gold-client/go/goldclient/goldclient.go

package goldclient

import (
	"bytes"
	"context"
	"crypto/md5"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"image"
	"image/png"
	"io/ioutil"
	"math"
	"net/url"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"time"

	"go.skia.org/infra/go/paramtools"
	"go.skia.org/infra/go/tiling"
	"go.skia.org/infra/gold-client/go/imgmatching"
	"go.skia.org/infra/golden/go/tracestore"
	"golang.org/x/sync/errgroup"

	"go.skia.org/infra/go/fileutil"
	"go.skia.org/infra/go/skerr"
	"go.skia.org/infra/go/sklog"
	"go.skia.org/infra/golden/go/diff"
	"go.skia.org/infra/golden/go/expectations"
	"go.skia.org/infra/golden/go/jsonio"
	"go.skia.org/infra/golden/go/types"
	"go.skia.org/infra/golden/go/web/frontend"
)

const (
	// stateFile is the name of the file that holds the state in the work directory
	// between calls
	stateFile = "result-state.json"

	// jsonTempFile is the temporary file that is created to upload results via gsutil.
	jsonTempFile = "dm.json"

	// digestsDirectory is the directory inside the work directory in which digests downloaded from
	// GCS will be cached.
	digestsDirectory = "digests"
)

// GoldClient is the uniform interface to communicate with the Gold service.
type GoldClient interface {
	// SetSharedConfig populates the config with details that will be shared
	// with all tests. This is safe to be called more than once, although
	// new settings will overwrite the old ones. This will cause the
	// baseline and known hashes to be (re-)downloaded from Gold.
	SetSharedConfig(sharedConfig jsonio.GoldResults, skipValidation bool) error

	// Test adds a test result to the current test run.
	//
	// The provided image will be uploaded to GCS if the hash of its pixels has not been seen before.
	// Using auth.SetDryRun(true) can prevent this.
	//
	// additionalKeys is an optional set of key/value pairs that apply to only this test. This is
	// typically a small amount of data (and can be nil). If there are many keys, they are likely
	// shared between tests and should be added in SetSharedConfig.
	//
	// optionalKeys can be used to specify a non-exact image matching algorithm and its parameters,
	// and any other optional keys specific to this test. This is optional and can be nil.
	// TODO(lovisolo): Explicitly mention key used to specify a non-exact image matching algorithm.
	//
	// If the GoldClient is configured for pass/fail mode, a JSON file will be uploaded to GCS with
	// the test results, and the returned boolean will indicate whether the test passed the
	// comparison against the baseline using the specified non-exact image matching algorithm, or
	// exact matching if none is specified.
	//
	// If the GoldClient is *not* configured for pass/fail mode, no JSON will be uploaded, and Test
	// will return true.
	//
	// An error is only returned if there was a technical problem in processing the test.
	Test(name types.TestName, imgFileName string, additionalKeys, optionalKeys map[string]string) (bool, error)

	// Check operates similarly to Test, except it does not persist anything about the call. That is,
	// the image will not be uploaded to Gold, only compared against the baseline.
	//
	// Argument optionalKeys is used to specify a non-exact image matching algorithm and its
	// parameters. TODO(lovisolo): Explicitly mention the optional key used for this.
	//
	// Argument keys is required if a non-exact image matching algorithm is specified. The test keys
	// are used to compute the ID of the trace from which to retrieve the most recent positive image
	// to be used as the basis of the non-exact image comparison.
	//
	// If both keys and optionalKeys are empty, an exact comparison will be carried out against the
	// baseline.
	Check(name types.TestName, imgFileName string, keys, optionalKeys map[string]string) (bool, error)

	// Diff computes a diff of the closest image to the given image file and puts it into outDir,
	// along with the closest image file itself.
	Diff(ctx context.Context, name types.TestName, corpus, imgFileName, outDir string) error

	// Finalize uploads the JSON file for all Test() calls previously seen.
	// A no-op if configured for pass/fail mode, since the JSON would have been uploaded
	// on the calls to Test().
	Finalize() error

	// Whoami makes a request to Gold's /json/whoami endpoint and returns the email address in the
	// response. For debugging purposes only.
	Whoami() (string, error)

