config: reject non-positive maxpeers

In this commit, we reject maxpeers values that cannot represent a usable
peer budget.

A zero value reached connmgr as TargetOutbound=0, where zero means to use
the default target of eight. The server then rejected every completed
peer and immediately created a replacement request. Rejecting the value
during configuration avoids that outbound reconnect loop.

Author: Roasbeef

config.go

@@ -95,6 +95,15 @@ func minUint32(a, b uint32) uint32 {
 	return b
 }
 
+// validateMaxPeers ensures btcd has a positive total peer budget.
+func validateMaxPeers(maxPeers int) error {
+	if maxPeers <= 0 {
+		return fmt.Errorf("maxpeers must be greater than zero: %d", maxPeers)
+	}
+
+	return nil
+}
+
 // config defines the configuration options for btcd.
 //
 // See loadConfig for details on the configuration load process.
@@ -129,7 +138,7 @@ type config struct {
 	Listeners            []string      `long:"listen" description:"Add an interface/port to listen for connections (default all interfaces port: 8333, testnet: 18333)"`
 	LogDir               string        `long:"logdir" description:"Directory to log output."`
 	MaxOrphanTxs         int           `long:"maxorphantx" description:"Max number of orphan transactions to keep in memory"`
-	MaxPeers             int           `long:"maxpeers" description:"Max number of inbound and outbound peers. Outbound slots for the configured peer mode are reserved before inbound capacity is calculated"`
+	MaxPeers             int           `long:"maxpeers" description:"Max number of inbound and outbound peers. Must be greater than zero. Outbound slots for the configured peer mode are reserved before inbound capacity is calculated"`
 	MiningAddrs          []string      `long:"miningaddr" description:"Add the specified payment address to the list of addresses to use for generated blocks -- At least one address is required if the generate option is set"`
 	MinRelayTxFee        float64       `long:"minrelaytxfee" description:"The minimum transaction fee in BTC/kB to be considered a non-zero fee."`
 	DisableBanning       bool          `long:"nobanning" description:"Disable banning of misbehaving peers"`
@@ -615,6 +624,13 @@ func loadConfig() (*config, []string, error) {
 		return nil, nil, err
 	}
 
+	if err := validateMaxPeers(cfg.MaxPeers); err != nil {
+		err := fmt.Errorf("%s: %w", funcName, err)
+		fmt.Fprintln(os.Stderr, err)
+		fmt.Fprintln(os.Stderr, usageMessage)
+		return nil, nil, err
+	}
+
 	// If mainnet is active, then we won't allow the stall handler to be
 	// disabled.
 	if activeNetParams.Params.Net == wire.MainNet && cfg.DisableStallHandler {

config_test.go

@@ -8,6 +8,30 @@ import (
 	"testing"
 )
 
+func TestValidateMaxPeers(t *testing.T) {
+	tests := []struct {
+		name     string
+		maxPeers int
+		wantErr  bool
+	}{
+		{name: "negative", maxPeers: -1, wantErr: true},
+		{name: "zero", wantErr: true},
+		{name: "positive", maxPeers: 1},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			err := validateMaxPeers(test.maxPeers)
+			if test.wantErr && err == nil {
+				t.Fatal("expected validation error")
+			}
+			if !test.wantErr && err != nil {
+				t.Fatalf("unexpected validation error: %v", err)
+			}
+		})
+	}
+}
+
 var (
 	rpcuserRegexp = regexp.MustCompile("(?m)^rpcuser=.+$")
 	rpcpassRegexp = regexp.MustCompile("(?m)^rpcpass=.+$")

sample-btcd.conf

@@ -106,7 +106,7 @@
 ; connect=fe80::1
 ; connect=[fe80::2]:8333
 
-; Maximum number of inbound and outbound peers.
+; Maximum number of inbound and outbound peers. Must be greater than zero.
 ; maxpeers=125
 
 ; Disable banning of misbehaving peers.