multi: Add assumevalid config option.

This adds an assumevalid config option which allows for specifying a
hash of an assume valid block or disabling assume valid.

The usage is as follows:

* (flag not specified): defaults to the hard-coded value in source
* --assumevalid=0: disable AssumeValid
* --assumevalid=[blockhash]: set AssumeValid to the specified block hash

config.go

@@ -195,6 +195,7 @@ type config struct {
 	// Chain related options.
 	DisableCheckpoints bool   `long:"nocheckpoints" description:"Disable built-in checkpoints.  Don't do this unless you know what you're doing"`
 	DumpBlockchain     string `long:"dumpblockchain" description:"Write blockchain as a flat file of blocks for use with addblock, to the specified filename"`
+	AssumeValid        string `long:"assumevalid" description:"Hash of an assume valid block.  Defaults to the hard-coded assume valid block that is updated periodically with new releases.  Set to 0 to disable.  Don't do this unless you know what you're doing"`
 
 	// Relay and mempool policy.
 	MinRelayTxFee    float64 `long:"minrelaytxfee" description:"The minimum transaction fee in DCR/kB to be considered a non-zero fee"`

doc.go

@@ -121,6 +121,11 @@ Application Options:
                                unless you know what you're doing
       --dumpblockchain=        Write blockchain as a flat file of blocks for use
                                with addblock, to the specified filename
+      --assumevalid=           Hash of an assume valid block.  Defaults to the
+                               hard-coded assume valid block that is updated
+                               periodically with new releases.  Set to 0 to
+                               disable.  Don't do this unless you know what
+                               you're doing
       --minrelaytxfee=         The minimum transaction fee in DCR/kB to be
                                considered a non-zero fee (default: 0.0001)
       --limitfreerelay=        Limit relay of transactions with no transaction

server.go

@@ -3470,12 +3470,24 @@ func newServer(ctx context.Context, listenAddrs []string, db database.DB,
 	}
 	s.feeEstimator = fe
 
-	// Only set assume valid block when checkpoints are enabled.  A new config
-	// option to disable assume valid will be introduced in a future commit and
-	// will be used instead of the disable checkpoints option.
+	// Set assume valid when enabled.
 	var assumeValid chainhash.Hash
-	if !cfg.DisableCheckpoints {
+	if cfg.AssumeValid != "0" {
+		// Default assume valid to the value specified by chain params.
 		assumeValid = s.chainParams.AssumeValid
+
+		// Override assume valid if specified by the config option.
+		if cfg.AssumeValid != "" {
+			hash, err := chainhash.NewHashFromStr(cfg.AssumeValid)
+			if err != nil {
+				err = fmt.Errorf("invalid hex for --assumevalid: %w", err)
+				return nil, err
+			}
+			assumeValid = *hash
+			srvrLog.Infof("Assume valid set to %v", assumeValid)
+		}
+	} else {
+		srvrLog.Info("Assume valid is disabled")
 	}
 
 	// Only configure checkpoints when enabled.