cmd/internal/objabi: add inverse of PathToPrefix

Add PrefixToPath, which can be used to convert a package path in a
symbol name back to the original package path.

For golang#61577.

Change-Id: Ifbe8c852a7f41ff9b81ad48b92a26a0e1b046777
Reviewed-on: https://go-review.googlesource.com/c/go/+/529557
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/cmd/internal/objabi/path.go

@@ -4,7 +4,11 @@
 
 package objabi
 
-import "strings"
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
 
 // PathToPrefix converts raw string to the prefix that will be used in the
 // symbol table. All control characters, space, '%' and '"', as well as
@@ -39,3 +43,36 @@ func PathToPrefix(s string) string {
 
 	return string(p)
 }
+
+// PrefixToPath is the inverse of PathToPrefix, replacing escape sequences with
+// the original character.
+func PrefixToPath(s string) (string, error) {
+	percent := strings.IndexByte(s, '%')
+	if percent == -1 {
+		return s, nil
+	}
+
+	p := make([]byte, 0, len(s))
+	for i := 0; i < len(s); {
+		if s[i] != '%' {
+			p = append(p, s[i])
+			i++
+			continue
+		}
+		if i+2 >= len(s) {
+			// Not enough characters remaining to be a valid escape
+			// sequence.
+			return "", fmt.Errorf("malformed prefix %q: escape sequence must contain two hex digits", s)
+		}
+
+		b, err := strconv.ParseUint(s[i+1:i+3], 16, 8)
+		if err != nil {
+			// Not a valid escape sequence.
+			return "", fmt.Errorf("malformed prefix %q: escape sequence %q must contain two hex digits", s, s[i:i+3])
+		}
+
+		p = append(p, byte(b))
+		i += 3
+	}
+	return string(p), nil
+}

src/cmd/internal/objabi/path_test.go

@@ -11,11 +11,11 @@ import (
 	"testing"
 )
 
-func TestPathToPrefix(t *testing.T) {
-	tests := []struct {
-		Path     string
-		Expected string
-	}{{"foo/bar/v1", "foo/bar/v1"},
+var escapeTests = []struct {
+		Path    string
+		Escaped string
+	}{
+		{"foo/bar/v1", "foo/bar/v1"},
 		{"foo/bar/v.1", "foo/bar/v%2e1"},
 		{"f.o.o/b.a.r/v1", "f.o.o/b.a.r/v1"},
 		{"f.o.o/b.a.r/v.1", "f.o.o/b.a.r/v%2e1"},
@@ -30,9 +30,38 @@ func TestPathToPrefix(t *testing.T) {
 		{"%foo%bar", "%25foo%25bar"},
 		{"\x01\x00\x7F☺", "%01%00%7f%e2%98%ba"},
 	}
+
+func TestPathToPrefix(t *testing.T) {
+	for _, tc := range escapeTests {
+		if got := PathToPrefix(tc.Path); got != tc.Escaped {
+			t.Errorf("expected PathToPrefix(%s) = %s, got %s", tc.Path, tc.Escaped, got)
+		}
+	}
+}
+
+func TestPrefixToPath(t *testing.T) {
+	for _, tc := range escapeTests {
+		got, err := PrefixToPath(tc.Escaped)
+		if err != nil {
+			t.Errorf("expected PrefixToPath(%s) err = nil, got %v", tc.Escaped, err)
+		}
+		if got != tc.Path {
+			t.Errorf("expected PrefixToPath(%s) = %s, got %s", tc.Escaped, tc.Path, got)
+		}
+	}
+}
+
+func TestPrefixToPathError(t *testing.T) {
+	tests := []string{
+		"foo%",
+		"foo%1",
+		"foo%%12",
+		"foo%1g",
+	}
 	for _, tc := range tests {
-		if got := PathToPrefix(tc.Path); got != tc.Expected {
-			t.Errorf("expected PathToPrefix(%s) = %s, got %s", tc.Path, tc.Expected, got)
+		_, err := PrefixToPath(tc)
+		if err == nil {
+			t.Errorf("expected PrefixToPath(%s) err != nil, got nil", tc)
 		}
 	}
 }