20318: Add comments about KeepClient/KeepGateway wiring.

[arvados.git] / sdk / go / arvados / client.go

// Copyright (C) The Arvados Authors. All rights reserved.
//
// SPDX-License-Identifier: Apache-2.0

package arvados

import (
        "bytes"
        "context"
        "crypto/rand"
        "crypto/tls"
        "encoding/json"
        "errors"
        "fmt"
        "io"
        "io/fs"
        "io/ioutil"
        "log"
        "math"
        "math/big"
        mathrand "math/rand"
        "net"
        "net/http"
        "net/url"
        "os"
        "regexp"
        "strconv"
        "strings"
        "sync"
        "sync/atomic"
        "time"

        "git.arvados.org/arvados.git/sdk/go/httpserver"
        "github.com/hashicorp/go-retryablehttp"
)

// A Client is an HTTP client with an API endpoint and a set of
// Arvados credentials.
//
// It offers methods for accessing individual Arvados APIs, and
// methods that implement common patterns like fetching multiple pages
// of results using List APIs.
type Client struct {
        // HTTP client used to make requests. If nil,
        // DefaultSecureClient or InsecureHTTPClient will be used.
        Client *http.Client `json:"-"`

        // Protocol scheme: "http", "https", or "" (https)
        Scheme string

        // Hostname (or host:port) of Arvados API server.
        APIHost string

        // User authentication token.
        AuthToken string

        // Accept unverified certificates. This works only if the
        // Client field is nil: otherwise, it has no effect.
        Insecure bool

        // Override keep service discovery with a list of base
        // URIs. (Currently there are no Client methods for
        // discovering keep services so this is just a convenience for
        // callers who use a Client to initialize an
        // arvadosclient.ArvadosClient.)
        KeepServiceURIs []string `json:",omitempty"`

        // HTTP headers to add/override in outgoing requests.
        SendHeader http.Header

        // Timeout for requests. NewClientFromConfig and
        // NewClientFromEnv return a Client with a default 5 minute
        // timeout. Within this time, retryable errors are
        // automatically retried with exponential backoff.
        //
        // To disable automatic retries, set Timeout to zero and use a
        // context deadline to establish a maximum request time.
        Timeout time.Duration

        // Maximum disk cache size in bytes or percent of total
        // filesystem size. If zero, use default, currently 10% of
        // filesystem size.
        DiskCacheSize ByteSizeOrPercent

        dd *DiscoveryDocument

        defaultRequestID string

        // APIHost and AuthToken were loaded from ARVADOS_* env vars
        // (used to customize "no host/token" error messages)
        loadedFromEnv bool

        // Track/limit concurrent outgoing API calls. Note this
        // differs from an outgoing connection limit (a feature
        // provided by http.Transport) when concurrent calls are
        // multiplexed on a single http2 connection.
        //
        // getRequestLimiter() should always be used, because this can
        // be nil.
        requestLimiter *requestLimiter

        last503 atomic.Value
}

// InsecureHTTPClient is the default http.Client used by a Client with
// Insecure==true and Client==nil.
var InsecureHTTPClient = &http.Client{
        Transport: &http.Transport{
                TLSClientConfig: &tls.Config{
                        InsecureSkipVerify: true}}}

// DefaultSecureClient is the default http.Client used by a Client otherwise.
var DefaultSecureClient = &http.Client{}

// NewClientFromConfig creates a new Client that uses the endpoints in
// the given cluster.
//
// AuthToken is left empty for the caller to populate.
func NewClientFromConfig(cluster *Cluster) (*Client, error) {
        ctrlURL := cluster.Services.Controller.ExternalURL
        if ctrlURL.Host == "" {
                return nil, fmt.Errorf("no host in config Services.Controller.ExternalURL: %v", ctrlURL)
        }
        var hc *http.Client
        if srvaddr := os.Getenv("ARVADOS_SERVER_ADDRESS"); srvaddr != "" {
                // When this client is used to make a request to
                // https://{ctrlhost}:port/ (any port), it dials the
                // indicated port on ARVADOS_SERVER_ADDRESS instead.
                //
                // This is invoked by arvados-server boot to ensure
                // that server->server traffic (e.g.,
                // keepproxy->controller) only hits local interfaces,
                // even if the Controller.ExternalURL host is a load
                // balancer / gateway and not a local interface
                // address (e.g., when running on a cloud VM).
                //
                // This avoids unnecessary delay/cost of routing
                // external traffic, and also allows controller to
                // recognize other services as internal clients based
                // on the connection source address.
                divertedHost := (*url.URL)(&cluster.Services.Controller.ExternalURL).Hostname()
                var dialer net.Dialer
                hc = &http.Client{
                        Transport: &http.Transport{
                                TLSClientConfig: &tls.Config{InsecureSkipVerify: cluster.TLS.Insecure},
                                DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
                                        host, port, err := net.SplitHostPort(addr)
                                        if err == nil && network == "tcp" && host == divertedHost {
                                                addr = net.JoinHostPort(srvaddr, port)
                                        }
                                        return dialer.DialContext(ctx, network, addr)
                                },
                        },
                }
        }
        return &Client{
                Client:         hc,
                Scheme:         ctrlURL.Scheme,
                APIHost:        ctrlURL.Host,
                Insecure:       cluster.TLS.Insecure,
                Timeout:        5 * time.Minute,
                DiskCacheSize:  cluster.Collections.WebDAVCache.DiskCacheSize,
                requestLimiter: &requestLimiter{maxlimit: int64(cluster.API.MaxConcurrentRequests / 4)},
        }, nil
}

