all: Use context in lib/dialer (#6177)

* all: Use context in lib/dialer

* a bit slimmer

* https://github.com/syncthing/syncthing/pull/5753

* bot

* missed adding debug.go

* errors.Cause

* simultaneous dialing

* anti-leak

lib/osutil/ping.go

@@ -7,6 +7,7 @@
 package osutil
 
 import (
+	"context"
 	"net/url"
 	"time"
 
@@ -16,9 +17,11 @@ import (
 // TCPPing returns the duration required to establish a TCP connection
 // to the given host. ICMP packets require root privileges, hence why we use
 // tcp.
-func TCPPing(address string) (time.Duration, error) {
+func TCPPing(ctx context.Context, address string) (time.Duration, error) {
 	start := time.Now()
-	conn, err := dialer.DialTimeout("tcp", address, time.Second)
+	ctx, cancel := context.WithTimeout(ctx, time.Second)
+	defer cancel()
+	conn, err := dialer.DialContext(ctx, "tcp", address)
 	if conn != nil {
 		conn.Close()
 	}
@@ -27,11 +30,11 @@ func TCPPing(address string) (time.Duration, error) {
 
 // GetLatencyForURL parses the given URL, tries opening a TCP connection to it
 // and returns the time it took to establish the connection.
-func GetLatencyForURL(addr string) (time.Duration, error) {
+func GetLatencyForURL(ctx context.Context, addr string) (time.Duration, error) {
 	uri, err := url.Parse(addr)
 	if err != nil {
 		return 0, err
 	}
 
-	return TCPPing(uri.Host)
+	return TCPPing(ctx, uri.Host)
 }