internal/coverage: refactor EmitTextual in preparation for bugfix

Refactor cformat.EmitTextual 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 #70244.

Change-Id: I1e6bcbfb5e68187d4d69d54b667e97bc1fdfa2d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/627315
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/covdata/dump.go

@@ -331,7 +331,7 @@ func (d *dstate) Finish() {
 			d.format.EmitFuncs(os.Stdout)
 		}
 		if d.textfmtoutf != nil {
-			if err := d.format.EmitTextual(d.textfmtoutf); err != nil {
+			if err := d.format.EmitTextual(nil, d.textfmtoutf); err != nil {
 				fatal("writing to %s: %v", *textfmtoutflag, err)
 			}
 		}

src/internal/coverage/cfile/testsupport.go

@@ -109,7 +109,7 @@ func ProcessCoverTestDir(dir string, cfile string, cm string, cpkg string, w io.
 
 	// Emit text output.
 	if tf != nil {
-		if err := ts.cf.EmitTextual(tf); err != nil {
+		if err := ts.cf.EmitTextual(nil, tf); err != nil {
 			return err
 		}
 		tfClosed = true

src/internal/coverage/cformat/fmt_test.go

@@ -47,8 +47,8 @@ func TestBasics(t *testing.T) {
 		fm.AddUnit("lit.go", "f3", true, u, 0)
 	}
 
-	var b1, b2, b3, b4 strings.Builder
-	if err := fm.EmitTextual(&b1); err != nil {
+	var b1, b2, b3, b4, b5 strings.Builder
+	if err := fm.EmitTextual(nil, &b1); err != nil {
 		t.Fatalf("EmitTextual returned %v", err)
 	}
 	wantText := strings.TrimSpace(`
@@ -64,6 +64,18 @@ lit.go:99.0,100.0 1 0`)
 		t.Errorf("emit text: got:\n%s\nwant:\n%s\n", gotText, wantText)
 	}
 
+	selected := []string{"my/pack2"}
+	if err := fm.EmitTextual(selected, &b5); err != nil {
+		t.Fatalf("EmitTextual returned %v", err)
+	}
+	wantText = strings.TrimSpace(`
+mode: atomic
+lit.go:99.0,100.0 1 0`)
+	gotText = strings.TrimSpace(b5.String())
+	if wantText != gotText {
+		t.Errorf("emit text: got:\n%s\nwant:\n%s\n", gotText, wantText)
+	}
+
 	// Percent output with no aggregation.
 	noCoverPkg := ""
 	if err := fm.EmitPercent(&b2, nil, noCoverPkg, false, false); err != nil {

src/internal/coverage/cformat/format.go

@@ -24,7 +24,7 @@ package cformat
 //			}
 //		}
 //		myformatter.EmitPercent(os.Stdout, nil, "", true, true)
-//		myformatter.EmitTextual(somefile)
+//		myformatter.EmitTextual(nil, somefile)
 //
 // These apis are linked into tests that are built with "-cover", and
 // called at the end of test execution to produce text output or
@@ -38,6 +38,7 @@ import (
 	"io"
 	"maps"
 	"slices"
+	"sort"
 	"strings"
 	"text/tabwriter"
 )
@@ -163,20 +164,31 @@ func (p *pstate) sortUnits(units []extcu) {
 	})
 }
 
-// EmitTextual writes the accumulated coverage data in the legacy
-// cmd/cover text format to the writer 'w'. We sort the data items by
+// EmitTextual writes the accumulated coverage data for 'pkgs' in the legacy
+// cmd/cover text format to the writer 'w'; if pkgs is empty, text output
+// is emitted for all packages recorded.  We sort the data items by
 // importpath, source file, and line number before emitting (this sorting
 // is not explicitly mandated by the format, but seems like a good idea
 // for repeatable/deterministic dumps).
-func (fm *Formatter) EmitTextual(w io.Writer) error {
+func (fm *Formatter) EmitTextual(pkgs []string, w io.Writer) error {
 	if fm.cm == coverage.CtrModeInvalid {
 		panic("internal error, counter mode unset")
 	}
+	if len(pkgs) == 0 {
+		pkgs = make([]string, 0, len(fm.pm))
+		for importpath := range fm.pm {
+			pkgs = append(pkgs, importpath)
+		}
+	}
 	if _, err := fmt.Fprintf(w, "mode: %s\n", fm.cm.String()); err != nil {
 		return err
 	}
-	for _, importpath := range slices.Sorted(maps.Keys(fm.pm)) {
+	sort.Strings(pkgs)
+	for _, importpath := range pkgs {
 		p := fm.pm[importpath]
+		if p == nil {
+			continue
+		}
 		units := make([]extcu, 0, len(p.unitTable))
 		for u := range p.unitTable {
 			units = append(units, u)