objstorage: fix readaheadState initialization

When I extracted out the readahead logic from the sstable Reader, I
accidentally omitted some initialization code - setting
`readaheadState.size` to `initialReadaheadSize`. Fixing.

objstorage/objstorageprovider/readahead.go

@@ -32,6 +32,12 @@ type readaheadState struct {
 	limit int64
 }
 
+func makeReadaheadState() readaheadState {
+	return readaheadState{
+		size: initialReadaheadSize,
+	}
+}
+
 func (rs *readaheadState) recordCacheHit(offset, blockLength int64) {
 	currentReadEnd := offset + blockLength
 	if rs.numReads >= minFileReadsForReadahead {

objstorage/objstorageprovider/readahead_test.go

@@ -15,7 +15,7 @@ import (
 )
 
 func TestMaybeReadahead(t *testing.T) {
-	var rs readaheadState
+	rs := makeReadaheadState()
 	datadriven.RunTest(t, "testdata/readahead", func(t *testing.T, d *datadriven.TestData) string {
 		cacheHit := false
 		switch d.Cmd {

objstorage/objstorageprovider/testdata/readahead

@@ -1,6 +1,3 @@
-reset
-----
-
 read
 2048, 16
 ----

objstorage/objstorageprovider/vfs_readable.go

@@ -73,6 +73,7 @@ func (r *fileReadable) Size() int64 {
 func (r *fileReadable) NewReadHandle(_ context.Context) objstorage.ReadHandle {
 	rh := readHandlePool.Get().(*vfsReadHandle)
 	rh.r = r
+	rh.rs = makeReadaheadState()
 	return rh
 }