azureblob: add support for managed identities

Fixes #3213

backend/azureblob/imds.go

@@ -0,0 +1,137 @@
+// +build !plan9,!solaris,!js,go1.13
+
+package azureblob
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"net/http"
+
+	"github.com/Azure/go-autorest/autorest/adal"
+	"github.com/pkg/errors"
+	"github.com/rclone/rclone/fs"
+	"github.com/rclone/rclone/fs/fshttp"
+)
+
+const (
+	azureResource      = "https://storage.azure.com"
+	imdsAPIVersion     = "2018-02-01"
+	msiEndpointDefault = "http://169.254.169.254/metadata/identity/oauth2/token"
+)
+
+// This custom type is used to add the port the test server has bound to
+// to the request context.
+type testPortKey string
+
+type msiIdentifierType int
+
+const (
+	msiClientID msiIdentifierType = iota
+	msiObjectID
+	msiResourceID
+)
+
+type userMSI struct {
+	Type  msiIdentifierType
+	Value string
+}
+
+type httpError struct {
+	Response *http.Response
+}
+
+func (e httpError) Error() string {
+	return fmt.Sprintf("HTTP error %v (%v)", e.Response.StatusCode, e.Response.Status)
+}
+
+// GetMSIToken attempts to obtain an MSI token from the Azure Instance
+// Metadata Service.
+func GetMSIToken(ctx context.Context, identity *userMSI) (adal.Token, error) {
+	// Attempt to get an MSI token; silently continue if unsuccessful.
+	// This code has been lovingly stolen from azcopy's OAuthTokenManager.
+	result := adal.Token{}
+	req, err := http.NewRequestWithContext(ctx, "GET", msiEndpointDefault, nil)
+	if err != nil {
+		fs.Debugf(nil, "Failed to create request: %v", err)
+		return result, err
+	}
+	params := req.URL.Query()
+	params.Set("resource", azureResource)
+	params.Set("api-version", imdsAPIVersion)
+
+	// Specify user-assigned identity if requested.
+	if identity != nil {
+		switch identity.Type {
+		case msiClientID:
+			params.Set("client_id", identity.Value)
+		case msiObjectID:
+			params.Set("object_id", identity.Value)
+		case msiResourceID:
+			params.Set("mi_res_id", identity.Value)
+		default:
+			// If this happens, the calling function and this one don't agree on
+			// what valid ID types exist.
+			return result, fmt.Errorf("unknown MSI identity type specified")
+		}
+	}
+	req.URL.RawQuery = params.Encode()
+
+	// The Metadata header is required by all calls to IMDS.
+	req.Header.Set("Metadata", "true")
+
+	// If this function is run in a test, query the test server instead of IMDS.
+	testPort, isTest := ctx.Value(testPortKey("testPort")).(int)
+	if isTest {
+		req.URL.Host = fmt.Sprintf("localhost:%d", testPort)
+		req.Host = req.URL.Host
+	}
+
+	// Send request
+	httpClient := fshttp.NewClient(ctx)
+	resp, err := httpClient.Do(req)
+	if err != nil {
+		return result, errors.Wrap(err, "MSI is not enabled on this VM")
+	}
+	defer func() { // resp and Body should not be nil
+		_, err = io.Copy(ioutil.Discard, resp.Body)
+		if err != nil {
+			fs.Debugf(nil, "Unable to drain IMDS response: %v", err)
+		}
+		err = resp.Body.Close()
+		if err != nil {
+			fs.Debugf(nil, "Unable to close IMDS response: %v", err)
+		}
+	}()
+	// Check if the status code indicates success
+	// The request returns 200 currently, add 201 and 202 as well for possible extension.
+	switch resp.StatusCode {
+	case 200, 201, 202:
+		break
+	default:
+		body, _ := ioutil.ReadAll(resp.Body)
+		fs.Errorf(nil, "Couldn't obtain OAuth token from IMDS; server returned status code %d and body: %v", resp.StatusCode, string(body))
+		return result, httpError{Response: resp}
+	}
+
+	b, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		return result, errors.Wrap(err, "Couldn't read IMDS response")
+	}
+	// Remove BOM, if any. azcopy does this so I'm following along.
+	b = bytes.TrimPrefix(b, []byte("\xef\xbb\xbf"))
+
+	// This would be a good place to persist the token if a large number of rclone
+	// invocations are being made in a short amount of time. If the token is
+	// persisted, the azureblob code will need to check for expiry before every
+	// storage API call.
+	err = json.Unmarshal(b, &result)
+	if err != nil {
+		return result, errors.Wrap(err, "Couldn't unmarshal IMDS response")
+	}
+
+	return result, nil
+}