[azidentity] Allow Identity Binding mode to be set via configuration options

sdk/azidentity/default_azure_credential_test.go

@@ -357,8 +357,8 @@ func TestDefaultAzureCredential_WorkloadIdentity(t *testing.T) {
 		t.Setenv(azureTokenCredentials, credNameWorkloadIdentity)
 		// these values should trigger validation errors if WorkloadIdentityCredential
 		// tries to configure identity binding mode...
-		t.Setenv(customtokenproxy.AzureKubernetesCAData, "not a valid cert")
-		t.Setenv(customtokenproxy.AzureKubernetesTokenProxy, "http://timeout.local&fail=yes#please")
+		t.Setenv(customtokenproxy.EnvAzureKubernetesCAData, "not a valid cert")
+		t.Setenv(customtokenproxy.EnvAzureKubernetesTokenProxy, "http://timeout.local&fail=yes#please")
 
 		cred, err := NewDefaultAzureCredential(&DefaultAzureCredentialOptions{
 			ClientOptions: policy.ClientOptions{Transport: &mockSTS{}},

sdk/azidentity/internal/customtokenproxy/configuration.go

@@ -0,0 +1,121 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+package customtokenproxy
+
+import (
+	"errors"
+	"fmt"
+	"net/url"
+	"os"
+
+	"github.com/Azure/azure-sdk-for-go/sdk/azcore/policy"
+	"github.com/Azure/azure-sdk-for-go/sdk/azidentity/internal/exported"
+)
+
+const (
+	EnvAzureKubernetesCAData     = "AZURE_KUBERNETES_CA_DATA"
+	EnvAzureKubernetesCAFile     = "AZURE_KUBERNETES_CA_FILE"
+	EnvAzureKubernetesSNIName    = "AZURE_KUBERNETES_SNI_NAME"
+	EnvAzureKubernetesTokenProxy = "AZURE_KUBERNETES_TOKEN_PROXY"
+)
+
+func readOptionsFromEnv() *exported.CustomTokenProxyOptions {
+	return &exported.CustomTokenProxyOptions{
+		TokenProxy: os.Getenv(EnvAzureKubernetesTokenProxy),
+		SNIName:    os.Getenv(EnvAzureKubernetesSNIName),
+		CAFile:     os.Getenv(EnvAzureKubernetesCAFile),
+		CAData:     os.Getenv(EnvAzureKubernetesCAData),
+	}
+}
+
+func backfillOptionsFromEnv(opts *exported.CustomTokenProxyOptions) {
+	if opts.CAData != "" || opts.CAFile != "" || opts.SNIName != "" || opts.TokenProxy != "" {
+		return
+	}
+
+	// only backfill if all fields are empty
+	*opts = *readOptionsFromEnv()
+}
+
+func parseTokenProxyURL(endpoint string) (*url.URL, error) {
+	tokenProxy, err := url.Parse(endpoint)
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse custom token proxy URL %q: %s", endpoint, err)
+	}
+	if tokenProxy.Scheme != "https" {
+		return nil, fmt.Errorf("custom token endpoint must use https scheme, got %q", tokenProxy.Scheme)
+	}
+	if tokenProxy.User != nil {
+		return nil, fmt.Errorf("custom token endpoint URL %q must not contain user info", tokenProxy)
+	}
+	if tokenProxy.RawQuery != "" {
+		return nil, fmt.Errorf("custom token endpoint URL %q must not contain a query", tokenProxy)
+	}
+	if tokenProxy.EscapedFragment() != "" {
+		return nil, fmt.Errorf("custom token endpoint URL %q must not contain a fragment", tokenProxy)
+	}
+	if tokenProxy.EscapedPath() == "" {
+		// if the path is empty, set it to "/" to avoid stripping the path from req.URL
+		tokenProxy.Path = "/"
+	}
+	return tokenProxy, nil
+}
+
+var (
+	errCustomEndpointSetWithoutTokenProxy = errors.New(
+		"AZURE_KUBERNETES_TOKEN_PROXY is not set but other custom endpoint-related settings are present",
+	)
+	errCustomEndpointMultipleCASourcesSet = errors.New(
+		"only one of AzureKubernetesCAFile or AzureKubernetesCAData can be specified",
+	)
+)
+
+func noopConfigure(*policy.ClientOptions) {
+	// no-op
+}
+
+// GetClientOptionsConfigurer returns a function that configures the client options to use the custom token proxy.
+func GetClientOptionsConfigurer(opts *exported.CustomTokenProxyOptions) (func(*policy.ClientOptions), error) {
+	if opts == nil {
+		return noopConfigure, nil
+	}
+
+	backfillOptionsFromEnv(opts)
+
+	if opts.TokenProxy == "" {
+		// custom token proxy is not set, while other Kubernetes-related environment variables are present,
+		// this is likely a configuration issue so erroring out to avoid misconfiguration
+		if opts.SNIName != "" || opts.CAFile != "" || opts.CAData != "" {
+			return nil, errCustomEndpointSetWithoutTokenProxy
+		}
+
+		return noopConfigure, nil
+	}
+
+	tokenProxy, err := parseTokenProxyURL(opts.TokenProxy)
+	if err != nil {
+		return nil, err
+	}
+
+	// CAFile and CAData are mutually exclusive, at most one can be set.
+	// If none of CAFile or CAData are set, the default system CA pool will be used.
+	if opts.CAFile != "" && opts.CAData != "" {
+		return nil, errCustomEndpointMultipleCASourcesSet
+	}
+
+	// preload the transport
+	t := &transport{
+		caFile:     opts.CAFile,
+		caData:     []byte(opts.CAData),
+		sniName:    opts.SNIName,
+		tokenProxy: tokenProxy,
+	}
+	if _, err := t.getTokenTransporter(); err != nil {
+		return nil, err
+	}
+
+	return func(clientOptions *policy.ClientOptions) {
+		clientOptions.Transport = t
+	}, nil
+}