Fix panic when not filling up zipfs's read buffer

zipfs/file.go

@@ -61,7 +61,7 @@ func (f *File) Read(p []byte) (n int, err error) {
 	}
 	err = f.fillBuffer(f.offset + int64(len(p)))
 	n = copy(p, f.buf[f.offset:])
-	f.offset += int64(len(p))
+	f.offset += int64(n)
 	return
 }
 

zipfs/file_test.go

@@ -0,0 +1,43 @@
+package zipfs
+
+import (
+	"archive/zip"
+	"io"
+	"testing"
+)
+
+func TestFileRead(t *testing.T) {
+	zrc, err := zip.OpenReader("testdata/small.zip")
+	if err != nil {
+		t.Fatal(err)
+	}
+	zfs := New(&zrc.Reader)
+	f, err := zfs.Open("smallFile")
+	if err != nil {
+		t.Fatal(err)
+	}
+	info, err := f.Stat()
+	if err != nil {
+		t.Fatal(err)
+	}
+	chunkSize := info.Size() * 2 // read with extra large buffer
+
+	buf := make([]byte, chunkSize)
+	n, err := f.Read(buf)
+	if err != io.EOF {
+		t.Fatal("Failed to read file to completion:", err)
+	}
+	if n != int(info.Size()) {
+		t.Errorf("Expected read length to be %d, found: %d", info.Size(), n)
+	}
+
+	// read a second time to check f.offset and f.buf are correct
+	buf = make([]byte, chunkSize)
+	n, err = f.Read(buf)
+	if err != io.EOF {
+		t.Fatal("Failed to read a fully read file:", err)
+	}
+	if n != 0 {
+		t.Errorf("Expected read length to be 0, found: %d", n)
+	}
+}