	// TriageAsPositive triages the given digest for the given test and optional changelist ID as
	// positive by making a request to Gold's /json/triage endpoint.
	TriageAsPositive(testName types.TestName, digest types.Digest, changeListId string) error

	// MostRecentPositiveDigest retrieves the most recent positive digest for the given trace via
	// Gold's /json/latestpositivedigest/{traceId} endpoint.
	MostRecentPositiveDigest(traceId tiling.TraceID) (types.Digest, error)
}

// GoldClientDebug contains some "optional" methods that can assist
// in debugging.
type GoldClientDebug interface {
	// DumpBaseline returns a human-readable representation of the baseline as a string.
	// This is a set of test names that each have a set of image
	// digests that each have exactly one types.Label.
	DumpBaseline() (string, error)
	// DumpKnownHashes returns a human-readable representation of the known image digests
	// which is a list of hashes.
	DumpKnownHashes() (string, error)
}

// CloudClient implements the GoldClient interface for the remote Gold service.
type CloudClient struct {
	// workDir is a temporary directory that has to exist between related calls
	workDir string

	// resultState keeps track of the all the information to generate and upload a valid result.
	resultState *resultState

	// ready caches the result of the isReady call so we avoid duplicate work.
	ready bool

	// these functions are overwritable by tests
	loadAndHashImage func(path string) ([]byte, types.Digest, error)
	now              func() time.Time

	// auth stores the authentication method to use.
	auth       AuthOpt
	httpClient HTTPClient
}

// GoldClientConfig is a config structure to configure GoldClient instances
type GoldClientConfig struct {
	// WorkDir is a temporary directory that caches data for one run with multiple calls to GoldClient
	WorkDir string

	// InstanceID is the id of the backend Gold instance
	InstanceID string

	// PassFailStep indicates whether each call to Test(...) should return a pass/fail value.
	PassFailStep bool

	// FailureFile is a file on disk that will contain newline-separated links to triage
	// any failures. Only written to if PassFailStep is true
	FailureFile string

	// OverrideGoldURL is optional and allows to override the GoldURL for testing.
	OverrideGoldURL string

	// UploadOnly is a mode where we don't check expectations against the server - i.e.
	// we just operate in upload mode.
	UploadOnly bool
}

// NewCloudClient returns an implementation of the GoldClient that relies on the Gold service.
// If a new instance is created for each call to Test, the arguments of the first call are
// preserved. They are cached in a JSON file in the work directory.
func NewCloudClient(authOpt AuthOpt, config GoldClientConfig) (*CloudClient, error) {
	// Make sure the workdir was given and exists.
	if config.WorkDir == "" {
		return nil, skerr.Fmt("no 'workDir' provided to NewCloudClient")
	}

	workDir, err := fileutil.EnsureDirExists(config.WorkDir)
	if err != nil {
		return nil, skerr.Wrapf(err, "setting up workdir %q", config.WorkDir)
	}

	if config.InstanceID == "" {
		return nil, skerr.Fmt("empty config passed into NewCloudClient")
	}

	ret := CloudClient{
		workDir:          workDir,
		auth:             authOpt,
		loadAndHashImage: loadAndHashImage,
		now:              defaultNow,

		resultState: newResultState(nil, &config),
	}
	if err := ret.setHttpClient(); err != nil {
		return nil, skerr.Wrapf(err, "setting http client")
	}

	if config.FailureFile != "" {
		if f, err := os.Create(config.FailureFile); err != nil {
			return nil, skerr.Wrapf(err, "making failure file %s", config.FailureFile)
		} else {
			if err := f.Close(); err != nil {
				return nil, skerr.Wrapf(err, "closing failure file %s", config.FailureFile)
			}
		}
	}

