azureblob: Stop Mkdir attempting to create existing containers

Before this change azureblob would attempt to create already existing
containers.  This causes problems with limited permissions keys.

This change checks the container exists before trying to create it in
the same way the s3 backend does.  This uses no more requests in the
usual case of the container existing.

See: https://forum.rclone.org/t/copying-individual-files-to-azure-blob-storage/8397

backend/azureblob/azureblob.go

@@ -754,6 +754,35 @@ func (f *Fs) Put(in io.Reader, src fs.ObjectInfo, options ...fs.OpenOption) (fs.
 	return fs, fs.Update(in, src, options...)
 }
 
+// Check if the container exists
+//
+// NB this can return incorrect results if called immediately after container deletion
+func (f *Fs) dirExists() (bool, error) {
+	options := azblob.ListBlobsSegmentOptions{
+		Details: azblob.BlobListingDetails{
+			Copy:             false,
+			Metadata:         false,
+			Snapshots:        false,
+			UncommittedBlobs: false,
+			Deleted:          false,
+		},
+		MaxResults: 1,
+	}
+	err := f.pacer.Call(func() (bool, error) {
+		ctx := context.Background()
+		_, err := f.cntURL.ListBlobsHierarchySegment(ctx, azblob.Marker{}, "", options)
+		return f.shouldRetry(err)
+	})
+	if err == nil {
+		return true, nil
+	}
+	// Check http error code along with service code, current SDK doesn't populate service code correctly sometimes
+	if storageErr, ok := err.(azblob.StorageError); ok && (storageErr.ServiceCode() == azblob.ServiceCodeContainerNotFound || storageErr.Response().StatusCode == http.StatusNotFound) {
+		return false, nil
+	}
+	return false, err
+}
+
 // Mkdir creates the container if it doesn't exist
 func (f *Fs) Mkdir(dir string) error {
 	f.containerOKMu.Lock()
@@ -761,6 +790,15 @@ func (f *Fs) Mkdir(dir string) error {
 	if f.containerOK {
 		return nil
 	}
+	if !f.containerDeleted {
+		exists, err := f.dirExists()
+		if err == nil {
+			f.containerOK = exists
+		}
+		if err != nil || exists {
+			return err
+		}
+	}
 
 	// now try to create the container
 	err := f.pacer.Call(func() (bool, error) {