// NewClientFromEnv creates a new Client that uses the default HTTP
// client, and loads API endpoint and credentials from ARVADOS_*
// environment variables (if set) and
// $HOME/.config/arvados/settings.conf (if readable).
//
// If a config exists in both locations, the environment variable is
// used.
//
// If there is an error (other than ENOENT) reading settings.conf,
// NewClientFromEnv logs the error to log.Default(), then proceeds as
// if settings.conf did not exist.
//
// Space characters are trimmed when reading the settings file, so
// these are equivalent:
//
//      ARVADOS_API_HOST=localhost\n
//      ARVADOS_API_HOST=localhost\r\n
//      ARVADOS_API_HOST = localhost \n
//      \tARVADOS_API_HOST = localhost\n
func NewClientFromEnv() *Client {
        vars := map[string]string{}
        home := os.Getenv("HOME")
        conffile := home + "/.config/arvados/settings.conf"
        if home == "" {
                // no $HOME => just use env vars
        } else if settings, err := os.ReadFile(conffile); errors.Is(err, fs.ErrNotExist) {
                // no config file => just use env vars
        } else if err != nil {
                // config file unreadable => log message, then use env vars
                log.Printf("continuing without loading %s: %s", conffile, err)
        } else {
                for _, line := range bytes.Split(settings, []byte{'\n'}) {
                        kv := bytes.SplitN(line, []byte{'='}, 2)
                        k := string(bytes.TrimSpace(kv[0]))
                        if len(kv) != 2 || !strings.HasPrefix(k, "ARVADOS_") {
                                // Same behavior as python sdk:
                                // silently skip leading # (comments),
                                // blank lines, typos, and non-Arvados
                                // vars.
                                continue
                        }
                        vars[k] = string(bytes.TrimSpace(kv[1]))
                }
        }
        for _, env := range os.Environ() {
                if !strings.HasPrefix(env, "ARVADOS_") {
                        continue
                }
                kv := strings.SplitN(env, "=", 2)
                if len(kv) == 2 {
                        vars[kv[0]] = kv[1]
                }
        }
        var svcs []string
        for _, s := range strings.Split(vars["ARVADOS_KEEP_SERVICES"], " ") {
                if s == "" {
                        continue
                } else if u, err := url.Parse(s); err != nil {
                        log.Printf("ARVADOS_KEEP_SERVICES: %q: %s", s, err)
                } else if !u.IsAbs() {
                        log.Printf("ARVADOS_KEEP_SERVICES: %q: not an absolute URI", s)
                } else {
                        svcs = append(svcs, s)
                }
        }
        var insecure bool
        if s := strings.ToLower(vars["ARVADOS_API_HOST_INSECURE"]); s == "1" || s == "yes" || s == "true" {
                insecure = true
        }
        return &Client{
                Scheme:          "https",
                APIHost:         vars["ARVADOS_API_HOST"],
                AuthToken:       vars["ARVADOS_API_TOKEN"],
                Insecure:        insecure,
                KeepServiceURIs: svcs,
                Timeout:         5 * time.Minute,
                loadedFromEnv:   true,
        }
}

var reqIDGen = httpserver.IDGenerator{Prefix: "req-"}

var nopCancelFunc context.CancelFunc = func() {}

var reqErrorRe = regexp.MustCompile(`net/http: invalid header `)