	// write it to disk
	if err := saveJSONFile(ret.getResultStatePath(), ret.resultState); err != nil {
		return nil, skerr.Wrapf(err, "writing the state to disk")
	}

	return &ret, nil
}

// LoadCloudClient returns a GoldClient that has previously been stored to disk
// in the path given by workDir.
func LoadCloudClient(authOpt AuthOpt, workDir string) (*CloudClient, error) {
	// Make sure the workdir was given and exists.
	if workDir == "" {
		return nil, skerr.Fmt("No 'workDir' provided to LoadCloudClient")
	}
	ret := CloudClient{
		workDir:          workDir,
		auth:             authOpt,
		loadAndHashImage: loadAndHashImage,
		now:              defaultNow,
	}
	var err error
	ret.resultState, err = loadStateFromJSON(ret.getResultStatePath())
	if err != nil {
		return nil, skerr.Wrapf(err, "loading state from disk")
	}
	if err = ret.setHttpClient(); err != nil {
		return nil, skerr.Wrapf(err, "setting http client")
	}

	return &ret, nil
}

// loadAndHashImage loads an image from disk and hashes the internal Pixel buffer. It returns
// the bytes of the encoded image and the MD5 hash of the pixels as hex encoded string.
func loadAndHashImage(fileName string) ([]byte, types.Digest, error) {
	// Load the image and save the bytes because we need to return them.
	imgBytes, err := ioutil.ReadFile(fileName)
	if err != nil {
		return nil, "", skerr.Wrapf(err, "loading file %s", fileName)
	}
	img, err := png.Decode(bytes.NewReader(imgBytes))
	if err != nil {
		return nil, "", skerr.Wrapf(err, "decoding PNG in file %s", fileName)
	}
	nrgbaImg := diff.GetNRGBA(img)
	// hash it
	s := md5.Sum(nrgbaImg.Pix)
	md5Hash := hex.EncodeToString(s[:])
	return imgBytes, types.Digest(md5Hash), nil
}

// defaultNow returns what time it is now in UTC
func defaultNow() time.Time {
	return time.Now().UTC()
}

// SetSharedConfig implements the GoldClient interface.
func (c *CloudClient) SetSharedConfig(sharedConfig jsonio.GoldResults, skipValidation bool) error {
	if !skipValidation {
		if err := sharedConfig.Validate(true); err != nil {
			return skerr.Wrapf(err, "invalid configuration")
		}
	}

	existingConfig := GoldClientConfig{
		WorkDir: c.workDir,
	}
	if c.resultState != nil {
		existingConfig.InstanceID = c.resultState.InstanceID
		existingConfig.PassFailStep = c.resultState.PerTestPassFail
		existingConfig.FailureFile = c.resultState.FailureFile
		existingConfig.OverrideGoldURL = c.resultState.GoldURL
		existingConfig.UploadOnly = c.resultState.UploadOnly
	}
	c.resultState = newResultState(&sharedConfig, &existingConfig)

	if !c.resultState.UploadOnly {
		// The GitHash may have changed (or been set for the first time),
		// So we can now load the baseline. We can also download the hashes
		// at this time, although we could have done it at any time before since
		// that does not depend on the GitHash we have.
		if err := c.downloadHashesAndBaselineFromGold(); err != nil {
			return skerr.Wrapf(err, "downloading from Gold")
		}
	}

	return saveJSONFile(c.getResultStatePath(), c.resultState)
}

// Test implements the GoldClient interface.
func (c *CloudClient) Test(name types.TestName, imgFileName string, additionalKeys, optionalKeys map[string]string) (bool, error) {
	if res, err := c.addTest(name, imgFileName, additionalKeys, optionalKeys); err != nil {
		return false, err
	} else {
		return res, saveJSONFile(c.getResultStatePath(), c.resultState)
	}
}

// addTest adds a test to results. If perTestPassFail is true it will also upload the result.
// Returns true if the test was added (and maybe uploaded) successfully.
func (c *CloudClient) addTest(name types.TestName, imgFileName string, additionalKeys, optionalKeys map[string]string) (bool, error) {
	if err := c.isReady(); err != nil {
		return false, skerr.Wrapf(err, "gold client not ready")
	}

	// Get an uploader. This is either based on an authenticated client or on gsutils.
	uploader, err := c.auth.GetGCSUploader()
	if err != nil {
		return false, skerr.Wrapf(err, "retrieving uploader")
	}

	// Load the PNG from disk and hash it.
	imgBytes, imgHash, err := c.loadAndHashImage(imgFileName)
	if err != nil {
		return false, skerr.Wrap(err)
	}

	// Add the result of this test.
	traceId := c.addResult(name, imgHash, additionalKeys, optionalKeys)

