consensus: implement Arbitrary for Header (#1235)

* consensus: implement Arbitrary for Header

* fix function call

* fn visibility

crates/consensus/src/header.rs

@@ -527,6 +527,83 @@ impl Decodable for Header {
     }
 }
 
+/// Generates a header which is valid __with respect to past and future forks__. This means, for
+/// example, that if the withdrawals root is present, the base fee per gas is also present.
+///
+/// If blob gas used were present, then the excess blob gas and parent beacon block root are also
+/// present. In this example, the withdrawals root would also be present.
+///
+/// This __does not, and should not guarantee__ that the header is valid with respect to __anything
+/// else__.
+#[cfg(any(test, feature = "arbitrary"))]
+pub(crate) const fn generate_valid_header(
+    mut header: Header,
+    eip_4844_active: bool,
+    blob_gas_used: u128,
+    excess_blob_gas: u128,
+    parent_beacon_block_root: B256,
+) -> Header {
+    // Clear all related fields if EIP-1559 is inactive
+    if header.base_fee_per_gas.is_none() {
+        header.withdrawals_root = None;
+    }
+
+    // Set fields based on EIP-4844 being active
+    if eip_4844_active {
+        header.blob_gas_used = Some(blob_gas_used);
+        header.excess_blob_gas = Some(excess_blob_gas);
+        header.parent_beacon_block_root = Some(parent_beacon_block_root);
+    } else {
+        header.blob_gas_used = None;
+        header.excess_blob_gas = None;
+        header.parent_beacon_block_root = None;
+    }
+
+    // Placeholder for future EIP adjustments
+    header.requests_root = None;
+
+    header
+}
+
+#[cfg(any(test, feature = "arbitrary"))]
+impl<'a> arbitrary::Arbitrary<'a> for Header {
+    fn arbitrary(u: &mut arbitrary::Unstructured<'a>) -> arbitrary::Result<Self> {
+        // Generate an arbitrary header, passing it to the generate_valid_header function to make
+        // sure it is valid _with respect to hardforks only_.
+        let base = Self {
+            parent_hash: u.arbitrary()?,
+            ommers_hash: u.arbitrary()?,
+            beneficiary: u.arbitrary()?,
+            state_root: u.arbitrary()?,
+            transactions_root: u.arbitrary()?,
+            receipts_root: u.arbitrary()?,
+            logs_bloom: u.arbitrary()?,
+            difficulty: u.arbitrary()?,
+            number: u.arbitrary()?,
+            gas_limit: u.arbitrary()?,
+            gas_used: u.arbitrary()?,
+            timestamp: u.arbitrary()?,
+            extra_data: u.arbitrary()?,
+            mix_hash: u.arbitrary()?,
+            nonce: u.arbitrary()?,
+            base_fee_per_gas: u.arbitrary()?,
+            blob_gas_used: u.arbitrary()?,
+            excess_blob_gas: u.arbitrary()?,
+            parent_beacon_block_root: u.arbitrary()?,
+            requests_root: u.arbitrary()?,
+            withdrawals_root: u.arbitrary()?,
+        };
+
+        Ok(generate_valid_header(
+            base,
+            u.arbitrary()?,
+            u.arbitrary()?,
+            u.arbitrary()?,
+            u.arbitrary()?,
+        ))
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;