cmd/doc: show the package clause always

If no writes to the package buffer happen, then the package clause
does not get printed. This is a bug for cases where a file just contains
the package clause.

We fix this by separating the printing of package clause to a new
function and calling it from (*pkgBuffer).Write as well as (*Package).flush.

Updates #31457

Change-Id: Ia3bd0ea3963274c460a45d1e37fafc6ee0a197f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/206128
Run-TryBot: Agniva De Sarker <agniva.quicksilver@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/doc/doc_test.go

@@ -211,6 +211,13 @@ var tests = []test{
 			`func \(unexportedType\)`,
 		},
 	},
+	// Package with just the package declaration. Issue 31457.
+	{
+		"only package declaration",
+		[]string{"-all", p + "/nested/empty"},
+		[]string{`package empty .*import`},
+		nil,
+	},
 	// Package dump -short
 	{
 		"full package with -short",

src/cmd/doc/pkg.go

@@ -53,14 +53,18 @@ type pkgBuffer struct {
 }
 
 func (pb *pkgBuffer) Write(p []byte) (int, error) {
-	if !pb.printed && len(p) > 0 {
+	pb.packageClause()
+	return pb.Buffer.Write(p)
+}
+
+func (pb *pkgBuffer) packageClause() {
+	if !pb.printed {
 		pb.printed = true
 		// Only show package clause for commands if requested explicitly.
 		if pb.pkg.pkg.Name != "main" || showCmd {
 			pb.pkg.packageClause()
 		}
 	}
-	return pb.Buffer.Write(p)
 }
 
 type PackageError string // type returned by pkg.Fatalf.
@@ -210,6 +214,8 @@ func (pkg *Package) Printf(format string, args ...interface{}) {
 }
 
 func (pkg *Package) flush() {
+	// Print the package clause in case it wasn't written already.
+	pkg.buf.packageClause()
 	_, err := pkg.writer.Write(pkg.buf.Bytes())
 	if err != nil {
 		log.Fatal(err)

src/cmd/doc/testdata/nested/empty/empty.go

@@ -0,0 +1 @@
+package empty