[Fix] Random reads won't re-use previous reader even if existing read…

…er can serve the request (#1683)

* use read full instead of read

* use variables in test

* review comment

internal/gcsx/random_reader.go

@@ -179,7 +179,7 @@ func (rr *randomReader) ReadAt(
 		if rr.reader != nil && rr.start < offset && offset-rr.start < maxReadSize {
 			bytesToSkip := int64(offset - rr.start)
 			p := make([]byte, bytesToSkip)
-			n, _ := rr.reader.Read(p)
+			n, _ := io.ReadFull(rr.reader, p)
 			rr.start += int64(n)
 		}
 

internal/gcsx/random_reader_test.go

@@ -222,6 +222,28 @@ func (t *RandomReaderTest) ExistingReader_WrongOffset() {
 	AssertNe(nil, err)
 }
 
+func (t *RandomReaderTest) ExistingReader_ReadAtOffsetAfterTheReaderPosition() {
+	var currentStartOffset int64 = 2
+	var readerLimit int64 = 15
+	var readAtOffset int64 = 10
+	var readSize int64 = 1
+	var expectedStartOffsetAfterRead = readAtOffset + readSize
+	// Simulate an existing reader.
+	rc := io.NopCloser(strings.NewReader(strings.Repeat("x", int(readerLimit))))
+	t.rr.wrapped.reader = rc
+	t.rr.wrapped.cancel = func() {}
+	t.rr.wrapped.start = currentStartOffset
+	t.rr.wrapped.limit = readerLimit
+
+	buf := make([]byte, readSize)
+	_, err := t.rr.ReadAt(buf, readAtOffset)
+
+	AssertEq(nil, err)
+	ExpectThat(rc, DeepEquals(t.rr.wrapped.reader))
+	ExpectEq(expectedStartOffsetAfterRead, t.rr.wrapped.start)
+	ExpectEq(readerLimit, t.rr.wrapped.limit)
+}
+
 func (t *RandomReaderTest) NewReaderReturnsError() {
 	ExpectCall(t.bucket, "NewReader")(Any(), Any()).
 		WillOnce(Return(nil, errors.New("taco")))