	// At this point the result should be correct for uploading.
	if err := c.resultState.SharedConfig.Validate(false); err != nil {
		return false, skerr.Wrapf(err, "invalid test config")
	}

	fmt.Printf("Given image with hash %s for test %s\n", imgHash, name)
	for expectHash, expectLabel := range c.resultState.Expectations[name] {
		fmt.Printf("Expectation for test: %s (%s)\n", expectHash, expectLabel.String())
	}

	var egroup errgroup.Group
	// Check against known hashes and upload if needed.
	if !c.resultState.KnownHashes[imgHash] {
		egroup.Go(func() error {
			gcsImagePath := c.resultState.getGCSImagePath(imgHash)
			if err := uploader.UploadBytes(context.TODO(), imgBytes, imgFileName, gcsImagePath); err != nil {
				return skerr.Fmt("Error uploading image %s to %s. Got: %s", imgFileName, gcsImagePath, err)
			}
			return nil
		})
	}

	// If we do per test pass/fail then upload the result and compare it to the baseline.
	ret := true
	if c.resultState.PerTestPassFail {
		egroup.Go(func() error {
			return c.uploadResultJSON(uploader)
		})

		egroup.Go(func() error {
			ret, err = c.matchImageAgainstBaseline(name, traceId, imgBytes, imgHash, optionalKeys)
			if err != nil {
				return skerr.Wrapf(err, "matching image against baseline")
			}

			if !ret {
				link := fmt.Sprintf("%s/detail?test=%s&digest=%s", c.resultState.GoldURL, name, imgHash)
				if c.resultState.SharedConfig.ChangeListID != "" {
					link += "&issue=" + c.resultState.SharedConfig.ChangeListID
				}
				link += "\n"
				fmt.Printf("Untriaged or negative image: %s", link)
				ff := c.resultState.FailureFile
				if ff != "" {
					f, err := os.OpenFile(ff, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
					if err != nil {
						return skerr.Fmt("could not open failure file %s: %s", ff, err)
					}
					if _, err := f.WriteString(link); err != nil {
						return skerr.Fmt("could not write to failure file %s: %s", ff, err)
					}
					if err := f.Close(); err != nil {
						return skerr.Fmt("could not close failure file %s: %s", ff, err)
					}
				}
			}

			return nil
		})
	}

	if err := egroup.Wait(); err != nil {
		return false, err
	}
	return ret, nil
}

// Check implements the GoldClient interface.
func (c *CloudClient) Check(name types.TestName, imgFileName string, keys, optionalKeys map[string]string) (bool, error) {
	if len(c.resultState.Expectations) == 0 {
		if err := c.downloadHashesAndBaselineFromGold(); err != nil {
			return false, skerr.Wrapf(err, "fetching baseline")
		}
		if err := saveJSONFile(c.getResultStatePath(), c.resultState); err != nil {
			return false, skerr.Wrapf(err, "writing the expectations to disk")
		}
	}

	// Load the PNG from disk and hash it.
	imgBytes, imgHash, err := c.loadAndHashImage(imgFileName)
	if err != nil {
		return false, skerr.Wrap(err)
	}
	fmt.Printf("Given image with hash %s for test %s\n", imgHash, name)
	for expectHash, expectLabel := range c.resultState.Expectations[name] {
		fmt.Printf("Expectation for test: %s (%s)\n", expectHash, expectLabel.String())
	}

	return c.matchImageAgainstBaseline(name, tracestore.TraceIDFromParams(keys), imgBytes, imgHash, optionalKeys)
}

// matchImageAgainstBaseline matches the given image against the baseline. A non-exact image
// matching algorithm will be used if one is specified via the optionalKeys; otherwise exact
// matching will be used (i.e. the image's MD5 hash will be matched against the hashes of the
// baseline images labeled as positive).
//
// It assumes that the baseline has already been downloaded from Gold.
//
// Returns true if the image matches the baseline, or false otherwise. A non-nil error is returned
// if there are any problems parsing or instantiating the specified image matching algorithm.
func (c *CloudClient) matchImageAgainstBaseline(testName types.TestName, traceId tiling.TraceID, imageBytes []byte, imageHash types.Digest, optionalKeys map[string]string) (bool, error) {
	// First we check whether the digest is a known positive or negative, regardless of the specified
	// image matching algorithm.
	if c.resultState.Expectations[testName][imageHash] == expectations.Positive {
		return true, nil
	}
	if c.resultState.Expectations[testName][imageHash] == expectations.Negative {
		return false, nil
	}

