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 #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>

1 files changed, 38 insertions, 1 deletions

diff --git a/src/cmd/internal/objabi/path.go b/src/cmd/internal/objabi/path.go
index 2a42179a36..30301b15f1 100644
--- a/src/cmd/internal/objabi/path.go
+++ b/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
+}