Config changes

- a source's interval must be greater than 0: a source is turned off with
  enabled, not with a zero interval, so JSCSourcesSource rejects an interval of
  0 or less (its validate is promoted to each source's section);
- max_message_size rejects -1: JSCServerWebSocket rejects a max_message_size
  of -1 (util.ByteSizeNoLimit), as a client could then exhaust the server's
  memory;
- every setting documents what 0 or empty means (zero/none, no limit or a
  default).

internal/config/conf_server_websocket.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"errors"
 	"reflect"
 	"time"
 
@@ -11,6 +12,10 @@ import (
 const JSCKServerWebSocket string = "websocket"
 
 // JSCServerWebSocket configures the server's websocket connections.
+// MaxMessageSize is the largest message read from a client; 0 means the
+// default (32KiB), and -1 (no limit) is rejected (see validate). A
+// WriteTimeout or PongTimeout of 0 or less means no limit, and a
+// PingInterval of 0 or less turns pings off.
 type JSCServerWebSocket struct {
 	EnableCompression bool             `cfg:"enable_compression" default:"false"`
 	MaxMessageSize    util.ByteSize    `cfg:"max_message_size"   default:"32KiB"`
@@ -40,3 +45,17 @@ func (c JSCServerWebSocket) ValueOf(key string) (any, error) {
 func (c *JSCServerWebSocket) SetValue(key string, value any) error {
 	return setValue(c, key, value)
 }
+
+// ---------- Section specific methods ----------
+
+// validate checks values before they are stored (see validator).
+// max_message_size can't be -1 (no limit): a client could then send a
+// message of any size, and the server holds each message in memory.
+func (c *JSCServerWebSocket) validate(key string, value any) error {
+	if key == "max_message_size" && value.(util.ByteSize) == util.ByteSizeNoLimit {
+		return errors.New(
+			"max_message_size can't be -1 (no limit): set an explicit size, or 0 for the default",
+		)
+	}
+	return nil
+}