cmd/cover: fix buglets in counter insertion

This patch has a couple of minor fixes to new-style counter insertion
(noticed these problems while working on the fix for issue 56370).

First, make sure that the function registration sequence (writing of
nctrs, pkgid, funcid to counter var prolog) comes prior to the first
counter update (they were reversed up to this point, due to an
artifact of the way cmd/internal/edit operates).

Second, fix up "per function" counter insertion mode (an experimental
feature disabled by default that adds just a single counter to each
function as opposed to one per basic block), which was failing to
insert the single counter in the right place.

Change-Id: Icfb613ca385647f35c0e52da2da8edeb2a506ab7
Reviewed-on: https://go-review.googlesource.com/c/go/+/444835
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: David Chase <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Than McIntosh <[email protected]>

Author: thanm

src/cmd/cover/cfg_test.go

@@ -129,7 +129,6 @@ func TestCoverWithCfg(t *testing.T) {
 		},
 	}
 
-	tag := "first"
 	var incfg string
 	for _, scenario := range scenarios {
 		// Instrument package "a", producing a set of instrumented output
@@ -138,6 +137,7 @@ func TestCoverWithCfg(t *testing.T) {
 		pname := "a"
 		mode := scenario.mode
 		gran := scenario.gran
+		tag := mode + "_" + gran
 		incfg = writePkgConfig(t, instdira, tag, ppath, pname, gran)
 		ofs, outcfg, _ := runPkgCover(t, instdira, tag, incfg, mode,
 			pfiles("a"), false)
@@ -158,6 +158,7 @@ func TestCoverWithCfg(t *testing.T) {
 	// Expect error if config file inaccessible/unreadable.
 	mode := "atomic"
 	errExpected := true
+	tag := "errors"
 	_, _, errmsg := runPkgCover(t, instdira, tag, "/not/a/file", mode,
 		pfiles("a"), errExpected)
 	want := "error reading pkgconfig file"

src/cmd/cover/cover.go

@@ -426,7 +426,9 @@ func (f *File) Visit(node ast.Node) ast.Visitor {
 		if *pkgcfg != "" {
 			f.preFunc(n, fname)
 		}
-		ast.Walk(f, n.Body)
+		if pkgconfig.Granularity != "perfunc" {
+			ast.Walk(f, n.Body)
+		}
 		if *pkgcfg != "" {
 			flit := true
 			f.postFunc(n, fname, flit, n.Body)
@@ -465,6 +467,13 @@ func (f *File) preFunc(fn ast.Node, fname string) {
 }
 
 func (f *File) postFunc(fn ast.Node, funcname string, flit bool, body *ast.BlockStmt) {
+
+	// Tack on single counter write if we are in "perfunc" mode.
+	singleCtr := ""
+	if pkgconfig.Granularity == "perfunc" {
+		singleCtr = "; " + f.newCounter(fn.Pos(), fn.Pos(), 1)
+	}
+
 	// record the length of the counter var required.
 	nc := len(f.fn.units) + coverage.FirstCtrOffset
 	f.pkg.counterLengths = append(f.pkg.counterLengths, nc)
@@ -504,12 +513,16 @@ func (f *File) postFunc(fn ast.Node, funcname string, flit bool, body *ast.Block
 	cv := f.fn.counterVar
 	regHook := hookWrite(cv, 0, strconv.Itoa(len(f.fn.units))) + " ; " +
 		hookWrite(cv, 1, mkPackageIdExpression()) + " ; " +
-		hookWrite(cv, 2, strconv.Itoa(int(funcId)))
+		hookWrite(cv, 2, strconv.Itoa(int(funcId))) + singleCtr
+
+	// Insert the registration sequence into the function. We want this sequence to
+	// appear before any counter updates, so use a hack to ensure that this edit
+	// applies before the edit corresponding to the prolog counter update.
 
-	// Insert the registration sequence into the function.
 	boff := f.offset(body.Pos())
-	ipos := f.fset.File(body.Pos()).Pos(boff + 1)
-	f.edit.Insert(f.offset(ipos), regHook+" ; ")
+	ipos := f.fset.File(body.Pos()).Pos(boff)
+	ip := f.offset(ipos)
+	f.edit.Replace(ip, ip+1, string(f.content[ipos-1])+regHook+" ; ")
 
 	f.fn.counterVar = ""
 }
@@ -661,7 +674,6 @@ func (f *File) newCounter(start, end token.Pos, numStmt int) string {
 			NxStmts: uint32(numStmt),
 		}
 		f.fn.units = append(f.fn.units, unit)
-
 	} else {
 		stmt = counterStmt(f, fmt.Sprintf("%s.Count[%d]", *varVar,
 			len(f.blocks)))