gcs: Fix download of "Content-Encoding: gzip" compressed objects

Before this change, if an object compressed with "Content-Encoding:
gzip" was downloaded, a length and hash mismatch would occur since the
as the go runtime automatically decompressed the object on download.

This change erases the length and hash on compressed objects so they
can be downloaded successfully, at the cost of not being able to check
the length or the hash of the downloaded object.

This also adds the --gcs-download-compressed flag to allow the
compressed files to be downloaded as-is providing compressed objects
with intact size and hash information.

Fixes #2658

backend/googlecloudstorage/googlecloudstorage.go

@@ -24,6 +24,7 @@ import (
 	"path"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/rclone/rclone/fs"
@@ -304,6 +305,21 @@ rclone does if you know the bucket exists already.
 `,
 			Default:  false,
 			Advanced: true,
+		}, {
+			Name: "download_compressed",
+			Help: `If set this will download compressed objects as-is.
+
+It is possible to upload objects to GCS with "Content-Encoding: gzip"
+set. Normally rclone will transparently decompress these files on
+download. This means that rclone can't check the hash or the size of
+the file as both of these refer to the compressed object.
+
+If this flag is set then rclone will download files with
+"Content-Encoding: gzip" as they are received. This means that rclone
+can check the size and hash but the file contents will be compressed.
+`,
+			Advanced: true,
+			Default:  false,
 		}, {
 			Name:     config.ConfigEncoding,
 			Help:     config.ConfigEncodingHelp,
@@ -327,21 +343,23 @@ type Options struct {
 	Location                  string               `config:"location"`
 	StorageClass              string               `config:"storage_class"`
 	NoCheckBucket             bool                 `config:"no_check_bucket"`
+	DownloadCompressed        bool                 `config:"download_compressed"`
 	Enc                       encoder.MultiEncoder `config:"encoding"`
 }
 
 // Fs represents a remote storage server
 type Fs struct {
-	name          string           // name of this remote
-	root          string           // the path we are working on if any
-	opt           Options          // parsed options
-	features      *fs.Features     // optional features
-	svc           *storage.Service // the connection to the storage server
-	client        *http.Client     // authorized client
-	rootBucket    string           // bucket part of root (if any)
-	rootDirectory string           // directory part of root (if any)
-	cache         *bucket.Cache    // cache of bucket status
-	pacer         *fs.Pacer        // To pace the API calls
+	name           string           // name of this remote
+	root           string           // the path we are working on if any
+	opt            Options          // parsed options
+	features       *fs.Features     // optional features
+	svc            *storage.Service // the connection to the storage server
+	client         *http.Client     // authorized client
+	rootBucket     string           // bucket part of root (if any)
+	rootDirectory  string           // directory part of root (if any)
+	cache          *bucket.Cache    // cache of bucket status
+	pacer          *fs.Pacer        // To pace the API calls
+	warnCompressed sync.Once        // warn once about compressed files
 }
 
 // Object describes a storage object
@@ -355,6 +373,7 @@ type Object struct {
 	bytes    int64     // Bytes in the object
 	modTime  time.Time // Modified time of the object
 	mimeType string
+	gzipped  bool // set if object has Content-Encoding: gzip
 }
 
 // ------------------------------------------------------------
@@ -975,6 +994,7 @@ func (o *Object) setMetaData(info *storage.Object) {
 	o.url = info.MediaLink
 	o.bytes = int64(info.Size)
 	o.mimeType = info.ContentType
+	o.gzipped = info.ContentEncoding == "gzip"
 
 	// Read md5sum
 	md5sumData, err := base64.StdEncoding.DecodeString(info.Md5Hash)
@@ -1013,6 +1033,15 @@ func (o *Object) setMetaData(info *storage.Object) {
 	} else {
 		o.modTime = modTime
 	}
+
+	// If gunzipping then size and md5sum are unknown
+	if o.gzipped && !o.fs.opt.DownloadCompressed {
+		o.bytes = -1
+		o.md5sum = ""
+		o.fs.warnCompressed.Do(func() {
+			fs.Logf(o.fs, "Decompressing 'Content-Encoding: gzip' compressed file. Use --gcs-download-compressed to override")
+		})
+	}
 }
 
 // readObjectInfo reads the definition for an object
@@ -1113,6 +1142,15 @@ func (o *Object) Open(ctx context.Context, options ...fs.OpenOption) (in io.Read
 		return nil, err
 	}
 	fs.FixRangeOption(options, o.bytes)
+	if o.gzipped && o.fs.opt.DownloadCompressed {
+		// Allow files which are stored on the cloud storage system
+		// compressed to be downloaded without being decompressed.  Note
+		// that setting this here overrides the automatic decompression
+		// in the Transport.
+		//
+		// See: https://cloud.google.com/storage/docs/transcoding
+		req.Header.Set("Accept-Encoding", "gzip")
+	}
 	fs.OpenOptionAddHTTPHeaders(req.Header, options)
 	var res *http.Response
 	err = o.fs.pacer.Call(func() (bool, error) {