Permit pathfinding to be disabled. (RIPD-271)

If you do not need pathfinding, you can disable it and save some
resources by setting path_search_max to zero in your config file.

doc/rippled-example.cfg

@@ -527,6 +527,9 @@
 # [path_search_max]
 #   When searching for paths, the minimum and maximum search aggressiveness.
 #
+#   If you do not need pathfinding, you can set path_search_max to zero to
+#   disable it and avoid some expensive bookkeeping.
+#
 #   The default for 'path_search_fast' is 2. The default for 'path_search_max' is 10.
 #
 # [path_search_old]

src/ripple/app/ledger/OrderBookDB.cpp

@@ -66,7 +66,11 @@ void OrderBookDB::setup(
         mSeq = seq;
     }
 
-    if (app_.config().RUN_STANDALONE)
+    if (app_.config().PATH_SEARCH_MAX == 0)
+    {
+        // nothing to do
+    }
+    else if (app_.config().RUN_STANDALONE)
         update(ledger);
     else
         app_.getJobQueue().addJob(
@@ -84,6 +88,12 @@ void OrderBookDB::update(
 
     JLOG (j_.debug) << "OrderBookDB::update>";
 
+    if (app_.config().PATH_SEARCH_MAX == 0)
+    {
+        // pathfinding has been disabled
+        return;
+    }
+
     // walk through the entire ledger looking for orderbook entries
     int books = 0;
 

src/ripple/rpc/handlers/PathFind.cpp

@@ -32,6 +32,9 @@ namespace ripple {
 
 Json::Value doPathFind (RPC::Context& context)
 {
+    if (context.app.config().PATH_SEARCH_MAX == 0)
+        return rpcError (rpcNOT_SUPPORTED);
+
     auto lpLedger = context.ledgerMaster.getClosedLedger();
 
     if (!context.params.isMember (jss::subcommand) ||

src/ripple/rpc/handlers/RipplePathFind.cpp

@@ -70,6 +70,9 @@ buildSrcCurrencies(AccountID const& account,
 // This interface is deprecated.
 Json::Value doRipplePathFind (RPC::Context& context)
 {
+    if (context.app.config().PATH_SEARCH_MAX == 0)
+        return rpcError (rpcNOT_SUPPORTED);
+
     RPC::LegacyPathFind lpf (context.role == Role::ADMIN, context.app);
     if (!lpf.isOk ())
         return rpcError (rpcTOO_BUSY);