- dns: limit the number of active goroutines for incoming requests processing

Close #2015

Squashed commit of the following:

commit 90ba06f1fce22a452b4d61db62bd950b976debd1
Merge: 9494b29b 473d8818
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Fri Aug 14 19:14:26 2020 +0300

    Merge remote-tracking branch 'origin/master' into max-go

commit 9494b29b65ae8fe593a487984bed051aa78e4ff9
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Fri Aug 14 17:03:00 2020 +0300

    + max_goroutines setting

commit 87071a5e0ed43be192a7755fb25764cd4519da5a
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Fri Aug 14 15:29:00 2020 +0300

    - dns: limit the number of active goroutines for incoming requests processing

dnsforward/config.go

@@ -81,6 +81,7 @@ type FilteringConfig struct {
 	AAAADisabled           bool     `yaml:"aaaa_disabled"`      // Respond with an empty answer to all AAAA requests
 	EnableDNSSEC           bool     `yaml:"enable_dnssec"`      // Set DNSSEC flag in outcoming DNS request
 	EnableEDNSClientSubnet bool     `yaml:"edns_client_subnet"` // Enable EDNS Client Subnet option
+	MaxGoroutines          uint32   `yaml:"max_goroutines"`     // Max. number of parallel goroutines for processing incoming requests
 }
 
 // TLSConfig is the TLS configuration for HTTPS, DNS-over-HTTPS, and DNS-over-TLS
@@ -144,6 +145,7 @@ func (s *Server) createProxyConfig() (proxy.Config, error) {
 		BeforeRequestHandler:   s.beforeRequestHandler,
 		RequestHandler:         s.handleDNSRequest,
 		EnableEDNSClientSubnet: s.conf.EnableEDNSClientSubnet,
+		MaxGoroutines:          int(s.conf.MaxGoroutines),
 	}
 
 	if s.conf.CacheSize != 0 {

dnsforward/dnsforward.go

@@ -176,6 +176,9 @@ func (s *Server) Prepare(config *ServerConfig) error {
 				return fmt.Errorf("DNS: invalid custom blocking IP address specified")
 			}
 		}
+		if s.conf.MaxGoroutines == 0 {
+			s.conf.MaxGoroutines = 50
+		}
 	}
 
 	// 2. Set default values in the case if nothing is configured