sure none of the implicit floor operations mess with the value.
 
-            Amount::from_sat(sats)
+            Some(Amount::from_sat(sats))
         }
 
         fn count_sigops_internal(&self, accurate: bool) -> usize {

bitcoin/src/blockdata/script/tests.rs

@@ -684,10 +684,10 @@ fn default_dust_value() {
     // well-known scriptPubKey types.
     let script_p2wpkh = Builder::new().push_int_unchecked(0).push_slice([42; 20]).into_script();
     assert!(script_p2wpkh.is_p2wpkh());
-    assert_eq!(script_p2wpkh.minimal_non_dust(), Amount::from_sat_unchecked(294));
+    assert_eq!(script_p2wpkh.minimal_non_dust(), Some(Amount::from_sat_unchecked(294)));
     assert_eq!(
         script_p2wpkh.minimal_non_dust_custom(FeeRate::from_sat_per_vb_unchecked(6)),
-        Amount::from_sat_unchecked(588)
+        Some(Amount::from_sat_unchecked(588))
     );
 
     let script_p2pkh = Builder::new()
@@ -698,10 +698,10 @@ fn default_dust_value() {
         .push_opcode(OP_CHECKSIG)
         .into_script();
     assert!(script_p2pkh.is_p2pkh());
-    assert_eq!(script_p2pkh.minimal_non_dust(), Amount::from_sat_unchecked(546));
+    assert_eq!(script_p2pkh.minimal_non_dust(), Some(Amount::from_sat_unchecked(546)));
     assert_eq!(
         script_p2pkh.minimal_non_dust_custom(FeeRate::from_sat_per_vb_unchecked(6)),
-        Amount::from_sat_unchecked(1092)
+        Some(Amount::from_sat_unchecked(1092))
     );
 }
 

bitcoin/src/blockdata/transaction.rs

@@ -183,8 +183,8 @@ crate::internal_macros::define_extension_trait! {
         /// To use a custom value, use [`minimal_non_dust_custom`].
         ///
         /// [`minimal_non_dust_custom`]: TxOut::minimal_non_dust_custom
-        fn minimal_non_dust(script_pubkey: ScriptBuf) -> Self {
-            TxOut { value: script_pubkey.minimal_non_dust(), script_pubkey }
+        fn minimal_non_dust(script_pubkey: ScriptBuf) -> Option<TxOut> {
+            Some(TxOut { value: script_pubkey.minimal_non_dust()?, script_pubkey })
         }
 
         /// Constructs a new `TxOut` with given script and the smallest possible `value` that is **not** dust
@@ -198,8 +198,8 @@ crate::internal_macros::define_extension_trait! {
         /// To use the default Bitcoin Core value, use [`minimal_non_dust`].
         ///
         /// [`minimal_non_dust`]: TxOut::minimal_non_dust
-        fn minimal_non_dust_custom(script_pubkey: ScriptBuf, dust_relay_fee: FeeRate) -> Self {
-            TxOut { value: script_pubkey.minimal_non_dust_custom(dust_relay_fee), script_pubkey }
+        fn minimal_non_dust_custom(script_pubkey: ScriptBuf, dust_relay_fee: FeeRate) -> Option<TxOut> {
+            Some(TxOut { value: script_pubkey.minimal_non_dust_custom(dust_relay_fee)?, script_pubkey })
         }
     }
 }