go/format: handle whitespace-only input correctly

Applied identical change to cmd/gofmt/internal.go.

Fixes #11275.

Change-Id: Icb4bf0460c94c9e2830dd0d62c69376774cbda30
Reviewed-on: https://go-review.googlesource.com/15154Reviewed-by: Alan Donovan <adonovan@google.com>

src/cmd/gofmt/internal.go

@@ -149,7 +149,17 @@ func format(
 	if err != nil {
 		return nil, err
 	}
-	res = append(res, sourceAdj(buf.Bytes(), cfg.Indent)...)
+	out := sourceAdj(buf.Bytes(), cfg.Indent)
+
+	// If the adjusted output is empty, the source
+	// was empty but (possibly) for white space.
+	// The result is the incoming source.
+	if len(out) == 0 {
+		return src, nil
+	}
+
+	// Otherwise, append output to leading space.
+	res = append(res, out...)
 
 	// Determine and append trailing space.
 	i = len(src)

src/go/format/format_test.go

@@ -72,6 +72,7 @@ func TestSource(t *testing.T) {
 }
 
 // Test cases that are expected to fail are marked by the prefix "ERROR".
+// The formatted result must look the same as the input for successful tests.
 var tests = []string{
 	// declaration lists
 	`import "go/format"`,
@@ -91,11 +92,23 @@ var tests = []string{
 	"\n\t\t\n\n\t\t\tx := 0\n\t\t\tconst s = `\n\t\tfoo\n`\n\n\n", // no indentation removed inside raw strings
 
 	// comments
-	"i := 5 /* Comment */",         // Issue 5551.
-	"\ta()\n//line :1",             // Issue 11276.
-	"\t//xxx\n\ta()\n//line :2",    // Issue 11276.
-	"\ta() //line :1\n\tb()\n",     // Issue 11276.
-	"x := 0\n//line :1\n//line :2", // Issue 11276.
+	"/* Comment */",
+	"\t/* Comment */ ",
+	"\n/* Comment */ ",
+	"i := 5 /* Comment */",         // issue #5551
+	"\ta()\n//line :1",             // issue #11276
+	"\t//xxx\n\ta()\n//line :2",    // issue #11276
+	"\ta() //line :1\n\tb()\n",     // issue #11276
+	"x := 0\n//line :1\n//line :2", // issue #11276
+
+	// whitespace
+	"",     // issue #11275
+	" ",    // issue #11275
+	"\t",   // issue #11275
+	"\t\t", // issue #11275
+	"\n",   // issue #11275
+	"\n\n", // issue #11275
+	"\t\n", // issue #11275
 
 	// erroneous programs
 	"ERROR1 + 2 +",

src/go/format/internal.go

@@ -149,7 +149,17 @@ func format(
 	if err != nil {
 		return nil, err
 	}
-	res = append(res, sourceAdj(buf.Bytes(), cfg.Indent)...)
+	out := sourceAdj(buf.Bytes(), cfg.Indent)
+
+	// If the adjusted output is empty, the source
+	// was empty but (possibly) for white space.
+	// The result is the incoming source.
+	if len(out) == 0 {
+		return src, nil
+	}
+
+	// Otherwise, append output to leading space.
+	res = append(res, out...)
 
 	// Determine and append trailing space.
 	i = len(src)