	// Extract the specified image matching algorithm from the optionalKeys (defaulting to exact
	// matching if none is specified) and obtain an instance of the imgmatching.Matcher if the
	// algorithm requires one (i.e. all but exact matching).
	algorithmName, matcher, err := imgmatching.MakeMatcher(optionalKeys)
	if err != nil {
		return false, skerr.Wrapf(err, "parsing image matching algorithm from optional keys")
	}

	// Nothing else to do if performing exact matching: we've already checked whether the image is a
	// known positive.
	if algorithmName == imgmatching.ExactMatching {
		return false, nil
	}

	// TODO(lovisolo): Remove once SobelFuzzyMatching is implemented.
	if algorithmName == imgmatching.SobelFuzzyMatching {
		return false, skerr.Fmt("image matching algorithm %q not yet supported", imgmatching.SobelFuzzyMatching)
	}

	// Decode test output PNG image.
	image, err := png.Decode(bytes.NewReader(imageBytes))
	if err != nil {
		return false, skerr.Wrapf(err, "decoding PNG image")
	}

	// Fetch the most recent positive digest.
	mostRecentPositiveDigest, err := c.MostRecentPositiveDigest(traceId)
	if err != nil {
		return false, skerr.Wrapf(err, "retrieving most recent positive image")
	}

	// Download from GCS the image corresponding to the most recent positive digest.
	mostRecentPositiveImage, _, err := c.getDigestFromCacheOrGCS(context.TODO(), mostRecentPositiveDigest)
	if err != nil {
		return false, skerr.Wrapf(err, "downloading most recent positive image from GCS")
	}

	// Return algorithm's output.
	return matcher.Match(mostRecentPositiveImage, image), nil
}

// Finalize implements the GoldClient interface.
func (c *CloudClient) Finalize() error {
	if err := c.isReady(); err != nil {
		return skerr.Fmt("Cannot finalize - client not ready: %s", err)
	}
	uploader, err := c.auth.GetGCSUploader()
	if err != nil {
		return skerr.Fmt("Error retrieving uploader: %s", err)
	}
	return c.uploadResultJSON(uploader)
}

// uploadResultJSON uploads the results (which live in SharedConfig, specifically
// SharedConfig.Results), to GCS.
func (c *CloudClient) uploadResultJSON(uploader GCSUploader) error {
	localFileName := filepath.Join(c.workDir, jsonTempFile)
	resultFilePath := c.resultState.getResultFilePath(c.now())
	if err := uploader.UploadJSON(context.TODO(), c.resultState.SharedConfig, localFileName, resultFilePath); err != nil {
		return skerr.Fmt("Error uploading JSON file to GCS path %s: %s", resultFilePath, err)
	}
	return nil
}

// setHttpClient sets authenticated httpClient, if authentication was configured via SetAuthConfig.
// It also retrieves a token of the configured source to make sure it works.
func (c *CloudClient) setHttpClient() error {
	// If no auth option was set, we return an unauthenticated client.
	client, err := c.auth.GetHTTPClient()
	if err != nil {
		return err
	}
	c.httpClient = client
	return nil
}

// saveAuthOpt assumes that auth has been set. It saves it to the work directory for retrieval
// during later calls.
func (c *CloudClient) saveAuthOpt() error {
	outFile := filepath.Join(c.workDir, authFile)
	return saveJSONFile(outFile, c.auth)
}

// isReady returns true if the instance is ready to accept test results (all necessary info has been
// configured)
func (c *CloudClient) isReady() error {
	if c.ready {
		return nil
	}

	// if resultState hasn't been set yet, then we are simply not ready.
	if c.resultState == nil {
		return skerr.Fmt("No result state object available")
	}

	// Check whether we have some means of uploading results
	if c.auth == nil {
		return skerr.Fmt("No authentication information provided.")
	}
	if err := c.auth.Validate(); err != nil {
		return skerr.Fmt("Invalid auth: %s", err)
	}

	// Check if the GoldResults instance is complete once results are added.
	if err := c.resultState.SharedConfig.Validate(true); err != nil {
		return skerr.Wrapf(err, "Gold results fields invalid")
	}

	c.ready = true
	return nil
}

// getResultStatePath returns the path of the temporary file where the state is cached as JSON
func (c *CloudClient) getResultStatePath() string {
	return filepath.Join(c.workDir, stateFile)
}

