fix(PageBufferReader): should conform to io.Reader interface (dgraph-…

…io#1935)

Fixes DGRAPHCORE-121

## Problem
The CI build was showing sporadic failures of TestPagebufferReader2 with
the error message: Received unexpected error: EOF
The result is sporadic because the test case relied on randomized
behavior. In particular, it would generate a read-buffer of some random
length, which could occasionally have a length of 0. When the length
is 0, we would encounter this error.

The cause is from the PageBufferReader having incorrect behavior for the
Read(p []byte) function: In particular, when p is empty, based on the
expected behavior of the Reader interface, this should return 0, nil. 
However, this is currently returning 0, EOF.

## Solution
I added a unit test to make sure we consider the empty buffer input in
every case (instead of just randomly every once in a while). This
doesn't actually provide a solution, but it makes sure that we don't end
up with a regression in this area.

At the point where we currently detect that we read 0 bytes, before just
returning 0, EOF, I add a check to make sure that the value p has
non-zero length.

y/y.go

@@ -508,7 +508,7 @@ func (r *PageBufferReader) Read(p []byte) (int, error) {
 		}
 	}
 
-	if read == 0 {
+	if read == 0 && len(p) > 0 {
 		return read, io.EOF
 	}
 

y/y_test.go

@@ -259,6 +259,28 @@ func TestPagebufferReader4(t *testing.T) {
 	require.Equal(t, n, 0)
 }
 
+// Test when reading into 0 length readBuffer
+func TestPagebufferReader5(t *testing.T) {
+	b := NewPageBuffer(32)
+	readerWhenEmptyPageBuffer := b.NewReaderAt(0)
+
+	readBuffer := []byte{} // Intentionally empty readBuffer.
+	n, err := readerWhenEmptyPageBuffer.Read(readBuffer)
+	require.NoError(t, err, "reading into empty buffer should return no error")
+	require.Equal(t, 0, n, "read into empty buffer should return 0 bytes")
+
+	var wb [20]byte
+	rand.Read(wb[:])
+	n, err = b.Write(wb[:])
+	require.Equal(t, n, len(wb), "length of buffer and length written should be equal")
+	require.NoError(t, err, "unable to write bytes to buffer")
+
+	readerWhenNonEmptyPageBuffer := b.NewReaderAt(0)
+	n, err = readerWhenNonEmptyPageBuffer.Read(readBuffer)
+	require.NoError(t, err, "reading into empty buffer should return no error")
+	require.Equal(t, 0, n, "read into empty buffer should return 0 bytes")
+}
+
 func TestSizeVarintForZero(t *testing.T) {
 	siz := sizeVarint(0)
 	require.Equal(t, 1, siz)