More explanatory error message from Listen (#4534)

* explain cryptic unix socket listener error related to process kill

#4533

* less ambiguous wording: clean up -> delete

* shorten error message explanation

* link back to pull request in comment for later archeaology

listeners.go

@@ -17,6 +17,7 @@ package caddy
 import (
 	"fmt"
 	"net"
+	"os"
 	"strconv"
 	"strings"
 	"sync"
@@ -39,6 +40,10 @@ func Listen(network, addr string) (net.Listener, error) {
 	sharedLn, _, err := listenerPool.LoadOrNew(lnKey, func() (Destructor, error) {
 		ln, err := net.Listen(network, addr)
 		if err != nil {
+			// https://github.com/caddyserver/caddy/pull/4534
+			if isUnixNetwork(network) && isListenBindAddressAlreadyInUseError(err) {
+				return nil, fmt.Errorf("%w: this can happen if Caddy was forcefully killed", err)
+			}
 			return nil, err
 		}
 		return &sharedListener{Listener: ln, key: lnKey}, nil
@@ -59,6 +64,10 @@ func ListenPacket(network, addr string) (net.PacketConn, error) {
 	sharedPc, _, err := listenerPool.LoadOrNew(lnKey, func() (Destructor, error) {
 		pc, err := net.ListenPacket(network, addr)
 		if err != nil {
+			// https://github.com/caddyserver/caddy/pull/4534
+			if isUnixNetwork(network) && isListenBindAddressAlreadyInUseError(err) {
+				return nil, fmt.Errorf("%w: this can happen if Caddy was forcefully killed", err)
+			}
 			return nil, err
 		}
 		return &sharedPacketConn{PacketConn: pc, key: lnKey}, nil
@@ -322,6 +331,20 @@ func isUnixNetwork(netw string) bool {
 	return netw == "unix" || netw == "unixgram" || netw == "unixpacket"
 }
 
+func isListenBindAddressAlreadyInUseError(err error) bool {
+	switch networkOperationError := err.(type) {
+	case *net.OpError:
+		switch syscallError := networkOperationError.Err.(type) {
+		case *os.SyscallError:
+			if syscallError.Syscall == "bind" {
+				return true
+			}
+		}
+	}
+
+	return false
+}
+
 // ParseNetworkAddress parses addr into its individual
 // components. The input string is expected to be of
 // the form "network/host:port-range" where any part is