cmd/compile/internal/importer: exportdata section ends with the last index of "\n$$\n"

This fixes a bug in the test only function Import where it looked for
the first instance of the string "\n$$\n" as the end of the exportdata
section. This should look for the last instance of "\n$$\n" within
the ar file.

Adds unit tests that demonstrate the error.

Added comments to tests that can correctly use the first instance.

Change-Id: I7a85afa41cf1c2902119516b757b7c6625d46d13
Reviewed-on: https://go-review.googlesource.com/c/go/+/626775
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>

src/cmd/compile/internal/importer/gcimporter.go

@@ -240,8 +240,8 @@ func Import(packages map[string]*types2.Package, path, srcDir string, lookup fun
 		// appropriate importer.
 		switch exportFormat {
 		case 'u':
-			// TODO(taking): Look into whether this should be LastIndex instead of Index.
-			s = s[:strings.Index(s, "\n$$\n")]
+			// exported strings may contain "\n$$\n" - search backwards
+			s = s[:strings.LastIndex(s, "\n$$\n")]
 			input := pkgbits.NewPkgDecoder(id, s)
 			pkg = ReadPackage(nil, packages, input)
 		default:

src/cmd/compile/internal/importer/gcimporter_test.go

@@ -204,6 +204,8 @@ func TestVersionHandling(t *testing.T) {
 		}
 		// 2) find export data
 		i := bytes.Index(data, []byte("\n$$B\n")) + 5
+		// Export data can contain "\n$$\n" in string constants, however,
+		// searching for the next end of section marker "\n$$\n" is good enough for testzs.
 		j := bytes.Index(data[i:], []byte("\n$$\n")) + i
 		if i < 0 || j < 0 || i > j {
 			t.Fatalf("export data section not found (i = %d, j = %d)", i, j)

src/cmd/compile/internal/importer/testdata/exports.go

@@ -26,6 +26,7 @@ const (
 	C8          = 42
 	C9  int     = 42
 	C10 float64 = 42
+	C11         = "\n$$\n" // an object file export data marker - export data extraction must not be led astray
 )
 
 type (

src/go/internal/gcimporter/gcimporter_test.go

@@ -329,6 +329,8 @@ func TestVersionHandling(t *testing.T) {
 		}
 		// 2) find export data
 		i := bytes.Index(data, []byte("\n$$B\n")) + 5
+		// Export data can contain "\n$$\n" in string constants, however,
+		// searching for the next end of section marker "\n$$\n" is good enough for tests.
 		j := bytes.Index(data[i:], []byte("\n$$\n")) + i
 		if i < 0 || j < 0 || i > j {
 			t.Fatalf("export data section not found (i = %d, j = %d)", i, j)

src/go/internal/gcimporter/testdata/exports.go

@@ -26,6 +26,7 @@ const (
 	C8          = 42
 	C9  int     = 42
 	C10 float64 = 42
+	C11         = "\n$$\n" // an object file export data marker - export data extraction must not be led astray
 )
 
 type (