Make index loading funcs use reader interfaces

Bonus fix: no longer use a bytes.Buffer to load the index.xml entry in the jar
entirely into memory

cmd/fdroidcl/main.go

@@ -157,10 +157,23 @@ func init() {
 	}
 }
 
+func indexPath(repoName string) string {
+	return repoName + ".jar"
+}
+
 func mustLoadIndex(repoName string) *fdroidcl.Index {
-	index, err := fdroidcl.LoadIndex(repoName)
+	path := indexPath(repoName)
+	f, err := os.Open(path)
+	if err != nil {
+		log.Fatalf("Could not open index file: %v", err)
+	}
+	stat, err := f.Stat()
+	if err != nil {
+		log.Fatalf("Could not stat index file: %v", err)
+	}
+	index, err := fdroidcl.LoadIndexJar(f, stat.Size())
 	if err != nil {
-		log.Fatalf("Could not load apps: %v", err)
+		log.Fatalf("Could not load index: %v", err)
 	}
 	return index
 }

index.go

@@ -5,7 +5,6 @@ package fdroidcl
 
 import (
 	"archive/zip"
-	"bytes"
 	"encoding/xml"
 	"errors"
 	"fmt"
@@ -231,32 +230,30 @@ func (al appList) Len() int           { return len(al) }
 func (al appList) Swap(i, j int)      { al[i], al[j] = al[j], al[i] }
 func (al appList) Less(i, j int) bool { return al[i].ID < al[j].ID }
 
-func LoadIndex(repoName string) (*Index, error) {
-	path := indexPath(repoName)
-	r, err := zip.OpenReader(path)
+func LoadIndexJar(r io.ReaderAt, size int64) (*Index, error) {
+	reader, err := zip.NewReader(r, size)
 	if err != nil {
 		return nil, err
 	}
-	defer r.Close()
-	buf := new(bytes.Buffer)
-
-	for _, f := range r.File {
+	var rc io.ReadCloser
+	for _, f := range reader.File {
 		if f.Name != "index.xml" {
 			continue
 		}
-		rc, err := f.Open()
+		rc, err = f.Open()
 		if err != nil {
 			return nil, err
 		}
-		if _, err = io.Copy(buf, rc); err != nil {
-			return nil, err
-		}
-		rc.Close()
 		break
 	}
+	defer rc.Close()
+	return LoadIndexXml(rc)
+}
 
+func LoadIndexXml(r io.Reader) (*Index, error) {
 	var index Index
-	if err := xml.Unmarshal(buf.Bytes(), &index); err != nil {
+	decoder := xml.NewDecoder(r)
+	if err := decoder.Decode(&index); err != nil {
 		return nil, err
 	}