Merge rust-bitcoin/rust-bitcoin#4554: Sane InputWeightPredition Arbitrary Type

8dd24cb67b Add Arbitrary type for InputWeightPrediction (yancy)

Pull request description:

  This type creates sane Arbitrary InputWeightPrediction types that do not panic.  To this end, when a custom type is created by calling new() or from_slice() constructor, only use values that would no greater than block size 4 MB of witness data or 1 MB of non-witness data.  This idea is from the discussion here  https://github.com/rust-bitcoin/rust-bitcoin/issues/4547

  I'd be up for a future PR that limits the constructors to no greater than 1MB or 4MB witness data, although I feel like this Arbitrary type will be less controversial.

  Furthermore, I did test this locally against my application and no panics where produced either through the constructor nor auxiliary methods `total_weight` or `witness_weight`.

ACKs for top commit:
  tcharding:
    ACK 8dd24cb67b
  apoelstra:
    ACK 8dd24cb67b948e619038b387d6c3bd79252a5de1; successfully ran local tests

Tree-SHA512: ff33f1a6ced4f68c4d236e362b1d03da405a8b9701cda0405405421ca21a563fa288c8065a9309e542fec0c8bc850119bcf93a2dc5e27677c42b35b1e5e52722

bitcoin/src/blockdata/transaction.rs

@@ -12,6 +12,8 @@
 
 use core::fmt;
 
+#[cfg(feature = "arbitrary")]
+use arbitrary::{Arbitrary, Unstructured};
 use hashes::sha256d;
 use internals::{compact_size, write_err, ToU64};
 use io::{BufRead, Write};
@@ -1141,6 +1143,38 @@ mod sealed {
     impl Sealed for super::Version {}
 }
 
+#[cfg(feature = "arbitrary")]
+impl<'a> Arbitrary<'a> for InputWeightPrediction {
+    fn arbitrary(u: &mut Unstructured<'a>) -> arbitrary::Result<Self> {
+        // limit script size to 4Mwu block size.
+        let max_block = Weight::MAX_BLOCK.to_wu() as usize;
+        let input_script_len = u.int_in_range(0..=max_block)?;
+        let remaining = max_block - input_script_len;
+
+        // create witness data if there is remaining space.
+        let mut witness_length = u.int_in_range(0..=remaining)?;
+        let mut witness_element_lengths = Vec::new();
+
+        // build vec of random witness element lengths.
+        while witness_length > 0 {
+            let elem = u.int_in_range(1..=witness_length)?;
+            witness_element_lengths.push(elem);
+            witness_length -= elem;
+        }
+
+        match u.int_in_range(0..=7)? {
+            0 => Ok(InputWeightPrediction::P2WPKH_MAX),
+            1 => Ok(InputWeightPrediction::NESTED_P2WPKH_MAX),
+            2 => Ok(InputWeightPrediction::P2PKH_COMPRESSED_MAX),
+            3 => Ok(InputWeightPrediction::P2PKH_UNCOMPRESSED_MAX),
+            4 => Ok(InputWeightPrediction::P2TR_KEY_DEFAULT_SIGHASH),
+            5 => Ok(InputWeightPrediction::P2TR_KEY_NON_DEFAULT_SIGHASH),
+            6 => Ok(InputWeightPrediction::new(input_script_len, witness_element_lengths)),
+            _ => Ok(InputWeightPrediction::from_slice(input_script_len, &witness_element_lengths)),
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use hex::FromHex;