Merge bitcoin/bitcoin#31190: TxDownloadManager followups

5dc94d1 fuzz fix: assert MAX_PEER_TX_ANNOUNCEMENTS is not exceeded (glozow)
8351562 [fuzz] allow negative time jumps in txdownloadman_impl (glozow)
917ab81 [doc] comment fixups from n30110 (glozow)

Pull request description:

  Addresses some remaining followups from #30110:
  - bitcoin/bitcoin#30110 (comment)
  - bitcoin/bitcoin#30110 (comment)
  - bitcoin/bitcoin#30110 (comment)

ACKs for top commit:
  naumenkogs:
    ACK bitcoin/bitcoin@5dc94d1
  instagibbs:
    ACK 5dc94d1
  theStack:
    ACK 5dc94d1

Tree-SHA512: 568de8822b2ba73407d2231d9c8c83e6c53fb929b598102b6135c16805752954b3b9b53f4e698856d4422fd8ac2f58ce7d033e9d8e101ed09986578b7605df66

Author: fanquake

src/node/txdownloadman.h

@@ -134,7 +134,8 @@ class TxDownloadManager {
     /** Deletes all txrequest announcements and orphans for a given peer. */
     void DisconnectedPeer(NodeId nodeid);
 
-    /** New inv has been received. May be added as a candidate to txrequest.
+    /** Consider adding this tx hash to txrequest. Should be called whenever a new inv has been received.
+     * Also called internally when a transaction is missing parents so that we can request them.
      * @param[in] p2p_inv     When true, only add this announcement if we don't already have the tx.
      * Returns true if this was a dropped inv (p2p_inv=true and we already have the tx), false otherwise. */
     bool AddTxAnnouncement(NodeId peer, const GenTxid& gtxid, std::chrono::microseconds now, bool p2p_inv);

src/node/txdownloadman_impl.h

@@ -160,7 +160,9 @@ class TxDownloadManagerImpl {
     void ConnectedPeer(NodeId nodeid, const TxDownloadConnectionInfo& info);
     void DisconnectedPeer(NodeId nodeid);
 
-    /** New inv has been received. May be added as a candidate to txrequest. */
+    /** Consider adding this tx hash to txrequest. Should be called whenever a new inv has been received.
+     * Also called internally when a transaction is missing parents so that we can request them.
+     */
     bool AddTxAnnouncement(NodeId peer, const GenTxid& gtxid, std::chrono::microseconds now, bool p2p_inv);
 
     /** Get getdata requests to send. */

src/test/fuzz/txdownloadman.cpp

@@ -287,7 +287,7 @@ static void CheckInvariants(const node::TxDownloadManagerImpl& txdownload_impl,
     // We should never have more than the maximum in-flight requests out for a peer.
     for (NodeId peer = 0; peer < NUM_PEERS; ++peer) {
         if (!HasRelayPermissions(peer)) {
-            Assert(txdownload_impl.m_txrequest.CountInFlight(peer) <= node::MAX_PEER_TX_REQUEST_IN_FLIGHT);
+            Assert(txdownload_impl.m_txrequest.Count(peer) <= node::MAX_PEER_TX_ANNOUNCEMENTS);
         }
     }
     txdownload_impl.m_txrequest.SanityCheck();
@@ -430,8 +430,9 @@ FUZZ_TARGET(txdownloadman_impl, .init = initialize)
             }
         );
 
-        // Jump ahead in time
-        time += fuzzed_data_provider.PickValueInArray(TIME_SKIPS);
+        auto time_skip = fuzzed_data_provider.PickValueInArray(TIME_SKIPS);
+        if (fuzzed_data_provider.ConsumeBool()) time_skip *= -1;
+        time += time_skip;
         CheckInvariants(txdownload_impl, max_orphan_count);
     }
     // Disconnect everybody, check that all data structures are empty.

src/test/txdownload_tests.cpp

@@ -27,7 +27,7 @@ struct Behaviors {
     bool m_ignore_inv_txid;
     bool m_ignore_inv_wtxid;
 
-    // Constructor. We are passing and casting ints because they are more readable in a table (see all_expected_results).
+    // Constructor. We are passing and casting ints because they are more readable in a table (see expected_behaviors).
     Behaviors(bool txid_rejects, bool wtxid_rejects, bool txid_recon, bool wtxid_recon, bool keep, bool txid_inv, bool wtxid_inv) :
         m_txid_in_rejects(txid_rejects),
         m_wtxid_in_rejects(wtxid_rejects),