// addResult adds the given test to the overall results and returns the ID of the affected trace.
func (c *CloudClient) addResult(name types.TestName, imgHash types.Digest, additionalKeys, optionalKeys map[string]string) tiling.TraceID {
	newResult := &jsonio.Result{
		Digest: imgHash,
		Key:    map[string]string{types.PrimaryKeyField: string(name)},

		// We need to specify this is a png, otherwise the backend will refuse
		// to ingest it.
		Options: map[string]string{"ext": "png"},
	}
	for k, v := range additionalKeys {
		newResult.Key[k] = v
	}
	for k, v := range optionalKeys {
		newResult.Options[k] = v
	}

	// Set the CorpusField (e.g. source_type) to the default value of the instanceID
	// if it is not set either on Key (via init) or additionalKeys (via add)
	if c.resultState.SharedConfig.Key[types.CorpusField] == "" && newResult.Key[types.CorpusField] == "" {
		newResult.Key[types.CorpusField] = c.resultState.InstanceID
	}
	c.resultState.SharedConfig.Results = append(c.resultState.SharedConfig.Results, newResult)

	// Compute trace ID from shared and test-specific keys. The latter overwrites the former in the
	// rare case of a conflict.
	traceParams := paramtools.Params{}
	traceParams.Add(c.resultState.SharedConfig.Key, newResult.Key)
	return tracestore.TraceIDFromParams(traceParams)
}

// downloadHashesAndBaselineFromGold downloads the hashes and baselines
// and stores them to resultState.
func (c *CloudClient) downloadHashesAndBaselineFromGold() error {
	// What hashes have we seen already (to avoid uploading them again).
	if err := c.resultState.loadKnownHashes(c.httpClient); err != nil {
		return err
	}

	fmt.Printf("Loaded %d known hashes\n", len(c.resultState.KnownHashes))

	// Fetch the baseline (may be empty but should not fail).
	if err := c.resultState.loadExpectations(c.httpClient); err != nil {
		return err
	}
	fmt.Printf("Loaded %d tests from the baseline\n", len(c.resultState.Expectations))

	return nil
}

// Diff fulfills the GoldClient interface.
func (c *CloudClient) Diff(ctx context.Context, name types.TestName, corpus, imgFileName, outDir string) error {
	if err := os.MkdirAll(outDir, os.ModePerm); err != nil {
		return skerr.Wrapf(err, "creating outdir %s", outDir)
	}

	// 1) Read in file, write it to outdir/ using correct hash.
	b, inputDigest, err := c.loadAndHashImage(imgFileName)
	if err != nil {
		return skerr.Wrapf(err, "reading input %s", imgFileName)
	}

	origFilePath := filepath.Join(outDir, fmt.Sprintf("input-%s.png", inputDigest))
	if err := ioutil.WriteFile(origFilePath, b, 0644); err != nil {
		return skerr.Wrapf(err, "writing to %s", origFilePath)
	}

	leftImg, err := png.Decode(bytes.NewReader(b))
	if err != nil {
		return skerr.Wrapf(err, "reading %s as png", origFilePath)
	}

	// 2) Check JSON endpoint digests to download
	u := fmt.Sprintf("%s/json/digests?test=%s&corpus=%s", c.resultState.GoldURL, url.QueryEscape(string(name)), url.QueryEscape(corpus))
	jb, err := getWithRetries(c.httpClient, u)
	if err != nil {
		return skerr.Wrapf(err, "reading images for test %s, corpus %s from gold (url: %s)", name, corpus, u)
	}

	var dlr frontend.DigestListResponse
	if err := json.Unmarshal(jb, &dlr); err != nil {
		return skerr.Wrapf(err, "invalid JSON from digests served from %s: %s", u, string(jb))
	}
	if len(dlr.Digests) == 0 {
		sklog.Warningf("Gold doesn't know of any digests that match %s and corpus %s", name, corpus)
		return nil
	}

	sklog.Infof("Going to compare %s.png against %d other images", inputDigest, len(dlr.Digests))

