Make the transaction job queue limit adjustable:

The job queue can impose limits of how many jobs of a particular
type can be queued.

This commit makes the previously hard-coded limit associated with
transactions configurable by the server's operator. Servers that
have increased memory capacity or which expect to see an influx
of transactions can increase the number of transactions their
server will be able to queue.

This commit fixes #3556.

cfg/rippled-example.cfg

@@ -453,6 +453,13 @@
 #
 #
 #
+# [max_transactions]
+#
+#   Configure the maximum number of transactions to have in the job queue
+#
+#   Must be a number between 100 and 1000, defaults to 250
+# 
+#
 # [overlay]
 #
 #   Controls settings related to the peer to peer overlay.

src/ripple/core/Config.h

@@ -173,6 +173,11 @@ public:
     // Compression
     bool COMPRESSION = false;
 
+    // Work queue limits
+    int MAX_TRANSACTIONS = 250;
+    static constexpr int MAX_JOB_QUEUE_TX = 1000;
+    static constexpr int MIN_JOB_QUEUE_TX = 100;
+
     // Amendment majority time
     std::chrono::seconds AMENDMENT_MAJORITY_TIME = defaultAmendmentMajorityTime;
 

src/ripple/core/ConfigSections.h

@@ -62,6 +62,7 @@ struct ConfigSection
 #define SECTION_IPS "ips"
 #define SECTION_IPS_FIXED "ips_fixed"
 #define SECTION_LEDGER_HISTORY "ledger_history"
+#define SECTION_MAX_TRANSACTIONS "max_transactions"
 #define SECTION_NETWORK_QUORUM "network_quorum"
 #define SECTION_NODE_SEED "node_seed"
 #define SECTION_NODE_SIZE "node_size"

src/ripple/core/impl/Config.cpp

@@ -488,6 +488,14 @@ Config::loadFromString(std::string const& fileContents)
         REDUCE_RELAY_SQUELCH = sec.value_or("squelch", false);
     }
 
+    if (getSingleSection(secConfig, SECTION_MAX_TRANSACTIONS, strTemp, j_))
+    {
+        MAX_TRANSACTIONS = std::clamp(
+            beast::lexicalCastThrow<int>(strTemp),
+            MIN_JOB_QUEUE_TX,
+            MAX_JOB_QUEUE_TX);
+    }
+
     if (getSingleSection(
             secConfig, SECTION_AMENDMENT_MAJORITY_TIME, strTemp, j_))
     {

src/ripple/overlay/impl/PeerImp.cpp

@@ -1512,9 +1512,8 @@ PeerImp::onMessage(std::shared_ptr<protocol::TMTransaction> const& m)
             }
         }
 
-        // The maximum number of transactions to have in the job queue.
-        constexpr int max_transactions = 250;
-        if (app_.getJobQueue().getJobCount(jtTRANSACTION) > max_transactions)
+        if (app_.getJobQueue().getJobCount(jtTRANSACTION) >
+            app_.config().MAX_TRANSACTIONS)
         {
             overlay_.incJqTransOverflow();
             JLOG(p_journal_.info()) << "Transaction queue is full";