// Do augments (*http.Client)Do(): adds Authorization and X-Request-Id
// headers, delays in order to comply with rate-limiting restrictions,
// and retries failed requests when appropriate.
func (c *Client) Do(req *http.Request) (*http.Response, error) {
        ctx := req.Context()
        if auth, _ := ctx.Value(contextKeyAuthorization{}).(string); auth != "" {
                req.Header.Add("Authorization", auth)
        } else if c.AuthToken != "" {
                req.Header.Add("Authorization", "OAuth2 "+c.AuthToken)
        }

        if req.Header.Get("X-Request-Id") == "" {
                var reqid string
                if ctxreqid, _ := ctx.Value(contextKeyRequestID{}).(string); ctxreqid != "" {
                        reqid = ctxreqid
                } else if c.defaultRequestID != "" {
                        reqid = c.defaultRequestID
                } else {
                        reqid = reqIDGen.Next()
                }
                if req.Header == nil {
                        req.Header = http.Header{"X-Request-Id": {reqid}}
                } else {
                        req.Header.Set("X-Request-Id", reqid)
                }
        }

        rreq, err := retryablehttp.FromRequest(req)
        if err != nil {
                return nil, err
        }

        cancel := nopCancelFunc
        var lastResp *http.Response
        var lastRespBody io.ReadCloser
        var lastErr error
        var checkRetryCalled int

        rclient := retryablehttp.NewClient()
        rclient.HTTPClient = c.httpClient()
        rclient.Backoff = exponentialBackoff
        if c.Timeout > 0 {
                rclient.RetryWaitMax = c.Timeout / 10
                rclient.RetryMax = 32
                ctx, cancel = context.WithDeadline(ctx, time.Now().Add(c.Timeout))
                rreq = rreq.WithContext(ctx)
        } else {
                rclient.RetryMax = 0
        }
        rclient.CheckRetry = func(ctx context.Context, resp *http.Response, respErr error) (bool, error) {
                checkRetryCalled++
                if c.getRequestLimiter().Report(resp, respErr) {
                        c.last503.Store(time.Now())
                }
                if c.Timeout == 0 {
                        return false, nil
                }
                if respErr != nil && reqErrorRe.MatchString(respErr.Error()) {
                        return false, nil
                }
                retrying, err := retryablehttp.DefaultRetryPolicy(ctx, resp, respErr)
                if retrying {
                        lastResp, lastRespBody, lastErr = resp, nil, respErr
                        if respErr == nil {
                                // Save the response and body so we
                                // can return it instead of "deadline
                                // exceeded". retryablehttp.Client
                                // will drain and discard resp.body,
                                // so we need to stash it separately.
                                buf, err := ioutil.ReadAll(resp.Body)
                                if err == nil {
                                        lastRespBody = io.NopCloser(bytes.NewReader(buf))
                                } else {
                                        lastResp, lastErr = nil, err
                                }
                        }
                }
                return retrying, err
        }
        rclient.Logger = nil

        limiter := c.getRequestLimiter()
        limiter.Acquire(ctx)
        if ctx.Err() != nil {
                limiter.Release()
                cancel()
                return nil, ctx.Err()
        }
        resp, err := rclient.Do(rreq)
        if (errors.Is(err, context.DeadlineExceeded) || errors.Is(err, context.Canceled)) && (lastResp != nil || lastErr != nil) {
                resp = lastResp
                err = lastErr
                if checkRetryCalled > 0 && err != nil {
                        // Mimic retryablehttp's "giving up after X
                        // attempts" message, even if we gave up
                        // because of time rather than maxretries.
                        err = fmt.Errorf("%s %s giving up after %d attempt(s): %w", req.Method, req.URL.String(), checkRetryCalled, err)
                }
                if resp != nil {
                        resp.Body = lastRespBody
                }
        }
        if err != nil {
                limiter.Release()
                cancel()
                return nil, err
        }
        // We need to call cancel() eventually, but we can't use
        // "defer cancel()" because the context has to stay alive
        // until the caller has finished reading the response body.
        resp.Body = cancelOnClose{
                ReadCloser: resp.Body,
                cancel: func() {
                        limiter.Release()
                        cancel()
                },
        }
        return resp, err
}

// Last503 returns the time of the most recent HTTP 503 (Service
// Unavailable) response. Zero time indicates never.
func (c *Client) Last503() time.Time {
        t, _ := c.last503.Load().(time.Time)
        return t
}

