os: treat "${}" in Expand like in Go 1.10

CL 103055 made it so that invalid parameter expansions, like "$|", did
not make the dollar sign silently disappear.

A few edge cases were not taken into account, such as "${}" and "${",
which were now printing just "$". For consistency and to not break
existing programs, go back to eating up the characters when invalid
syntax is encountered.

For completeness, add a "$" test case too, even though its behavior is
unchanged by this CL.

Fixes #26135.

Change-Id: I5d25db9a8356dc6047a8502e318355113a99b247
Reviewed-on: https://go-review.googlesource.com/121636
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/env.go

@@ -24,8 +24,12 @@ func Expand(s string, mapping func(string) string) string {
 			}
 			buf = append(buf, s[i:j]...)
 			name, w := getShellName(s[j+1:])
-			// If the name is empty, keep the $.
-			if name == "" {
+			if name == "" && w > 0 {
+				// Encountered invalid syntax; eat the
+				// characters.
+			} else if name == "" {
+				// Valid syntax, but $ was not followed by a
+				// name. Leave the dollar character untouched.
 				buf = append(buf, s[j])
 			} else {
 				buf = append(buf, mapping(name)...)
@@ -74,10 +78,13 @@ func getShellName(s string) (string, int) {
 		// Scan to closing brace
 		for i := 1; i < len(s); i++ {
 			if s[i] == '}' {
+				if i == 1 {
+					return "", 2 // Bad syntax; eat "${}"
+				}
 				return s[1:i], i + 1
 			}
 		}
-		return "", 1 // Bad syntax; just eat the brace.
+		return "", 1 // Bad syntax; eat "${"
 	case isShellSpecialVar(s[0]):
 		return s[0:1], 1
 	}

src/os/env_test.go

@@ -51,6 +51,10 @@ var expandTests = []struct {
 	{"A$$$#$1$H$home_1*B", "APIDNARGSARGUMENT1(Value of H)/usr/foo*B"},
 	{"start$+middle$^end$", "start$+middle$^end$"},
 	{"mixed$|bag$$$", "mixed$|bagPID$"},
+	{"$", "$"},
+	{"$}", "$}"},
+	{"${", ""},  // invalid syntax; eat up the characters
+	{"${}", ""}, // invalid syntax; eat up the characters
 }
 
 func TestExpand(t *testing.T) {