	// 3a) Download those from bucket (or use from working directory cache). We download them with
	//    the same credentials that let us upload them.
	smallestCombined := float32(math.MaxFloat32)
	var closestRightDigest types.Digest

	// we have to keep the raw bytes (i.e. we cannot simply re-encoded closestDiffImg to disk)
	// because golang does not support fancy image things like color spaces, so re-encoding it
	// would lose that data.
	var closestRightImg []byte
	var closestDiffImg image.Image
	for _, d := range dlr.Digests {
		rightImg, db, err := c.getDigestFromCacheOrGCS(ctx, d)
		if err != nil {
			return skerr.Wrap(err)
		}

		// 3b) Compare each of the images to the given image, looking for the smallest combined
		//     diff metric.
		dm, diffImg := diff.PixelDiff(leftImg, rightImg)
		cdm := diff.CombinedDiffMetric(dm, nil, nil)
		if cdm < smallestCombined {
			smallestCombined = cdm
			closestDiffImg = diffImg
			closestRightImg = db
			closestRightDigest = d
		}
	}
	sklog.Infof("Digest %s was closest (combined metric of %f)", closestRightDigest, smallestCombined)

	// 4) Write closest image and the diff to that image to the output directory.
	o := filepath.Join(outDir, fmt.Sprintf("closest-%s.png", closestRightDigest))
	if err := ioutil.WriteFile(o, closestRightImg, 0644); err != nil {
		return skerr.Wrapf(err, "writing closest image to %s", o)
	}

	o = filepath.Join(outDir, "diff.png")
	diffFile, err := os.Create(o)
	if err != nil {
		return skerr.Wrapf(err, "opening %s for writing", o)
	}
	if err := png.Encode(diffFile, closestDiffImg); err != nil {
		return skerr.Wrapf(err, "encoding diff image")
	}

	return skerr.Wrap(diffFile.Close())
}

// getDigestFromCacheOrGCS downloads from GCS the PNG file corresponding to the given digest, and
// returns the decoded image.Image and raw PNG file as a byte slice.
//
// The downloaded image is cached on disk. Subsequent calls for the same digest will load the
// cached image from disk.
func (c *CloudClient) getDigestFromCacheOrGCS(ctx context.Context, digest types.Digest) (image.Image, []byte, error) {
	// Make sure the local cache directory exists.
	cachePath := filepath.Join(c.workDir, digestsDirectory)
	if err := os.MkdirAll(cachePath, os.ModePerm); err != nil {
		return nil, nil, skerr.Wrapf(err, "creating digests directory %s", cachePath)
	}

	// Path where the digest should be cached.
	digestPath := filepath.Join(cachePath, string(digest)+".png")

	// Try to read digest from the local cache.
	digestBytes, err := ioutil.ReadFile(digestPath)
	if err != nil && !os.IsNotExist(err) {
		return nil, nil, skerr.Wrapf(err, "reading file %s", digestPath)
	}

	// Download and cache digest if it wasn't found on the cache.
	if os.IsNotExist(err) {
		downloader, err := c.auth.GetGCSDownloader()
		if err != nil {
			return nil, nil, skerr.Wrap(err)
		}

		// Download digest.
		digestGcsPath := c.resultState.getGCSImagePath(digest)
		digestBytes, err = downloader.Download(ctx, digestGcsPath, cachePath)
		if err != nil {
			return nil, nil, skerr.Wrapf(err, "downloading %s", digestGcsPath)
		}

		// Cache digest.
		if err := ioutil.WriteFile(digestPath, digestBytes, 0644); err != nil {
			return nil, nil, skerr.Wrapf(err, "caching to %s", digestPath)
		}
	}

	// Decode PNG file.
	image, err := png.Decode(bytes.NewReader(digestBytes))
	if err != nil {
		return nil, nil, skerr.Wrapf(err, "decoding PNG file at %s", digestPath)
	}

	return image, digestBytes, nil
}

// Whoami fulfills the GoldClient interface.
func (c *CloudClient) Whoami() (string, error) {
	jsonBytes, err := getWithRetries(c.httpClient, c.resultState.GoldURL+"/json/whoami")
	if err != nil {
		return "", skerr.Wrapf(err, "making request to %s/json/whoami", c.resultState.GoldURL)
	}

