Add tstring free test

Commit cd179d3
("Ensure proper validation checks are done before freeing a tstring")
was supposed to have included a test.  Add it.

Signed-off-by: Eugene Loh <eugene.loh@oracle.com>
Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>

libdtrace/dt_cg.c

@@ -877,6 +877,7 @@ dt_cg_tstring_xfree(dt_pcb_t *pcb, uint64_t offset)
 	}
 
 	assert(i < DT_TSTRING_SLOTS);
+	assert(ts->in_use != 0);
 
 	ts->in_use = 0;
 }

test/unittest/codegen/tst.tstring_free.d

@@ -0,0 +1,29 @@
+/*
+ * Oracle Linux DTrace.
+ * Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
+ * Licensed under the Universal Permissive License v 1.0 as shown at
+ * http://oss.oracle.com/licenses/upl.
+ */
+
+/*
+ * Stress test some tstring-free cases.
+ */
+
+#pragma D option quiet
+
+BEGIN {
+	trace(
+	    (x = strjoin("abc", "def")) ==
+	    (y = strjoin("abc", "def")) ? 0 : 1);
+	trace(strjoin(
+	    (x = strjoin("abc", "def")),
+	    (y = strjoin("ABC", "DEF"))));
+	trace(substr(
+	    (x = strjoin("123", "456")),
+	    2));
+	trace(x = strjoin("abc", "def"));
+	trace(x =
+	    (y = strjoin("ABC", "DEF")));
+
+	exit(0);
+}

test/unittest/codegen/tst.tstring_free.r

@@ -0,0 +1 @@
+0abcdefABCDEF3456abcdefABCDEF