cmd/go: open correct path when loading embeds from root directory

The existing implementation of `load.resolveEmbed`
uses an expression like `path[len(pkgdir)+1:]`.
Though the `+1` is intended to remove a prefix slash,
the expression returns an incorrect path when `pkgdir`
is "/". (ex.: when removing "/" from "/foo", want "foo",
but got "oo")

It seems that `str.TrimFilePathPrefix` would solve
the problem, but the function contains the same bug.

So, this commit fixes `str.TrimFilePathPrefix` then
applies it to `load.resolveEmbed` to solve the issue.
The fix is quite simple. First, remove prefix. Then
check whether the remained first letter is equal to
`filepath.Separator`. If so, remove it then return.

Fixed #49570

Change-Id: I26ab727ee4dfcbf51ed9bd0a573957ced2154515
Reviewed-on: https://go-review.googlesource.com/c/go/+/396694
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/internal/load/pkg.go

@@ -2056,7 +2056,8 @@ func resolveEmbed(pkgdir string, patterns []string) (files []string, pmap map[st
 		// then there may be other things lying around, like symbolic links or .git directories.)
 		var list []string
 		for _, file := range match {
-			rel := filepath.ToSlash(file[len(pkgdir)+1:]) // file, relative to p.Dir
+			// relative path to p.Dir which begins without prefix slash
+			rel := filepath.ToSlash(str.TrimFilePathPrefix(file, pkgdir))
 
 			what := "file"
 			info, err := fsys.Lstat(file)
@@ -2106,7 +2107,7 @@ func resolveEmbed(pkgdir string, patterns []string) (files []string, pmap map[st
 					if err != nil {
 						return err
 					}
-					rel := filepath.ToSlash(path[len(pkgdir)+1:])
+					rel := filepath.ToSlash(str.TrimFilePathPrefix(path, pkgdir))
 					name := info.Name()
 					if path != file && (isBadEmbedName(name) || ((name[0] == '.' || name[0] == '_') && !all)) {
 						// Ignore bad names, assuming they won't go into modules.

src/cmd/go/internal/str/path.go

@@ -58,8 +58,11 @@ func TrimFilePathPrefix(s, prefix string) string {
 	if !HasFilePathPrefix(s, prefix) {
 		return s
 	}
-	if len(s) == len(prefix) {
-		return ""
+	trimmed := s[len(prefix):]
+	if len(trimmed) == 0 || trimmed[0] != filepath.Separator {
+		// Prefix either is equal to s, or ends with a separator
+		// (for example, if it is exactly "/").
+		return trimmed
 	}
-	return s[len(prefix)+1:]
+	return trimmed[1:]
 }

src/cmd/go/internal/str/str_test.go

@@ -5,6 +5,8 @@
 package str
 
 import (
+	"os"
+	"runtime"
 	"testing"
 )
 
@@ -27,3 +29,72 @@ func TestFoldDup(t *testing.T) {
 		}
 	}
 }
+
+type trimFilePathPrefixTest struct {
+	s, prefix, want string
+}
+
+func TestTrimFilePathPrefixSlash(t *testing.T) {
+	if os.PathSeparator != '/' {
+		t.Skipf("test requires slash-separated file paths")
+	}
+	tests := []trimFilePathPrefixTest{
+		{"/foo", "", "foo"},
+		{"/foo", "/", "foo"},
+		{"/foo", "/foo", ""},
+		{"/foo/bar", "/foo", "bar"},
+		{"/foo/bar", "/foo/", "bar"},
+		// if prefix is not s's prefix, return s
+		{"/foo", "/bar", "/foo"},
+		{"/foo", "/foo/bar", "/foo"},
+	}
+
+	for _, tt := range tests {
+		if got := TrimFilePathPrefix(tt.s, tt.prefix); got != tt.want {
+			t.Errorf("TrimFilePathPrefix(%q, %q) = %q, want %q", tt.s, tt.prefix, got, tt.want)
+		}
+	}
+}
+
+func TestTrimFilePathPrefixWindows(t *testing.T) {
+	if runtime.GOOS != "windows" {
+		t.Skipf("test requires Windows file paths")
+	}
+	tests := []trimFilePathPrefixTest{
+		{`C:\foo`, `C:`, `foo`},
+		{`C:\foo`, `C:\`, `foo`},
+		{`C:\foo`, `C:\foo`, ``},
+		{`C:\foo\bar`, `C:\foo`, `bar`},
+		{`C:\foo\bar`, `C:\foo\`, `bar`},
+		// if prefix is not s's prefix, return s
+		{`C:\foo`, `C:\bar`, `C:\foo`},
+		{`C:\foo`, `C:\foo\bar`, `C:\foo`},
+		// if volumes are different, return s
+		{`C:\foo`, ``, `C:\foo`},
+		{`C:\foo`, `\foo`, `C:\foo`},
+		{`C:\foo`, `D:\foo`, `C:\foo`},
+
+		//UNC path
+		{`\\host\share\foo`, `\\host\share`, `foo`},
+		{`\\host\share\foo`, `\\host\share\`, `foo`},
+		{`\\host\share\foo`, `\\host\share\foo`, ``},
+		{`\\host\share\foo\bar`, `\\host\share\foo`, `bar`},
+		{`\\host\share\foo\bar`, `\\host\share\foo\`, `bar`},
+		// if prefix is not s's prefix, return s
+		{`\\host\share\foo`, `\\host\share\bar`, `\\host\share\foo`},
+		{`\\host\share\foo`, `\\host\share\foo\bar`, `\\host\share\foo`},
+		// if either host or share name is different, return s
+		{`\\host\share\foo`, ``, `\\host\share\foo`},
+		{`\\host\share\foo`, `\foo`, `\\host\share\foo`},
+		{`\\host\share\foo`, `\\host\other\`, `\\host\share\foo`},
+		{`\\host\share\foo`, `\\other\share\`, `\\host\share\foo`},
+		{`\\host\share\foo`, `\\host\`, `\\host\share\foo`},
+		{`\\host\share\foo`, `\share\`, `\\host\share\foo`},
+	}
+
+	for _, tt := range tests {
+		if got := TrimFilePathPrefix(tt.s, tt.prefix); got != tt.want {
+			t.Errorf("TrimFilePathPrefix(%q, %q) = %q, want %q", tt.s, tt.prefix, got, tt.want)
+		}
+	}
+}