implement file sources

If the file is not cached we have to get it from somewhere. For that I
implemented a webdav file source which gets the file from reva.

pkg/thumbnails/imgsource/imgsource.go

@@ -0,0 +1,33 @@
+package imgsource
+
+import "image"
+
+// Source defines the interface for image sources
+type Source interface {
+	Get(path string, ctx SourceContext) (image.Image, error)
+}
+
+// NewContext creates a new SourceContext instance
+func NewContext() SourceContext {
+	return SourceContext{
+		m: make(map[string]interface{}),
+	}
+}
+
+// SourceContext is used to pass source specific parameters
+type SourceContext struct {
+	m map[string]interface{}
+}
+
+// GetString tries to cast the value to a string
+func (s SourceContext) GetString(key string) string {
+	if s, ok := s.m[key].(string); ok {
+		return s
+	}
+	return ""
+}
+
+// Set sets a value
+func (s SourceContext) Set(key string, val interface{}) {
+	s.m[key] = val
+}

pkg/thumbnails/imgsource/webdav.go

@@ -0,0 +1,41 @@
+package imgsource
+
+import (
+	"fmt"
+	"image"
+	"net/http"
+	"net/url"
+	"path"
+)
+
+// WebDav implements the Source interface for webdav services
+type WebDav struct {
+	Basepath string
+}
+
+const (
+	// WebDavAuth is the parameter name for the autorization token
+	WebDavAuth = "Authorization"
+)
+
+// Get downloads the file from a webdav service
+func (s WebDav) Get(file string, ctx SourceContext) (image.Image, error) {
+	u, _ := url.Parse(s.Basepath)
+	u.Path = path.Join(u.Path, file)
+	req, err := http.NewRequest(http.MethodGet, u.String(), nil)
+	if err != nil {
+		return nil, fmt.Errorf("could not get the file %s error: %s", file, err.Error())
+	}
+
+	auth := ctx.GetString(WebDavAuth)
+	req.Header.Add("Authorization", auth)
+
+	client := &http.Client{}
+	resp, err := client.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("could not get the file %s error: %s", file, err.Error())
+	}
+
+	img, _, _ := image.Decode(resp.Body)
+	return img, nil
+}