internal/coverage: refactor EmitPercent in preparation for bugfix

Refactor cformat.EmitPercent to accept a package filter (list of
packages to report). This is a no-op in terms of exposed coverage
functionality, but we will need this feature in a subsequent patch.

Updates #65570.

Change-Id: I04ddc624a634837ea31c12ec395aa1295a0ea1f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/592204
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/covdata/dump.go

@@ -325,7 +325,7 @@ func (d *dstate) Finish() {
 	// d.format maybe nil here if the specified input dir was empty.
 	if d.format != nil {
 		if d.cmd == percentMode {
-			d.format.EmitPercent(os.Stdout, "", false, false)
+			d.format.EmitPercent(os.Stdout, nil, "", false, false)
 		}
 		if d.cmd == funcMode {
 			d.format.EmitFuncs(os.Stdout)

src/internal/coverage/cfile/testsupport.go

@@ -103,7 +103,7 @@ func ProcessCoverTestDir(dir string, cfile string, cm string, cpkg string, w io.
 	}
 
 	// Emit percent.
-	if err := ts.cf.EmitPercent(w, cpkg, true, true); err != nil {
+	if err := ts.cf.EmitPercent(w, nil, cpkg, true, true); err != nil {
 		return err
 	}
 

src/internal/coverage/cformat/fmt_test.go

@@ -66,7 +66,7 @@ lit.go:99.0,100.0 1 0`)
 
 	// Percent output with no aggregation.
 	noCoverPkg := ""
-	if err := fm.EmitPercent(&b2, noCoverPkg, false, false); err != nil {
+	if err := fm.EmitPercent(&b2, nil, noCoverPkg, false, false); err != nil {
 		t.Fatalf("EmitPercent returned %v", err)
 	}
 	wantPercent := strings.Fields(`
@@ -81,7 +81,7 @@ lit.go:99.0,100.0 1 0`)
 
 	// Percent mode with aggregation.
 	withCoverPkg := " in ./..."
-	if err := fm.EmitPercent(&b3, withCoverPkg, false, true); err != nil {
+	if err := fm.EmitPercent(&b3, nil, withCoverPkg, false, true); err != nil {
 		t.Fatalf("EmitPercent returned %v", err)
 	}
 	wantPercent = strings.Fields(`
@@ -110,6 +110,24 @@ total		(statements)	62.5%`)
 		t.Logf("perc2 is %s\n", b3.String())
 		t.Logf("funcs is %s\n", b4.String())
 	}
+
+	// Percent output with specific packages selected.
+	{
+		var b strings.Builder
+		selpkgs := []string{"foo/bar", "my/pack1"}
+		if err := fm.EmitPercent(&b, selpkgs, noCoverPkg, false, false); err != nil {
+			t.Fatalf("EmitPercent returned %v", err)
+		}
+		wantPercent := strings.Fields(`
+       	my/pack1		coverage: 66.7% of statements
+`)
+		gotPercent := strings.Fields(b.String())
+		if !slices.Equal(wantPercent, gotPercent) {
+			t.Errorf("emit percent: got:\n%+v\nwant:\n%+v\n",
+				gotPercent, wantPercent)
+		}
+	}
+
 }
 
 func TestEmptyPackages(t *testing.T) {
@@ -122,7 +140,7 @@ func TestEmptyPackages(t *testing.T) {
 	{
 		var b strings.Builder
 		noCoverPkg := ""
-		if err := fm.EmitPercent(&b, noCoverPkg, true, false); err != nil {
+		if err := fm.EmitPercent(&b, nil, noCoverPkg, true, false); err != nil {
 			t.Fatalf("EmitPercent returned %v", err)
 		}
 		wantPercent := strings.Fields(`
@@ -140,7 +158,7 @@ func TestEmptyPackages(t *testing.T) {
 	{
 		var b strings.Builder
 		noCoverPkg := ""
-		if err := fm.EmitPercent(&b, noCoverPkg, true, true); err != nil {
+		if err := fm.EmitPercent(&b, nil, noCoverPkg, true, true); err != nil {
 			t.Fatalf("EmitPercent returned %v", err)
 		}
 		wantPercent := strings.Fields(`

src/internal/coverage/cformat/format.go

@@ -23,7 +23,7 @@ package cformat
 //				}
 //			}
 //		}
-//		myformatter.EmitPercent(os.Stdout, "", true, true)
+//		myformatter.EmitPercent(os.Stdout, nil, "", true, true)
 //		myformatter.EmitTextual(somefile)
 //
 // These apis are linked into tests that are built with "-cover", and
@@ -199,17 +199,21 @@ func (fm *Formatter) EmitTextual(w io.Writer) error {
 	return nil
 }
 
-// EmitPercent writes out a "percentage covered" string to the writer 'w'.
-func (fm *Formatter) EmitPercent(w io.Writer, covpkgs string, noteEmpty bool, aggregate bool) error {
-	pkgs := make([]string, 0, len(fm.pm))
-	for importpath := range fm.pm {
-		pkgs = append(pkgs, importpath)
+// EmitPercent writes out a "percentage covered" string to the writer
+// 'w', selecting the set of packages in 'pkgs' and suffixing the
+// printed string with 'inpkgs'.
+func (fm *Formatter) EmitPercent(w io.Writer, pkgs []string, inpkgs string, noteEmpty bool, aggregate bool) error {
+	if len(pkgs) == 0 {
+		pkgs = make([]string, 0, len(fm.pm))
+		for importpath := range fm.pm {
+			pkgs = append(pkgs, importpath)
+		}
 	}
 
 	rep := func(cov, tot uint64) error {
 		if tot != 0 {
 			if _, err := fmt.Fprintf(w, "coverage: %.1f%% of statements%s\n",
-				100.0*float64(cov)/float64(tot), covpkgs); err != nil {
+				100.0*float64(cov)/float64(tot), inpkgs); err != nil {
 				return err
 			}
 		} else if noteEmpty {
@@ -224,6 +228,9 @@ func (fm *Formatter) EmitPercent(w io.Writer, covpkgs string, noteEmpty bool, ag
 	var totalStmts, coveredStmts uint64
 	for _, importpath := range pkgs {
 		p := fm.pm[importpath]
+		if p == nil {
+			continue
+		}
 		if !aggregate {
 			totalStmts, coveredStmts = 0, 0
 		}