fix go coverage when parsing blocks out of order

src/test/coverage_test.go

@@ -156,6 +156,34 @@ func TestParseBlocks(t *testing.T) {
 	assertLine(t, lines, 3, core.Covered)
 }
 
+func TestParseBlocksLineZero(t *testing.T) {
+	lines := parseBlocks([]cover.ProfileBlock{
+		{StartLine: 0, EndLine: 1, Count: 1},
+		{StartLine: 1, EndLine: 2, Count: 1},
+	})
+	if len(lines) != 2 {
+		t.Errorf("Wrong number of lines, should have been %d, was %d", 2, len(lines))
+	}
+	assertLine(t, lines, 1, core.Covered)
+	assertLine(t, lines, 2, core.Covered)
+}
+
+func TestParseBlocksUnsorted(t *testing.T) {
+	lines := parseBlocks([]cover.ProfileBlock{
+		{StartLine: 5, EndLine: 10, Count: 1},
+		{StartLine: 1, EndLine: 3, Count: 1},
+	})
+	if len(lines) != 10 {
+		t.Errorf("Wrong number of lines, should have been %d, was %d", 10, len(lines))
+	}
+	assertLine(t, lines, 1, core.Covered)
+	assertLine(t, lines, 2, core.Covered)
+	assertLine(t, lines, 3, core.Covered)
+	assertLine(t, lines, 4, core.NotExecutable)
+	assertLine(t, lines, 5, core.Covered)
+	assertLine(t, lines, 10, core.Covered)
+}
+
 func assertLine(t *testing.T, lines []core.LineCoverage, i int, expected core.LineCoverage) {
 	t.Helper()
 	i-- // 1-indexed

src/test/go_coverage.go

@@ -34,10 +34,18 @@ func parseBlocks(blocks []cover.ProfileBlock) []core.LineCoverage {
 	if len(blocks) == 0 {
 		return nil
 	}
-	lastLine := blocks[len(blocks)-1].EndLine
+	var lastLine int
+	for _, block := range blocks {
+		if block.EndLine > lastLine {
+			lastLine = block.EndLine
+		}
+	}
 	ret := make([]core.LineCoverage, lastLine)
 	for _, block := range blocks {
 		for line := block.StartLine - 1; line < block.EndLine; line++ {
+			if line < 0 {
+				continue
+			}
 			if block.Count > 0 {
 				ret[line] = core.Covered
 			} else {