pkg/configfetcher/metadata_retriever.go

// Copyright 2021 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package configfetcher

// Implementation of the MetadataRetriever interface for running on an actual
// GCE instance. This implementation allows us to reduce the number of calls
// to metadata server, thus reducing the cpu utilization of device_policy_manager.
// If we were using go pkg for metadata server(https://pkg.go.dev/cloud.google.com/go/compute/metadata),
// we can use two functions: InstanceAttributes() and  InstanceAttributeValue().
// This would have resulted in multiple calls to metadata server.
// Current implementation allows us to call metadata server once,
// and then parse the raw metadata on our side, thus reducing the cpu
// utilization.

// API for querying the metadata server is specified here:
// https://cloud.google.com/compute/docs/metadata

import (
	"fmt"
	"io/ioutil"
	"net/http"
	"strings"
)

// Non-exported constants for querying the metadata server.
const (
	// Header to include in every request to the metadata server.
	requestHeaderKey   = "Metadata-Flavor"
	requestHeaderValue = "Google"

	// Note that there are 2 types of metadata entries: endpoint and
	// directory. Endpoint maps to a single value, while directory contains
	// a list of key value pairs.
	// URL path for metadata directory. Note that the trailing slash
	// indicates that the entry is a directory instead of an endpoint.
	// Without it the request will just be forwared by the metadata
	// server with a 301 status code.
	// The leading slash is required since these entries are relative to the
	// metadata server URL of the form http://.../...
	instanceCustomDataDirectory = "/instance/attributes/"
	projectCustomDataDirectory  = "/project/attributes/"
)

// getEntry performs a HTTP GET request for the metadata entry specified by
// its url path, which is relative to the metadata server's URL.
// It returns the body of the response if and only if the status code of the
// response is 200.
//
// Args:
//	entry: the path of the metadata entry, e.g. /instance/attributes/cos-update-strategy
//	metadataURL: the standard url to fetch metadata from.
// Returns:
//	resp: the response body of the HTTP GET request.
//	etag: "ETag" field of the response head.
//	err: error.
func getEntry(entry, metadataURL string) ([]byte, string, error) {
	url := metadataURL + entry

	// Construct HTTP client to make the request with the necessary headers.
	client := &http.Client{
		// Use default policy to stop after 10 consecutive requests.
		CheckRedirect: nil,
	}
	req, err := http.NewRequest("GET", url, nil)
	if err != nil {
		return nil, "", err
	}
	req.Header.Add(requestHeaderKey, requestHeaderValue)

	// Make the request and read the entire response.
	resp, err := client.Do(req)
	if resp != nil && resp.Body != nil {
		defer resp.Body.Close()
	}
	if err != nil {
		return nil, "", err
	}
	if resp.StatusCode != http.StatusOK {
		return nil, "", fmt.Errorf("request for %s returned status %d",
			entry, resp.StatusCode)
	}

	body, err := ioutil.ReadAll(resp.Body)

	return body, resp.Header.Get("etag"), err
}

// FetchMetadata fetches the content and the ETag of the instance custom
// metadata directory. If the parameter 'lastEtag' is not empty, FetchMetadata
// won't return until the directory is updated (compared to 'lastEtag').
// Otherwise, it returns immediately without waiting.
func FetchMetadata(lastEtag string, gceMetadataURL string) (
	string, string, error) {
	path := instanceCustomDataDirectory + "?recursive=true"
	if lastEtag != "" {
		path += "&wait_for_change=true&last_etag=" + lastEtag
	}

	resp, etag, err := getEntry(path, gceMetadataURL)
	if err != nil {
		return "", "", err
	}

	return strings.TrimSpace(string(resp)), etag, nil
}