	whoami := map[string]string{}
	if err := json.Unmarshal(jsonBytes, &whoami); err != nil {
		return "", skerr.Wrapf(err, "parsing JSON response from %s/json/whoami", c.resultState.GoldURL)
	}

	email, ok := whoami["whoami"]
	if !ok {
		return "", skerr.Wrapf(err, `JSON response from %s/json/whoami does not contain key "whoami"`, c.resultState.GoldURL)
	}

	return email, nil
}

// TriageAsPositive fulfills the GoldClient interface.
func (c *CloudClient) TriageAsPositive(testName types.TestName, digest types.Digest, changeListId string) error {
	// Build TriageRequest struct and encode it into JSON.
	triageRequest := &frontend.TriageRequest{
		TestDigestStatus: map[types.TestName]map[types.Digest]string{testName: {digest: expectations.Positive.String()}},
		ChangeListID:     changeListId,
	}
	jsonTriageRequest, err := json.Marshal(triageRequest)
	if err != nil {
		return skerr.Wrapf(err, `encoding frontend.TriageRequest into JSON for test %q, digest %q and issue %q`, testName, digest, changeListId)
	}

	// Make /json/triage request. Response is always empty.
	_, err = post(c.httpClient, c.resultState.GoldURL+"/json/triage", "application/json", bytes.NewReader(jsonTriageRequest))
	if err != nil {
		return skerr.Wrapf(err, `making POST request to %s/json/triage for test %q, digest %q and issue %q`, c.resultState.GoldURL, testName, digest, changeListId)
	}

	return nil
}

// MostRecentPositiveDigest fulfills the GoldClient interface.
func (c *CloudClient) MostRecentPositiveDigest(traceId tiling.TraceID) (types.Digest, error) {
	endpointUrl := c.resultState.GoldURL + "/json/latestpositivedigest/" + string(traceId)

	jsonBytes, err := getWithRetries(c.httpClient, endpointUrl)
	if err != nil {
		return "", skerr.Wrapf(err, "making request to %s", endpointUrl)
	}

	mostRecentPositiveDigest := frontend.MostRecentPositiveDigestResponse{}
	if err := json.Unmarshal(jsonBytes, &mostRecentPositiveDigest); err != nil {
		return "", skerr.Wrapf(err, "unmarshalling JSON response from %s", endpointUrl)
	}

	return mostRecentPositiveDigest.Digest, nil
}

// DumpBaseline fulfills the GoldClientDebug interface
func (c *CloudClient) DumpBaseline() (string, error) {
	if c.resultState == nil || c.resultState.Expectations == nil {
		return "", errors.New("Not instantiated - call init?")
	}
	return stringifyBaseline(c.resultState.Expectations), nil
}

func stringifyBaseline(b map[types.TestName]map[types.Digest]expectations.Label) string {
	names := make([]string, 0, len(b))
	for testName := range b {
		names = append(names, string(testName))
	}
	sort.Strings(names)
	s := strings.Builder{}
	for _, testName := range names {
		digestMap := b[types.TestName(testName)]
		digests := make([]string, 0, len(digestMap))
		for d := range digestMap {
			digests = append(digests, string(d))
		}
		sort.Strings(digests)
		_, _ = fmt.Fprintf(&s, "%s:\n", testName)
		for _, d := range digests {
			_, _ = fmt.Fprintf(&s, "\t%s : %s\n", d, digestMap[types.Digest(d)].String())
		}
	}
	return s.String()
}

// DumpKnownHashes fulfills the GoldClientDebug interface
func (c *CloudClient) DumpKnownHashes() (string, error) {
	if c.resultState == nil || c.resultState.KnownHashes == nil {
		return "", errors.New("Not instantiated - call init?")
	}
	var hashes []string
	for h := range c.resultState.KnownHashes {
		hashes = append(hashes, string(h))
	}
	sort.Strings(hashes)
	s := strings.Builder{}
	_, _ = s.WriteString("Hashes:\n\t")
	_, _ = s.WriteString(strings.Join(hashes, "\n\t"))
	_, _ = s.WriteString("\n")
	return s.String(), nil
}

// Make sure CloudClient fulfills the GoldClient interface
var _ GoldClient = (*CloudClient)(nil)

// Make sure CloudClient fulfills the GoldClientDebug interface
var _ GoldClientDebug = (*CloudClient)(nil)