// globalRequestLimiter entries (one for each APIHost) don't have a
// hard limit on outgoing connections, but do add a delay and reduce
// concurrency after 503 errors.
var (
        globalRequestLimiter     = map[string]*requestLimiter{}
        globalRequestLimiterLock sync.Mutex
)

// Get this client's requestLimiter, or a global requestLimiter
// singleton for c's APIHost if this client doesn't have its own.
func (c *Client) getRequestLimiter() *requestLimiter {
        if c.requestLimiter != nil {
                return c.requestLimiter
        }
        globalRequestLimiterLock.Lock()
        defer globalRequestLimiterLock.Unlock()
        limiter := globalRequestLimiter[c.APIHost]
        if limiter == nil {
                limiter = &requestLimiter{}
                globalRequestLimiter[c.APIHost] = limiter
        }
        return limiter
}

// cancelOnClose calls a provided CancelFunc when its wrapped
// ReadCloser's Close() method is called.
type cancelOnClose struct {
        io.ReadCloser
        cancel context.CancelFunc
}

func (coc cancelOnClose) Close() error {
        err := coc.ReadCloser.Close()
        coc.cancel()
        return err
}

func isRedirectStatus(code int) bool {
        switch code {
        case http.StatusMovedPermanently, http.StatusFound, http.StatusSeeOther, http.StatusTemporaryRedirect, http.StatusPermanentRedirect:
                return true
        default:
                return false
        }
}

const minExponentialBackoffBase = time.Second

// Implements retryablehttp.Backoff using the server-provided
// Retry-After header if available, otherwise nearly-full jitter
// exponential backoff (similar to
// https://aws.amazon.com/blogs/architecture/exponential-backoff-and-jitter/),
// in all cases respecting the provided min and max.
func exponentialBackoff(min, max time.Duration, attemptNum int, resp *http.Response) time.Duration {
        if attemptNum > 0 && min < minExponentialBackoffBase {
                min = minExponentialBackoffBase
        }
        var t time.Duration
        if resp != nil && (resp.StatusCode == http.StatusTooManyRequests || resp.StatusCode == http.StatusServiceUnavailable) {
                if s := resp.Header.Get("Retry-After"); s != "" {
                        if sleep, err := strconv.ParseInt(s, 10, 64); err == nil {
                                t = time.Second * time.Duration(sleep)
                        } else if stamp, err := time.Parse(time.RFC1123, s); err == nil {
                                t = stamp.Sub(time.Now())
                        }
                }
        }
        if t == 0 {
                jitter := mathrand.New(mathrand.NewSource(int64(time.Now().Nanosecond()))).Float64()
                t = min + time.Duration((math.Pow(2, float64(attemptNum))*float64(min)-float64(min))*jitter)
        }
        if t < min {
                return min
        } else if t > max {
                return max
        } else {
                return t
        }
}

// DoAndDecode performs req and unmarshals the response (which must be
// JSON) into dst. Use this instead of RequestAndDecode if you need
// more control of the http.Request object.
//
// If the response status indicates an HTTP redirect, the Location
// header value is unmarshalled to dst as a RedirectLocation
// key/field.
func (c *Client) DoAndDecode(dst interface{}, req *http.Request) error {
        resp, err := c.Do(req)
        if err != nil {
                return err
        }
        defer resp.Body.Close()
        buf, err := ioutil.ReadAll(resp.Body)
        if err != nil {
                return err
        }
        switch {
        case resp.StatusCode == http.StatusNoContent:
                return nil
        case resp.StatusCode == http.StatusOK && dst == nil:
                return nil
        case resp.StatusCode == http.StatusOK:
                return json.Unmarshal(buf, dst)

        // If the caller uses a client with a custom CheckRedirect
        // func, Do() might return the 3xx response instead of
        // following it.
        case isRedirectStatus(resp.StatusCode) && dst == nil:
                return nil
        case isRedirectStatus(resp.StatusCode):
                // Copy the redirect target URL to dst.RedirectLocation.
                buf, err := json.Marshal(map[string]string{"redirect_location": resp.Header.Get("Location")})
                if err != nil {
                        return err
                }
                return json.Unmarshal(buf, dst)

