internal/coverage/encodecounter: followup changes from code review

This patch contains a small set of changes with fixes for some
issues that surfaced during the code review for CL 484535. Due
to an error on my part, these never got included in the final version
that was checked in (I rebased, mailed the rebase, but then never
mailed the final patch set with the changes). This patch sends
the remaining bits and pieces.

Updates golang#59563.

Change-Id: I87dc05a83f8e44c8bfe7203bc2b035defc817af9
Reviewed-on: https://go-review.googlesource.com/c/go/+/492981
Run-TryBot: Than McIntosh <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

src/internal/coverage/encodecounter/encode.go

@@ -28,7 +28,6 @@ type CoverageDataWriter struct {
 	w       *bufio.Writer
 	csh     coverage.CounterSegmentHeader
 	tmp     []byte
-	nfuncs  uint64
 	cflavor coverage.CounterFlavor
 	segs    uint32
 	debug   bool
@@ -86,17 +85,25 @@ func padToFourByteBoundary(ws *slicewriter.WriteSeeker) error {
 }
 
 func (cfw *CoverageDataWriter) patchSegmentHeader(ws *slicewriter.WriteSeeker) error {
+	// record position
+	off, err := ws.Seek(0, io.SeekCurrent)
+	if err != nil {
+		return fmt.Errorf("error seeking in patchSegmentHeader: %v", err)
+	}
+	// seek back to start so that we can update the segment header
 	if _, err := ws.Seek(0, io.SeekStart); err != nil {
 		return fmt.Errorf("error seeking in patchSegmentHeader: %v", err)
 	}
-	cfw.csh.FcnEntries = cfw.nfuncs
-	cfw.nfuncs = 0
 	if cfw.debug {
 		fmt.Fprintf(os.Stderr, "=-= writing counter segment header: %+v", cfw.csh)
 	}
 	if err := binary.Write(ws, binary.LittleEndian, cfw.csh); err != nil {
 		return err
 	}
+	// ... and finally return to the original offset.
+	if _, err := ws.Seek(off, io.SeekStart); err != nil {
+		return fmt.Errorf("error seeking in patchSegmentHeader: %v", err)
+	}
 	return nil
 }
 
@@ -167,8 +174,7 @@ func (cfw *CoverageDataWriter) AppendSegment(args map[string]string, visitor Cou
 		cfw.stab.Lookup(v)
 	}
 
-	var swws slicewriter.WriteSeeker
-	ws := &swws
+	ws := &slicewriter.WriteSeeker{}
 	if err = cfw.writeSegmentPreamble(args, ws); err != nil {
 		return err
 	}
@@ -253,7 +259,7 @@ func (cfw *CoverageDataWriter) writeCounters(visitor CounterVisitor, ws *slicewr
 
 	// Write out entries for each live function.
 	emitter := func(pkid uint32, funcid uint32, counters []uint32) error {
-		cfw.nfuncs++
+		cfw.csh.FcnEntries++
 		if err := wrval(uint32(len(counters))); err != nil {
 			return err
 		}