Make max number of orphan blocks kept in memory a startup parameter (fixes #4253)

src/init.cpp

@@ -206,6 +206,7 @@ std::string HelpMessage(HelpMessageMode hmm)
     strUsage += "  -dbcache=<n>           " + strprintf(_("Set database cache size in megabytes (%d to %d, default: %d)"), nMinDbCache, nMaxDbCache, nDefaultDbCache) + "\n";
     strUsage += "  -keypool=<n>           " + _("Set key pool size to <n> (default: 100)") + "\n";
     strUsage += "  -loadblock=<file>      " + _("Imports blocks from external blk000??.dat file") + " " + _("on startup") + "\n";
+    strUsage += "  -maxorphanblocks=<n>   " + strprintf(_("Keep at most <n> unconnectable blocks in memory (default: %u)"), DEFAULT_MAX_ORPHAN_BLOCKS) + "\n";
     strUsage += "  -par=<n>               " + strprintf(_("Set the number of script verification threads (%u to %d, 0 = auto, <0 = leave that many cores free, default: %d)"), -(int)boost::thread::hardware_concurrency(), MAX_SCRIPTCHECK_THREADS, DEFAULT_SCRIPTCHECK_THREADS) + "\n";
     strUsage += "  -pid=<file>            " + _("Specify pid file (default: bitcoind.pid)") + "\n";
     strUsage += "  -reindex               " + _("Rebuild block chain index from current blk000??.dat files") + " " + _("on startup") + "\n";

src/main.cpp

@@ -1160,7 +1160,7 @@ uint256 static GetOrphanRoot(const uint256& hash)
 // Remove a random orphan block (which does not have any dependent orphans).
 void static PruneOrphanBlocks()
 {
-    if (mapOrphanBlocksByPrev.size() <= MAX_ORPHAN_BLOCKS)
+    if (mapOrphanBlocksByPrev.size() <= (size_t)std::max((int64_t)0, GetArg("-maxorphanblocks", DEFAULT_MAX_ORPHAN_BLOCKS)))
         return;
 
     // Pick a random orphan block.

src/main.h

@@ -45,8 +45,8 @@ static const unsigned int MAX_STANDARD_TX_SIZE = 100000;
 static const unsigned int MAX_BLOCK_SIGOPS = MAX_BLOCK_SIZE/50;
 /** The maximum number of orphan transactions kept in memory */
 static const unsigned int MAX_ORPHAN_TRANSACTIONS = MAX_BLOCK_SIZE/100;
-/** The maximum number of orphan blocks kept in memory */
-static const unsigned int MAX_ORPHAN_BLOCKS = 750;
+/** Default for -maxorphanblocks, maximum number of orphan blocks kept in memory */
+static const unsigned int DEFAULT_MAX_ORPHAN_BLOCKS = 750;
 /** The maximum size of a blk?????.dat file (since 0.8) */
 static const unsigned int MAX_BLOCKFILE_SIZE = 0x8000000; // 128 MiB
 /** The pre-allocation chunk size for blk?????.dat files (since 0.8) */