        default:
                return newTransactionError(req, resp, buf)
        }
}

// Convert an arbitrary struct to url.Values. For example,
//
//      Foo{Bar: []int{1,2,3}, Baz: "waz"}
//
// becomes
//
//      url.Values{`bar`:`{"a":[1,2,3]}`,`Baz`:`waz`}
//
// params itself is returned if it is already an url.Values.
func anythingToValues(params interface{}) (url.Values, error) {
        if v, ok := params.(url.Values); ok {
                return v, nil
        }
        // TODO: Do this more efficiently, possibly using
        // json.Decode/Encode, so the whole thing doesn't have to get
        // encoded, decoded, and re-encoded.
        j, err := json.Marshal(params)
        if err != nil {
                return nil, err
        }
        var generic map[string]interface{}
        dec := json.NewDecoder(bytes.NewBuffer(j))
        dec.UseNumber()
        err = dec.Decode(&generic)
        if err != nil {
                return nil, err
        }
        urlValues := url.Values{}
        for k, v := range generic {
                if v, ok := v.(string); ok {
                        urlValues.Set(k, v)
                        continue
                }
                if v, ok := v.(json.Number); ok {
                        urlValues.Set(k, v.String())
                        continue
                }
                if v, ok := v.(bool); ok {
                        if v {
                                urlValues.Set(k, "true")
                        } else {
                                // "foo=false", "foo=0", and "foo="
                                // are all taken as true strings, so
                                // don't send false values at all --
                                // rely on the default being false.
                        }
                        continue
                }
                j, err := json.Marshal(v)
                if err != nil {
                        return nil, err
                }
                if bytes.Equal(j, []byte("null")) {
                        // don't add it to urlValues at all
                        continue
                }
                urlValues.Set(k, string(j))
        }
        return urlValues, nil
}

// RequestAndDecode performs an API request and unmarshals the
// response (which must be JSON) into dst. Method and body arguments
// are the same as for http.NewRequest(). The given path is added to
// the server's scheme/host/port to form the request URL. The given
// params are passed via POST form or query string.
//
// path must not contain a query string.
func (c *Client) RequestAndDecode(dst interface{}, method, path string, body io.Reader, params interface{}) error {
        return c.RequestAndDecodeContext(context.Background(), dst, method, path, body, params)
}

// RequestAndDecodeContext does the same as RequestAndDecode, but with a context
func (c *Client) RequestAndDecodeContext(ctx context.Context, dst interface{}, method, path string, body io.Reader, params interface{}) error {
        if body, ok := body.(io.Closer); ok {
                // Ensure body is closed even if we error out early
                defer body.Close()
        }
        if c.APIHost == "" {
                if c.loadedFromEnv {
                        return errors.New("ARVADOS_API_HOST and/or ARVADOS_API_TOKEN environment variables are not set")
                }
                return errors.New("arvados.Client cannot perform request: APIHost is not set")
        }
        urlString := c.apiURL(path)
        urlValues, err := anythingToValues(params)
        if err != nil {
                return err
        }
        if dst == nil {
                if urlValues == nil {
                        urlValues = url.Values{}
                }
                urlValues["select"] = []string{`["uuid"]`}
        }
        if urlValues == nil {
                // Nothing to send
        } else if body != nil || ((method == "GET" || method == "HEAD") && len(urlValues.Encode()) < 1000) {
                // Send params in query part of URL
                u, err := url.Parse(urlString)
                if err != nil {
                        return err
                }
                u.RawQuery = urlValues.Encode()
                urlString = u.String()
        } else {
                body = strings.NewReader(urlValues.Encode())
        }
        req, err := http.NewRequest(method, urlString, body)
        if err != nil {
                return err
        }
        if (method == "GET" || method == "HEAD") && body != nil {
                req.Header.Set("X-Http-Method-Override", method)
                req.Method = "POST"
        }
        req = req.WithContext(ctx)
        req.Header.Set("Content-type", "application/x-www-form-urlencoded")
        for k, v := range c.SendHeader {
                req.Header[k] = v
        }
        return c.DoAndDecode(dst, req)
}

type resource interface {
        resourceName() string
}

// UpdateBody returns an io.Reader suitable for use as an http.Request
// Body for a create or update API call.
func (c *Client) UpdateBody(rsc resource) io.Reader {
        j, err := json.Marshal(rsc)
        if err != nil {
                // Return a reader that returns errors.
                r, w := io.Pipe()
                w.CloseWithError(err)
                return r
        }
        v := url.Values{rsc.resourceName(): {string(j)}}
        return bytes.NewBufferString(v.Encode())
}

