@@ -80,7 +80,7 @@ func (n *networkRouter) postNetworkCreate(ctx context.Context, w http.ResponseWr

 		return err

 	}

-	if _, err := n.clusterProvider.GetNetwork(create.Name); err == nil {

+	if nws, err := n.clusterProvider.GetNetworksByName(create.Name); err == nil && len(nws) > 0 {

 		return libnetwork.NetworkNameError(create.Name)

 	}

@@ -1556,8 +1556,7 @@ func (c *Cluster) GetNetwork(input string) (apitypes.NetworkResource, error) {

 	return convert.BasicNetworkFromGRPC(*network), nil

 }

-// GetNetworks returns all current cluster managed networks.

-func (c *Cluster) GetNetworks() ([]apitypes.NetworkResource, error) {

+func (c *Cluster) getNetworks(filters *swarmapi.ListNetworksRequest_Filters) ([]apitypes.NetworkResource, error) {

 	c.RLock()

 	defer c.RUnlock()

@@ -1568,7 +1567,7 @@ func (c *Cluster) GetNetworks() ([]apitypes.NetworkResource, error) {

 	ctx, cancel := c.getRequestContext()

 	defer cancel()

-	r, err := c.client.ListNetworks(ctx, &swarmapi.ListNetworksRequest{})

+	r, err := c.client.ListNetworks(ctx, &swarmapi.ListNetworksRequest{Filters: filters})

 	if err != nil {

 		return nil, err

 	}

@@ -1582,6 +1581,21 @@ func (c *Cluster) GetNetworks() ([]apitypes.NetworkResource, error) {

 	return networks, nil

 }

+// GetNetworks returns all current cluster managed networks.

+func (c *Cluster) GetNetworks() ([]apitypes.NetworkResource, error) {

+	return c.getNetworks(nil)

+}

+

+// GetNetworksByName returns cluster managed networks by name.

+// It is ok to have multiple networks here. #18864

+func (c *Cluster) GetNetworksByName(name string) ([]apitypes.NetworkResource, error) {

+	// Note that swarmapi.GetNetworkRequest.Name is not functional.

+	// So we cannot just use that with c.GetNetwork.

+	return c.getNetworks(&swarmapi.ListNetworksRequest_Filters{

+		Names: []string{name},

+	})

+}

+

 func attacherKey(target, containerID string) string {

 	return containerID + ":" + target

 }

@@ -1184,3 +1184,50 @@ func (s *DockerTrustedSwarmSuite) TestTrustedServiceUpdate(c *check.C) {

 	c.Assert(err, check.NotNil, check.Commentf(out))

 	c.Assert(string(out), checker.Contains, "Error: remote trust data does not exist", check.Commentf(out))

 }

+

+// Test case for issue #27866, which did not allow NW name that is the prefix of a swarm NW ID.

+// e.g. if the ingress ID starts with "n1", it was impossible to create a NW named "n1".

+func (s *DockerSwarmSuite) TestSwarmNetworkCreateIssue27866(c *check.C) {

+	d := s.AddDaemon(c, true, true)

+	out, err := d.Cmd("network", "inspect", "-f", "{{.Id}}", "ingress")

+	c.Assert(err, checker.IsNil, check.Commentf("out: %v", out))

+	ingressID := strings.TrimSpace(out)

+	c.Assert(ingressID, checker.Not(checker.Equals), "")

+

+	// create a network of which name is the prefix of the ID of an overlay network

+	// (ingressID in this case)

+	newNetName := ingressID[0:2]

+	out, err = d.Cmd("network", "create", "--driver", "overlay", newNetName)

+	// In #27866, it was failing because of "network with name %s already exists"

+	c.Assert(err, checker.IsNil, check.Commentf("out: %v", out))

+	out, err = d.Cmd("network", "rm", newNetName)

+	c.Assert(err, checker.IsNil, check.Commentf("out: %v", out))

+}

+

+// Test case for https://github.com/docker/docker/pull/27938#issuecomment-265768303

+// This test creates two networks with the same name sequentially, with various drivers.

+// Since the operations in this test are done sequentially, the 2nd call should fail with

+// "network with name FOO already exists".

+// Note that it is to ok have multiple networks with the same name if the operations are done

+// in parallel. (#18864)

+func (s *DockerSwarmSuite) TestSwarmNetworkCreateDup(c *check.C) {

+	d := s.AddDaemon(c, true, true)

+	drivers := []string{"bridge", "overlay"}

+	for i, driver1 := range drivers {

+		nwName := fmt.Sprintf("network-test-%d", i)

+		for _, driver2 := range drivers {

+			c.Logf("Creating a network named %q with %q, then %q",

+				nwName, driver1, driver2)

+			out, err := d.Cmd("network", "create", "--driver", driver1, nwName)

+			c.Assert(err, checker.IsNil, check.Commentf("out: %v", out))

+			out, err = d.Cmd("network", "create", "--driver", driver2, nwName)

+			c.Assert(out, checker.Contains,

+				fmt.Sprintf("network with name %s already exists", nwName))

+			c.Assert(err, checker.NotNil)

+			c.Logf("As expected, the attempt to network %q with %q failed: %s",

+				nwName, driver2, out)

+			out, err = d.Cmd("network", "rm", nwName)

+			c.Assert(err, checker.IsNil, check.Commentf("out: %v", out))

+		}

+	}

+}