fix panic from eval {} inside /(?{...})/

GH #19680

Normally in code like

    eval {.... };

then even if the eval is the last statement in the file or sub, the
OP_LEAVETRY isn't the last op in the execution path: it's followed
by an OP_LEAVE or OP_LEAVESUB or whatever, which will be the op to
resume execution from after an exception is caught.

However, if the eval is the *last* thing within a regex code block:

    /(?{ ...; eval {....}; })/

then the op_next pointer of the OP_LEAVETRY op is actually NULL.

This confused S_docatch(), which wrongly assumed that a NULL
PL_restartop indicated that the caught exception should be rethrown,
popping execution back to the outer perl_run() call and hence leading to
the confused panic warning:

    "panic: restartop in perl_run"

The fix is to to separate out the "do we need to re-throw" test,
(PL_restartjmpenv != PL_top_env), from the "no more ops so no need to
re-enter the runops loop" test, (!PL_restartop).

Author: iabyn

pp_ctl.c

@@ -3350,8 +3350,11 @@ S_docatch(pTHX_ Perl_ppaddr_t firstpp)
         CALLRUNOPS(aTHX);
         break;
     case 3:
-        /* die caught by an inner eval - continue inner loop */
-        if (PL_restartop && PL_restartjmpenv == PL_top_env) {
+        if (PL_restartjmpenv == PL_top_env) {
+            /* die caught by an inner eval - continue inner loop */
+
+            if (!PL_restartop)
+                break;
             PL_restartjmpenv = NULL;
             PL_op = PL_restartop;
             PL_restartop = 0;

t/re/pat_re_eval.t

@@ -24,7 +24,7 @@ BEGIN {
 
 our @global;
 
-plan tests => 506;  # Update this when adding/deleting tests.
+plan tests => 507;  # Update this when adding/deleting tests.
 
 run_tests() unless caller;
 
@@ -1332,6 +1332,15 @@ sub run_tests {
         pass("SvTEMP 2");
     }
 
+    # GH #19680 "panic: restartop in perl_run"
+    # The eval block embedded within the (?{}) - but with no more code
+    # following it - causes the next op after the OP_LEAVETRY to be NULL
+    # (not even an OP_LEAVE). This confused the exception-catching and
+    # rethrowing code: it was incorrectly rethrowing the exception rather
+    # than just stopping at that point.
+
+    ok("test" =~ m{^ (?{eval {die "boo!"}}) test $}x, "GH #19680");
+
 } # End of sub run_tests
 
 1;