// WithRequestID returns a new shallow copy of c that sends the given
// X-Request-Id value (instead of a new randomly generated one) with
// each subsequent request that doesn't provide its own via context or
// header.
func (c *Client) WithRequestID(reqid string) *Client {
        cc := *c
        cc.defaultRequestID = reqid
        return &cc
}

func (c *Client) httpClient() *http.Client {
        switch {
        case c.Client != nil:
                return c.Client
        case c.Insecure:
                return InsecureHTTPClient
        default:
                return DefaultSecureClient
        }
}

func (c *Client) apiURL(path string) string {
        scheme := c.Scheme
        if scheme == "" {
                scheme = "https"
        }
        // Double-slash in URLs tend to cause subtle hidden problems
        // (e.g., they can behave differently when a load balancer is
        // in the picture). Here we ensure exactly one "/" regardless
        // of whether the given APIHost or path has a superfluous one.
        return scheme + "://" + strings.TrimSuffix(c.APIHost, "/") + "/" + strings.TrimPrefix(path, "/")
}

// DiscoveryDocument is the Arvados server's description of itself.
type DiscoveryDocument struct {
        BasePath                     string              `json:"basePath"`
        DefaultCollectionReplication int                 `json:"defaultCollectionReplication"`
        BlobSignatureTTL             int64               `json:"blobSignatureTtl"`
        GitURL                       string              `json:"gitUrl"`
        Schemas                      map[string]Schema   `json:"schemas"`
        Resources                    map[string]Resource `json:"resources"`
        Revision                     string              `json:"revision"`
}

type Resource struct {
        Methods map[string]ResourceMethod `json:"methods"`
}

type ResourceMethod struct {
        HTTPMethod string         `json:"httpMethod"`
        Path       string         `json:"path"`
        Response   MethodResponse `json:"response"`
}

type MethodResponse struct {
        Ref string `json:"$ref"`
}

type Schema struct {
        UUIDPrefix string `json:"uuidPrefix"`
}

// DiscoveryDocument returns a *DiscoveryDocument. The returned object
// should not be modified: the same object may be returned by
// subsequent calls.
func (c *Client) DiscoveryDocument() (*DiscoveryDocument, error) {
        if c.dd != nil {
                return c.dd, nil
        }
        var dd DiscoveryDocument
        err := c.RequestAndDecode(&dd, "GET", "discovery/v1/apis/arvados/v1/rest", nil, nil)
        if err != nil {
                return nil, err
        }
        c.dd = &dd
        return c.dd, nil
}

var pdhRegexp = regexp.MustCompile(`^[0-9a-f]{32}\+\d+$`)

func (c *Client) modelForUUID(dd *DiscoveryDocument, uuid string) (string, error) {
        if pdhRegexp.MatchString(uuid) {
                return "Collection", nil
        }
        if len(uuid) != 27 {
                return "", fmt.Errorf("invalid UUID: %q", uuid)
        }
        infix := uuid[6:11]
        var model string
        for m, s := range dd.Schemas {
                if s.UUIDPrefix == infix {
                        model = m
                        break
                }
        }
        if model == "" {
                return "", fmt.Errorf("unrecognized type portion %q in UUID %q", infix, uuid)
        }
        return model, nil
}

func (c *Client) KindForUUID(uuid string) (string, error) {
        dd, err := c.DiscoveryDocument()
        if err != nil {
                return "", err
        }
        model, err := c.modelForUUID(dd, uuid)
        if err != nil {
                return "", err
        }
        return "arvados#" + strings.ToLower(model[:1]) + model[1:], nil
}

func (c *Client) PathForUUID(method, uuid string) (string, error) {
        dd, err := c.DiscoveryDocument()
        if err != nil {
                return "", err
        }
        model, err := c.modelForUUID(dd, uuid)
        if err != nil {
                return "", err
        }
        var resource string
        for r, rsc := range dd.Resources {
                if rsc.Methods["get"].Response.Ref == model {
                        resource = r
                        break
                }
        }
        if resource == "" {
                return "", fmt.Errorf("no resource for model: %q", model)
        }
        m, ok := dd.Resources[resource].Methods[method]
        if !ok {
                return "", fmt.Errorf("no method %q for resource %q", method, resource)
        }
        path := dd.BasePath + strings.Replace(m.Path, "{uuid}", uuid, -1)
        if path[0] == '/' {
                path = path[1:]
        }
        return path, nil
}

var maxUUIDInt = (&big.Int{}).Exp(big.NewInt(36), big.NewInt(15), nil)

func RandomUUID(clusterID, infix string) string {
        n, err := rand.Int(rand.Reader, maxUUIDInt)
        if err != nil {
                panic(err)
        }
        nstr := n.Text(36)
        for len(nstr) < 15 {
                nstr = "0" + nstr
        }
        return clusterID + "-" + infix + "-" + nstr
}