ktest: Treat undefined self-reference as empty

Config variables are expanded when they are assigned. A first-time append
such as:

  VAR := ${VAR} foo

leaves the literal ${VAR} in the stored value because VAR has not been
defined yet. Later expansions then carry the self-reference forward instead
of behaving like an empty prefix.

Drop an unescaped self-reference when the variable has no current value,
and trim the outer whitespace left behind. Keep escaped \${VAR} references
unchanged so literal text still works.

Cc: John Hawley <[email protected]>
Cc: Andrea Righi <[email protected]>
Cc: Marcos Paulo de Souza <[email protected]>
Cc: Matthieu Baerts <[email protected]>
Cc: Fernando Fernandez Mancera <[email protected]>
Cc: Pedro Falcato <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Ricardo B. Marlière <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>

Author: rbmarliere

tools/testing/ktest/ktest.pl

@@ -910,6 +910,14 @@ sub set_variable {
     if (defined($command_tmp_vars{$lvalue})) {
 	return;
     }
+
+    # If a variable is undefined, treat an unescaped self-reference as empty.
+    if (!defined($variable{$lvalue})) {
+	$rvalue =~ s/(?<!\\)\$\{\Q$lvalue\E\}//g;
+	$rvalue =~ s/^\s+//;
+	$rvalue =~ s/\s+$//;
+    }
+
     if ($rvalue =~ /^\s*$/) {
 	delete $variable{$lvalue};
     } else {