net/dnscache: use parent context to perform lookup (#48)

Upstream PR: tailscale#11935

Author: kylecarbs

net/dnscache/dnscache.go

@@ -222,7 +222,7 @@ func (r *Resolver) LookupIP(ctx context.Context, host string) (ip, v6 netip.Addr
 	}
 
 	ch := r.sf.DoChan(host, func() (ret ipRes, _ error) {
-		ip, ip6, allIPs, err := r.lookupIP(host)
+		ip, ip6, allIPs, err := r.lookupIP(ctx, host)
 		if err != nil {
 			return ret, err
 		}
@@ -283,30 +283,30 @@ func (r *Resolver) lookupTimeoutForHost(host string) time.Duration {
 	return 10 * time.Second
 }
 
-func (r *Resolver) lookupIP(host string) (ip, ip6 netip.Addr, allIPs []netip.Addr, err error) {
+func (r *Resolver) lookupIP(ctx context.Context, host string) (ip, ip6 netip.Addr, allIPs []netip.Addr, err error) {
 	if ip, ip6, allIPs, ok := r.lookupIPCache(host); ok {
 		r.dlogf("%q found in cache as %v", host, ip)
 		return ip, ip6, allIPs, nil
 	}
 
-	ctx, cancel := context.WithTimeout(context.Background(), r.lookupTimeoutForHost(host))
-	defer cancel()
-	ips, err := r.fwd().LookupNetIP(ctx, "ip", host)
+	lookupCtx, lookupCancel := context.WithTimeout(ctx, r.lookupTimeoutForHost(host))
+	defer lookupCancel()
+	ips, err := r.fwd().LookupNetIP(lookupCtx, "ip", host)
 	if err != nil || len(ips) == 0 {
 		if resolver, ok := r.cloudHostResolver(); ok {
 			r.dlogf("resolving %q via cloud resolver", host)
-			ips, err = resolver.LookupNetIP(ctx, "ip", host)
+			ips, err = resolver.LookupNetIP(lookupCtx, "ip", host)
 		}
 	}
 	if (err != nil || len(ips) == 0) && r.LookupIPFallback != nil {
-		ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
-		defer cancel()
+		lookupCtx, lookupCancel := context.WithTimeout(ctx, 30*time.Second)
+		defer lookupCancel()
 		if err != nil {
 			r.dlogf("resolving %q using fallback resolver due to error", host)
 		} else {
 			r.dlogf("resolving %q using fallback resolver due to no returned IPs", host)
 		}
-		ips, err = r.LookupIPFallback(ctx, host)
+		ips, err = r.LookupIPFallback(lookupCtx, host)
 	}
 	if err != nil {
 		return netip.Addr{}, netip.Addr{}, nil, err