Fix node auto-configuration code:

The `node_size` configuration option is used to automatically
configure various parameters (cache sizes, timeouts, etc) for
the server.

A previous commit included changes that caused incorrect values
to be returned which can result in sub-optimal performance that
can manifest as difficulty syncing to the network, or increased
disk I/O and/or memory usage. The problem was introduced with
commit 66fad62e66.

This commit, if merged, fixes the code to ensure that the correct
values are returned and introduces a compile-time check to prevent
this issue from reoccurring.

src/ripple/core/impl/Config.cpp

@@ -32,12 +32,61 @@
 #include <boost/format.hpp>
 #include <boost/regex.hpp>
 #include <boost/system/error_code.hpp>
+#include <algorithm>
 #include <fstream>
 #include <iostream>
 #include <iterator>
 
 namespace ripple {
 
+inline constexpr
+std::array<std::pair<SizedItem, std::array<int, 5>>, 11>
+sizedItems
+{{
+    // FIXME: We should document each of these items, explaining exactly what
+    //        they control and whether there exists an explicit config option
+    //        that can be used to override the default.
+    { SizedItem::sweepInterval,
+        {{      10,      30,       60,       90,      120  }} },
+    { SizedItem::treeCacheSize,
+        {{  128000,  256000,   512000,   768000,  2048000  }} },
+    { SizedItem::treeCacheAge,
+        {{      30,      60,       90,      120,      900  }} },
+    { SizedItem::ledgerSize,
+        {{      32,     128,      256,      384,      768  }} },
+    { SizedItem::ledgerAge,
+        {{      30,      90,      180,      240,      900  }} },
+    { SizedItem::ledgerFetch,
+        {{       2,       3,        4,        5,        8  }} },
+    { SizedItem::nodeCacheSize,
+        {{   16384,   32768,   131072,   262144,   524288  }} },
+    { SizedItem::nodeCacheAge,
+        {{      60,      90,      120,      900,     1800  }} },
+    { SizedItem::hashNodeDBCache,
+        {{       4,      12,       24,       64,      128  }} },
+    { SizedItem::txnDBCache,
+        {{       4,      12,       24,       64,      128  }} },
+    { SizedItem::lgrDBCache,
+        {{       4,       8,       16,       32,      128  }} },
+}};
+
+// Ensure that the order of entries in the table corresponds to the
+// order of entries in the enum:
+static_assert([]() constexpr -> bool
+{
+    std::underlying_type_t<SizedItem> idx = 0;
+
+    for (auto const& i : sizedItems)
+    {
+        if (static_cast<std::underlying_type_t<SizedItem>>(i.first) != idx)
+            return false;
+
+        ++idx;
+    }
+
+    return true;
+}(), "Mismatch between sized item enum & array indices");
+
 //
 // TODO: Check permissions on config file before using it.
 //
@@ -329,14 +378,8 @@ void Config::loadFromString (std::string const& fileContents)
         else if (boost::iequals(strTemp, "huge"))
             NODE_SIZE = 4;
         else
-        {
-            NODE_SIZE = beast::lexicalCastThrow <int> (strTemp);
-
-            if (NODE_SIZE < 0)
-                NODE_SIZE = 0;
-            else if (NODE_SIZE > 4)
-                NODE_SIZE = 4;
-        }
+            NODE_SIZE = std::min<std::size_t>(4,
+                beast::lexicalCastThrow<std::size_t>(strTemp));
     }
 
     if (getSingleSection (secConfig, SECTION_SIGNING_SUPPORT, strTemp, j_))
@@ -594,4 +637,13 @@ boost::filesystem::path Config::getDebugLogFile () const
     return log_file;
 }
 
+int
+Config::getValueFor(SizedItem item, boost::optional<std::size_t> node) const
+{
+    auto const index = static_cast<std::underlying_type_t<SizedItem>>(item);
+    assert(index < sizedItems.size());
+    assert(!node || *node <= 4);
+    return sizedItems.at(index).second.at(node.value_or(NODE_SIZE));
+}
+
 } // ripple