Add option -alerts to opt out of alert system

Make it possible to opt-out of the centralized alert system by providing
an option `-noalerts` or `-alerts=0`. The default remains unchanged.

This is a gentler form of bitcoin#6260, in which I went a bit overboard by
removing the alert system completely.

I intend to add this to the GUI options in another pull after this.

src/init.cpp

@@ -266,6 +266,7 @@ std::string HelpMessage(HelpMessageMode mode)
     // Do not translate _(...) -help-debug options, Many technical terms, and only a very small audience, so is unnecessary stress to translators.
     string strUsage = HelpMessageGroup(_("Options:"));
     strUsage += HelpMessageOpt("-?", _("This help message"));
+    strUsage += HelpMessageOpt("-alerts", strprintf(_("Receive and display P2P network alerts (default: %u)"), DEFAULT_ALERTS));
     strUsage += HelpMessageOpt("-alertnotify=<cmd>", _("Execute command when a relevant alert is received or we see a really long fork (%s in cmd is replaced by message)"));
     strUsage += HelpMessageOpt("-blocknotify=<cmd>", _("Execute command when the best block changes (%s in cmd is replaced by block hash)"));
     strUsage += HelpMessageOpt("-checkblocks=<n>", strprintf(_("How many blocks to check at startup (default: %u, 0 = all)"), 288));
@@ -865,6 +866,8 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
     fIsBareMultisigStd = GetBoolArg("-permitbaremultisig", true);
     nMaxDatacarrierBytes = GetArg("-datacarriersize", nMaxDatacarrierBytes);
 
+    fAlerts = GetBoolArg("-alerts", DEFAULT_ALERTS);
+
     // ********************************************************* Step 4: application initialization: dir lock, daemonize, pidfile, debug log
 
     // Initialize elliptic curve code

src/main.cpp

@@ -61,6 +61,7 @@ bool fCheckBlockIndex = false;
 bool fCheckpointsEnabled = true;
 size_t nCoinCacheUsage = 5000 * 300;
 uint64_t nPruneTarget = 0;
+bool fAlerts = DEFAULT_ALERTS;
 
 /** Fees smaller than this (in satoshi) are considered zero fee (for relaying and mining) */
 CFeeRate minRelayTxFee = CFeeRate(1000);
@@ -4622,7 +4623,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
     }
 
 
-    else if (strCommand == "alert")
+    else if (fAlerts && strCommand == "alert")
     {
         CAlert alert;
         vRecv >> alert;

src/main.h

@@ -52,6 +52,8 @@ static const unsigned int DEFAULT_BLOCK_MAX_SIZE = 750000;
 static const unsigned int DEFAULT_BLOCK_MIN_SIZE = 0;
 /** Default for -blockprioritysize, maximum space for zero/low-fee transactions **/
 static const unsigned int DEFAULT_BLOCK_PRIORITY_SIZE = 50000;
+/** Default for accepting alerts from the P2P network. */
+static const bool DEFAULT_ALERTS = true;
 /** The maximum size for transactions we're willing to relay/mine */
 static const unsigned int MAX_STANDARD_TX_SIZE = 100000;
 /** Maximum number of signature check operations in an IsStandard() P2SH script */
@@ -113,6 +115,7 @@ extern bool fCheckBlockIndex;
 extern bool fCheckpointsEnabled;
 extern size_t nCoinCacheUsage;
 extern CFeeRate minRelayTxFee;
+extern bool fAlerts;
 
 /** Best header we've seen so far (used for getheaders queries' starting points). */
 extern CBlockIndex *pindexBestHeader;