cmd/cover: fix sorting of profile segment boundaries, again

This is a refinement of CL 114855, which fixed the empty clause case,
but broke some other cases where segment boundaries can coincide
for other reasons.

Fixes #25767.

Change-Id: I2a387c83f9d651c8358f3e11b03f6167af0eb8bf
Reviewed-on: https://go-review.googlesource.com/116976
Run-TryBot: David Symonds <dsymonds@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/cover/profile.go

@@ -153,6 +153,7 @@ type Boundary struct {
 	Start  bool    // Is this the start of a block?
 	Count  int     // Event count from the cover profile.
 	Norm   float64 // Count normalized to [0..1].
+	Index  int     // Order in input file.
 }
 
 // Boundaries returns a Profile as a set of Boundary objects within the provided src.
@@ -168,8 +169,10 @@ func (p *Profile) Boundaries(src []byte) (boundaries []Boundary) {
 	divisor := math.Log(float64(max))
 
 	// boundary returns a Boundary, populating the Norm field with a normalized Count.
+	index := 0
 	boundary := func(offset int, start bool, count int) Boundary {
-		b := Boundary{Offset: offset, Start: start, Count: count}
+		b := Boundary{Offset: offset, Start: start, Count: count, Index: index}
+		index++
 		if !start || count == 0 {
 			return b
 		}
@@ -209,10 +212,9 @@ func (b boundariesByPos) Len() int      { return len(b) }
 func (b boundariesByPos) Swap(i, j int) { b[i], b[j] = b[j], b[i] }
 func (b boundariesByPos) Less(i, j int) bool {
 	if b[i].Offset == b[j].Offset {
-		// Boundaries at the same offset should be ordered Start < !Start.
-		// They represent empty sections of code (e.g. a switch/select clause
-		// without a body).
-		return b[i].Start && !b[j].Start
+		// Boundaries at the same offset should be ordered according to
+		// their original position.
+		return b[i].Index < b[j].Index
 	}
 	return b[i].Offset < b[j].Offset
 }

src/cmd/cover/testdata/html/html.go

@@ -1,5 +1,7 @@
 package html
 
+import "fmt"
+
 // This file is tested by html_test.go.
 // The comments below are markers for extracting the annotated source
 // from the HTML output.
@@ -16,3 +18,13 @@ func f() {
 }
 
 // END f
+
+// https://golang.org/issue/25767
+// START g
+func g() {
+	if false {
+		fmt.Printf("Hello")
+	}
+}
+
+// END g

src/cmd/cover/testdata/html/html.golden

@@ -8,3 +8,11 @@ func f() <span class="cov8" title="1">{
 }
 
 // END f
+// START g
+func g() <span class="cov8" title="1">{
+	if false </span><span class="cov0" title="0">{
+		fmt.Printf("Hello")
+	}</span>
+}
+
+// END g

src/cmd/cover/testdata/html/html_test.go

@@ -4,4 +4,5 @@ import "testing"
 
 func TestAll(t *testing.T) {
 	f()
+	g()
 }