build: fix host-gateway handling

Signed-off-by: CrazyMax <[email protected]>

build/build.go

@@ -641,8 +641,14 @@ func toSolveOpt(ctx context.Context, node builder.Node, multiDriver bool, opt Op
 		return nil, nil, errors.Errorf("network mode %q not supported by buildkit - you can define a custom network for your builder using the network driver-opt in buildx create", opt.NetworkMode)
 	}
 
+	// retrieve host gateway IP from node driver
+	hostGatewayIP, err := nodeDriver.HostGatewayIP(ctx)
+	if err != nil {
+		return nil, nil, err
+	}
+
 	// setup extrahosts
-	extraHosts, err := toBuildkitExtraHosts(opt.ExtraHosts, nodeDriver.IsMobyDriver())
+	extraHosts, err := toBuildkitExtraHosts(opt.ExtraHosts, hostGatewayIP)
 	if err != nil {
 		return nil, nil, err
 	}

build/utils.go

@@ -57,7 +57,7 @@ func isArchive(header []byte) bool {
 }
 
 // toBuildkitExtraHosts converts hosts from docker key:value format to buildkit's csv format
-func toBuildkitExtraHosts(inp []string, mobyDriver bool) (string, error) {
+func toBuildkitExtraHosts(inp []string, hgip net.IP) (string, error) {
 	if len(inp) == 0 {
 		return "", nil
 	}
@@ -67,11 +67,16 @@ func toBuildkitExtraHosts(inp []string, mobyDriver bool) (string, error) {
 		if !ok || host == "" || ip == "" {
 			return "", errors.Errorf("invalid host %s", h)
 		}
-		// Skip IP address validation for "host-gateway" string with moby driver
-		if !mobyDriver || ip != mobyHostGatewayName {
-			if net.ParseIP(ip) == nil {
-				return "", errors.Errorf("invalid host %s", h)
+		// If the IP Address is a "host-gateway", replace this value with the
+		// IP address provided by the worker's label.
+		if ip == mobyHostGatewayName {
+			if hgip == nil {
+				return "", errors.Errorf("unable to derive the IP value for host-gateway")
 			}
+			ip = hgip.String()
+		}
+		if net.ParseIP(ip) == nil {
+			return "", errors.Errorf("invalid host %s", h)
 		}
 		hosts = append(hosts, host+"="+ip)
 	}

driver/manager.go

@@ -2,17 +2,17 @@ package driver
 
 import (
 	"context"
+	"net"
 	"os"
 	"sort"
 	"strings"
 	"sync"
 
-	"k8s.io/client-go/rest"
-
 	dockerclient "github.com/docker/docker/client"
 	"github.com/moby/buildkit/client"
 	specs "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/pkg/errors"
+	"k8s.io/client-go/rest"
 )
 
 type Factory interface {
@@ -154,6 +154,9 @@ type DriverHandle struct {
 	features                map[Feature]bool
 	historyAPISupportedOnce sync.Once
 	historyAPISupported     bool
+	hostGatewayIPOnce       sync.Once
+	hostGatewayIP           net.IP
+	hostGatewayIPErr        error
 }
 
 func (d *DriverHandle) Client(ctx context.Context) (*client.Client, error) {
@@ -178,3 +181,34 @@ func (d *DriverHandle) HistoryAPISupported(ctx context.Context) bool {
 	})
 	return d.historyAPISupported
 }
+
+func (d *DriverHandle) HostGatewayIP(ctx context.Context) (net.IP, error) {
+	d.hostGatewayIPOnce.Do(func() {
+		if !d.Driver.IsMobyDriver() {
+			return
+		}
+		c, err := d.Client(ctx)
+		if err != nil {
+			d.hostGatewayIPErr = err
+			return
+		}
+		workers, err := c.ListWorkers(ctx)
+		if err != nil {
+			d.hostGatewayIPErr = err
+			return
+		}
+		if len(workers) == 0 {
+			return
+		}
+		// TODO: Use HostGatewayIP const from github.com/docker/docker/builder/builder-next/worker/label pkg
+		if v, ok := workers[0].Labels["org.mobyproject.buildkit.worker.hostgatewayip"]; ok && v != "" {
+			ip := net.ParseIP(v)
+			if ip == nil {
+				d.hostGatewayIPErr = errors.Wrapf(err, "failed to parse host gateway IP: %s", v)
+				return
+			}
+			d.hostGatewayIP = ip
+		}
+	})
+	return d.hostGatewayIP, d.hostGatewayIPErr
+}