Fix Script::dust_value()'s calculation for non-P2*PKH script_pubkeys

The dust calculations added were only valid for P2WPKH and P2PKH
outputs, and somehow this fact was missed in review, despite the
upstream Core code being linked to and looked at by two reviewers
and the author (me).

Someday I will grow eyeballs, but that day is not today.

src/blockdata/script.rs

@@ -400,23 +400,31 @@ impl Script {
                                opcodes::All::from(self.0[0]).classify() == opcodes::Class::IllegalOp)
     }
 
-    /// The minimum value an output to a witness script must have in order to be
+    /// The minimum value an output with a P2WPKH script_pubkey must have in order to be
     /// broadcastable on today's bitcoin network.
-    pub const WITNESS_OUTPUT_DUST_THRESHOLD: u64 = 294;
+    pub const P2WPKH_OUTPUT_DUST_THRESHOLD: u64 = 294;
 
-    /// The minimum value an output to a non-witness script must have in order to be
+    /// The minimum value an output with a P2PKH script_pubkey must have in order to be
     /// broadcastable on today's bitcoin network.
-    pub const LEGACY_OUTPUT_DUST_THRESHOLD: u64 = 546;
+    pub const P2PKH_OUTPUT_DUST_THRESHOLD: u64 = 546;
 
     /// Gets the minimum value an output with this script should have in order to be
     /// broadcastable on today's bitcoin network.
     pub fn dust_value(&self) -> u64 {
+        // 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
+        // broadcasted/confirmed.
+        3 * // The default dust relay fee is 3000 satoshi/kB (ie 3 sat/vByte)
         if self.is_op_return() {
             0
         } else if self.is_witness_program() {
-            Self::WITNESS_OUTPUT_DUST_THRESHOLD
+            32 + 4 + 1 + (107 / 4) + 4 + // The spend cost copied from Core
+            8 + // The serialized size of the TxOut's amount field
+            self.consensus_encode(&mut ::std::io::sink()).unwrap() as u64 // The serialized size of this script_pubkey
         } else {
-            Self::LEGACY_OUTPUT_DUST_THRESHOLD
+            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 ::std::io::sink()).unwrap() as u64 // The serialized size of this script_pubkey
         }
     }
 
@@ -1229,5 +1237,21 @@ mod test {
 		let spending = Vec::from_hex("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").unwrap();
 		spent.verify(0, 18393430, spending.as_slice()).unwrap();
 	}
+
+    #[test]
+    fn defult_dust_value_tests() {
+        let script_p2wpkh = Builder::new().push_int(0).push_slice(&[42; 20]).into_script();
+        assert!(script_p2wpkh.is_v0_p2wpkh());
+        assert_eq!(script_p2wpkh.dust_value(), Script::P2WPKH_OUTPUT_DUST_THRESHOLD);
+        let script_p2pkh = Builder::new()
+            .push_opcode(opcodes::all::OP_DUP)
+            .push_opcode(opcodes::all::OP_HASH160)
+            .push_slice(&[42; 20])
+            .push_opcode(opcodes::all::OP_EQUALVERIFY)
+            .push_opcode(opcodes::all::OP_CHECKSIG)
+            .into_script();
+        assert!(script_p2pkh.is_p2pkh());
+        assert_eq!(script_p2pkh.dust_value(), Script::P2PKH_OUTPUT_DUST_THRESHOLD);
+    }
 }