net: store a zero-parent header only for genesis

changelog.d/zero-prev-header.md

@@ -0,0 +1,8 @@
+Fixed
+
+- **A header with an all-zero parent is stored only for genesis.** Any
+  other such header was written to the header store with no proof-of-work
+  or context check, from an unrequested block, a `headers` message, or
+  `submitheader`. New rows are now refused, as in Core. An unrequested
+  `block` with such a header gets the usual misbehavior penalty, and
+  `submitheader` returns `Must submit previous header`.

crates/rbitcoin-net/src/chain.rs

@@ -1409,7 +1409,8 @@ impl ChainHub {
         let prev = header.prev_blockhash;
         let prev_bytes = prev.to_byte_array();
         // Same for the parent, plus a body parked with no header row yet.
-        let prev_known = prev_bytes == [0u8; 32]
+        // An all-zero parent is known only to genesis.
+        let prev_known = (prev_bytes == [0u8; 32] && hash == self.params.genesis_hash)
             || self
                 .query
                 .get_header_by_hash(&prev_bytes)
@@ -1516,6 +1517,13 @@ impl ChainHub {
     ) -> Result<Fk, NetError> {
         let prev_bytes = header.prev_blockhash.to_byte_array();
         let prev_fk = if prev_bytes == [0u8; 32] {
+            // Only genesis has no parent. Any other all-zero parent is
+            // missing, as in Core, and gets no row.
+            if header.block_hash() != self.params.genesis_hash {
+                return Err(NetError::Consensus(
+                    "header parent unknown — ensure parent before child".into(),
+                ));
+            }
             Fk::NULL
         } else if let Some(n) = in_batch.get(&prev_bytes) {
             n.fk
@@ -6064,6 +6072,23 @@ mod tests {
             .to_string()
             .contains("header parent unknown"));
         assert_eq!(hub.query.store().header_count(), after_ok);
+        // Only genesis has an all-zero parent. Any other zero-parent header
+        // is an orphan, as in Core, and writes no row.
+        let mut zero = b4.header;
+        zero.prev_blockhash = BlockHash::from_byte_array([0u8; 32]);
+        rbitcoin_consensus::grind_regtest_pow(&mut zero);
+        let err = hub.ensure_header(&zero).unwrap_err();
+        assert!(err.to_string().contains("header parent unknown"), "{err}");
+        let err = hub.ensure_headers_batch(&[zero, b4.header]).unwrap_err();
+        assert!(err.to_string().contains("header parent unknown"), "{err}");
+        assert_eq!(
+            hub.process_submitted_header(&zero).unwrap_err(),
+            "Must submit previous header"
+        );
+        assert_eq!(hub.query.store().header_count(), after_ok);
+        assert!(!hub.knows_header(&zero.block_hash()));
+        hub.ensure_header(&rbitcoin_consensus::genesis_block(&hub.params).header)
+            .expect("genesis still writes its zero-parent row");
         let _ = std::fs::remove_dir_all(&dir);
     }
 

crates/rbitcoin-net/src/peer_header_dos_journey.rs

@@ -727,6 +727,25 @@ async fn header_reject_punishes_except_time(hub: &crate::chain::ChainHub) {
         "an unrequested header with bad pow must disconnect"
     );
 
+    // Core: a missing parent is `prev-blk-not-found` and Misbehaving. An
+    // all-zero parent is missing too unless the block is genesis.
+    let mut zero_prev = rbitcoin_consensus::mine_regtest_paying(
+        gen,
+        tip_time.saturating_add(2),
+        next_h,
+        op_true(),
+        vec![],
+    );
+    zero_prev.header.prev_blockhash = zero_stop();
+    rbitcoin_consensus::grind_regtest_pow(&mut zero_prev.header);
+    let mut follow = PeerFollowState::new();
+    push(hub, &out_tx, &mut follow, None, NetworkMessage::Block(zero_prev.clone())).await;
+    assert!(
+        follow.ban_score >= BAN_SCORE_THRESHOLD,
+        "an unrequested block with an all-zero parent must disconnect"
+    );
+    assert!(!hub.knows_header(&zero_prev.block_hash()));
+
     let now = u32::try_from(
         std::time::SystemTime::now()
             .duration_since(std::time::UNIX_EPOCH)

docs/external_findings/030-header-accept.md

@@ -9,6 +9,13 @@ accepted. A non-genesis block with an all-zero previous hash is not held
 and does not disconnect the tip. Header work is summed with a checked
 add. A zero `nBits` target is not passed to `Header::work`.
 
+Header intake also refuses a new zero-parent non-genesis row, from a
+single header, a `headers` batch, or `submitheader`. An unrequested block
+with such a header gets the usual misbehavior penalty. Rows written by
+older versions stay in the store.
+
 **Regression:** `rbitcoin-net`
 `chain::tests::hostile_peer_session`,
+`chain::tests::ensure_headers_batch_chain_and_missing_parent`,
+`peer::tests::peer_header_dos_and_self_announce`,
 `most_work::tests::sum_work_and_work_better`