Fix reader for large index files.

Currently the offsets are cast into uint32 even though the index can
grow larger than 4GiB.

Signed-off-by: Goutham Veeramachaneni <cs14btech11014@iith.ac.in>

index/index.go

@@ -143,7 +143,11 @@ func NewWriter(fn string) (*Writer, error) {
 	if err != nil {
 		return nil, err
 	}
-	defer df.Close() // close for flatform windows
+	defer df.Close() // Close for platform windows.
+
+	if err := os.RemoveAll(fn); err != nil {
+		return nil, errors.Wrap(err, "remove any existing index at path")
+	}
 
 	f, err := os.OpenFile(fn, os.O_CREATE|os.O_WRONLY, 0666)
 	if err != nil {
@@ -530,8 +534,8 @@ type Reader struct {
 	c io.Closer
 
 	// Cached hashmaps of section offsets.
-	labels   map[string]uint32
-	postings map[labels.Label]uint32
+	labels   map[string]uint64
+	postings map[labels.Label]uint64
 	// Cache of read symbols. Strings that are returned when reading from the
 	// block are always backed by true strings held in here rather than
 	// strings that are backed by byte slices from the mmap'd index file. This
@@ -595,8 +599,8 @@ func newReader(b ByteSlice, c io.Closer, version int) (*Reader, error) {
 		b:        b,
 		c:        c,
 		symbols:  map[uint32]string{},
-		labels:   map[string]uint32{},
-		postings: map[labels.Label]uint32{},
+		labels:   map[string]uint64{},
+		postings: map[labels.Label]uint64{},
 		crc32:    newCRC32(),
 		version:  version,
 	}
@@ -617,7 +621,7 @@ func newReader(b ByteSlice, c io.Closer, version int) (*Reader, error) {
 	}
 	var err error
 
-	err = r.readOffsetTable(r.toc.labelIndicesTable, func(key []string, off uint32) error {
+	err = r.readOffsetTable(r.toc.labelIndicesTable, func(key []string, off uint64) error {
 		if len(key) != 1 {
 			return errors.Errorf("unexpected key length %d", len(key))
 		}
@@ -627,7 +631,7 @@ func newReader(b ByteSlice, c io.Closer, version int) (*Reader, error) {
 	if err != nil {
 		return nil, errors.Wrap(err, "read label index table")
 	}
-	err = r.readOffsetTable(r.toc.postingsTable, func(key []string, off uint32) error {
+	err = r.readOffsetTable(r.toc.postingsTable, func(key []string, off uint64) error {
 		if len(key) != 2 {
 			return errors.Errorf("unexpected key length %d", len(key))
 		}
@@ -780,7 +784,7 @@ func (r *Reader) readSymbols(off int) error {
 // readOffsetTable reads an offset table at the given position calls f for each
 // found entry.f
 // If f returns an error it stops decoding and returns the received error,
-func (r *Reader) readOffsetTable(off uint64, f func([]string, uint32) error) error {
+func (r *Reader) readOffsetTable(off uint64, f func([]string, uint64) error) error {
 	d := r.decbufAt(int(off))
 	cnt := d.be32()
 
@@ -791,7 +795,7 @@ func (r *Reader) readOffsetTable(off uint64, f func([]string, uint32) error) err
 		for i := 0; i < keyCount; i++ {
 			keys = append(keys, d.uvarintStr())
 		}
-		o := uint32(d.uvarint())
+		o := d.uvarint64()
 		if d.err() != nil {
 			break
 		}