Fix bug around transaction requests

If a transaction is already in-flight when a peer announces a new tx to us, we
schedule a time in the future to reconsider whether to download. At that future
time, there was a bug that would prevent transactions from being rescheduled
for potential download again (ie if the transaction was still in-flight at the
time of reconsideration, such as from some other peer). Fix this.

src/net_processing.cpp

@@ -700,6 +700,24 @@ void UpdateTxRequestTime(const uint256& txid, int64_t request_time) EXCLUSIVE_LO
     }
 }
 
+int64_t CalculateTxGetDataTime(const uint256& txid, int64_t current_time, bool use_inbound_delay) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
+{
+    int64_t process_time;
+    int64_t last_request_time = GetTxRequestTime(txid);
+    // First time requesting this tx
+    if (last_request_time == 0) {
+        process_time = current_time;
+    } else {
+        // Randomize the delay to avoid biasing some peers over others (such as due to
+        // fixed ordering of peer processing in ThreadMessageHandler)
+        process_time = last_request_time + GETDATA_TX_INTERVAL + GetRand(MAX_GETDATA_RANDOM_DELAY);
+    }
+
+    // We delay processing announcements from inbound peers
+    if (use_inbound_delay) process_time += INBOUND_PEER_TX_DELAY;
+
+    return process_time;
+}
 
 void RequestTx(CNodeState* state, const uint256& txid, int64_t nNow) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
 {
@@ -713,19 +731,9 @@ void RequestTx(CNodeState* state, const uint256& txid, int64_t nNow) EXCLUSIVE_L
     }
     peer_download_state.m_tx_announced.insert(txid);
 
-    int64_t process_time;
-    int64_t last_request_time = GetTxRequestTime(txid);
-    // First time requesting this tx
-    if (last_request_time == 0) {
-        process_time = nNow;
-    } else {
-        // Randomize the delay to avoid biasing some peers over others (such as due to
-        // fixed ordering of peer processing in ThreadMessageHandler)
-        process_time = last_request_time + GETDATA_TX_INTERVAL + GetRand(MAX_GETDATA_RANDOM_DELAY);
-    }
-
-    // We delay processing announcements from non-preferred (eg inbound) peers
-    if (!state->fPreferredDownload) process_time += INBOUND_PEER_TX_DELAY;
+    // Calculate the time to try requesting this transaction. Use
+    // fPreferredDownload as a proxy for outbound peers.
+    int64_t process_time = CalculateTxGetDataTime(txid, nNow, !state->fPreferredDownload);
 
     peer_download_state.m_tx_process_time.emplace(process_time, txid);
 }
@@ -3967,7 +3975,10 @@ bool PeerLogicValidation::SendMessages(CNode* pto)
 
         auto& tx_process_time = state.m_tx_download.m_tx_process_time;
         while (!tx_process_time.empty() && tx_process_time.begin()->first <= nNow && state.m_tx_download.m_tx_in_flight.size() < MAX_PEER_TX_IN_FLIGHT) {
-            const uint256& txid = tx_process_time.begin()->second;
+            const uint256 txid = tx_process_time.begin()->second;
+            // Erase this entry from tx_process_time (it may be added back for
+            // processing at a later time, see below)
+            tx_process_time.erase(tx_process_time.begin());
             CInv inv(MSG_TX | GetFetchFlags(pto), txid);
             if (!AlreadyHave(inv)) {
                 // If this transaction was last requested more than 1 minute ago,
@@ -3987,14 +3998,14 @@ bool PeerLogicValidation::SendMessages(CNode* pto)
                     // up processing to happen after the download times out
                     // (with a slight delay for inbound peers, to prefer
                     // requests to outbound peers).
-                    RequestTx(&state, txid, nNow);
+                    int64_t next_process_time = CalculateTxGetDataTime(txid, nNow, !state.fPreferredDownload);
+                    tx_process_time.emplace(next_process_time, txid);
                 }
             } else {
                 // We have already seen this transaction, no need to download.
                 state.m_tx_download.m_tx_announced.erase(inv.hash);
                 state.m_tx_download.m_tx_in_flight.erase(inv.hash);
             }
-            tx_process_time.erase(tx_process_time.begin());
         }