internal/coverage: implement conforming Seek method in slicereader

Implement a real Seek() method in the slicereader helper (prior to
this it had a simplified SeekTo function), so that slicereader's will
satisfy the ReadSeeker interface (needed in a subsequent patch).

Change-Id: I832e3ec1e34d0f8c6b5edf390470f6f943c6ece0
Reviewed-on: https://go-review.googlesource.com/c/go/+/495438
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/internal/coverage/decodemeta/decode.go

@@ -14,6 +14,7 @@ import (
 	"internal/coverage"
 	"internal/coverage/slicereader"
 	"internal/coverage/stringtab"
+	"io"
 	"os"
 )
 
@@ -55,7 +56,9 @@ func (d *CoverageMetaDataDecoder) readHeader() error {
 func (d *CoverageMetaDataDecoder) readStringTable() error {
 	// Seek to the correct location to read the string table.
 	stringTableLocation := int64(coverage.CovMetaHeaderSize + 4*d.hdr.NumFuncs)
-	d.r.SeekTo(stringTableLocation)
+	if _, err := d.r.Seek(stringTableLocation, io.SeekStart); err != nil {
+		return err
+	}
 
 	// Read the table itself.
 	d.strtab = stringtab.NewReader(d.r)
@@ -88,7 +91,9 @@ func (d *CoverageMetaDataDecoder) ReadFunc(fidx uint32, f *coverage.FuncDesc) er
 
 	// Seek to the correct location to read the function offset and read it.
 	funcOffsetLocation := int64(coverage.CovMetaHeaderSize + 4*fidx)
-	d.r.SeekTo(funcOffsetLocation)
+	if _, err := d.r.Seek(funcOffsetLocation, io.SeekStart); err != nil {
+		return err
+	}
 	foff := d.r.ReadUint32()
 
 	// Check assumptions
@@ -97,7 +102,10 @@ func (d *CoverageMetaDataDecoder) ReadFunc(fidx uint32, f *coverage.FuncDesc) er
 	}
 
 	// Seek to the correct location to read the function.
-	d.r.SeekTo(int64(foff))
+	floc := int64(foff)
+	if _, err := d.r.Seek(floc, io.SeekStart); err != nil {
+		return err
+	}
 
 	// Preamble containing number of units, file, and function.
 	numUnits := uint32(d.r.ReadULEB128())

src/internal/coverage/slicereader/slicereader.go

@@ -6,6 +6,8 @@ package slicereader
 
 import (
 	"encoding/binary"
+	"fmt"
+	"io"
 	"unsafe"
 )
 
@@ -38,8 +40,31 @@ func (r *Reader) Read(b []byte) (int, error) {
 	return amt, nil
 }
 
-func (r *Reader) SeekTo(off int64) {
-	r.off = off
+func (r *Reader) Seek(offset int64, whence int) (ret int64, err error) {
+	switch whence {
+	case io.SeekStart:
+		if offset < 0 || offset > int64(len(r.b)) {
+			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", offset, len(r.b))
+		}
+		r.off = offset
+		return offset, nil
+	case io.SeekCurrent:
+		newoff := r.off + offset
+		if newoff < 0 || newoff > int64(len(r.b)) {
+			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", newoff, len(r.b))
+		}
+		r.off = newoff
+		return r.off, nil
+	case io.SeekEnd:
+		newoff := int64(len(r.b)) + offset
+		if newoff < 0 || newoff > int64(len(r.b)) {
+			return 0, fmt.Errorf("invalid seek: new offset %d (out of range [0 %d]", newoff, len(r.b))
+		}
+		r.off = newoff
+		return r.off, nil
+	}
+	// other modes are not supported
+	return 0, fmt.Errorf("unsupported seek mode %d", whence)
 }
 
 func (r *Reader) Offset() int64 {

src/internal/coverage/slicereader/slr_test.go

@@ -6,6 +6,7 @@ package slicereader
 
 import (
 	"encoding/binary"
+	"io"
 	"testing"
 )
 
@@ -64,10 +65,12 @@ func TestSliceReader(t *testing.T) {
 		if gs2 != s2 {
 			t.Fatalf("readStr got %s want %s", gs2, s2)
 		}
-		slr.SeekTo(4)
+		if _, err := slr.Seek(4, io.SeekStart); err != nil {
+			t.Fatal(err)
+		}
 		off := slr.Offset()
 		if off != 4 {
-			t.Fatalf("Offset(0 returned %d wanted 4", off)
+			t.Fatalf("Offset() returned %d wanted 4", off)
 		}
 		g64 = slr.ReadUint64()
 		if g64 != e64 {