[flang][runtime] Enforce restrictions on unlimited format repetition

[klausler]

Summary

A repeated format item group with an unlimited ('*') repetition count
can appear only as the last item at the top level of a format; it can't
be nested in more parentheses and it can't be followed by anything
else.

flang/runtime/format-implementation.h

if (ch == '-' || ch == '+' || (ch >= '0' && ch <= '9')) {
       unlimited = true;
       ch = GetNextChar(context);
       if (ch != '(') {
         ReportBadFormat(context,
             "Invalid FORMAT: '*' may appear only before '('",
             maybeReversionPoint);
         return 0;
       }
+      if (height_ != 1) {
+        ReportBadFormat(context,
+            "Invalid FORMAT: '*' must be nested in exactly one set of "
+            "parentheses",
+            maybeReversionPoint);
+        return 0;
+      }
     }
     ch = Capitalize(ch);
     if (ch == '(') {
       if (height_ >= maxHeight_) {
         ReportBadFormat(context,
             "FORMAT stack overflow: too many nested parentheses",
             maybeReversionPoint);
         return 0;
if (ch == '(') {
         }
         context.AdvanceRecord(); // implied / before rightmost )
       }
       auto restart{stack_[height_ - 1].start};
       if (format_[restart] == '(') {
         ++restart;
       }
       if (stack_[height_ - 1].remaining == Iteration::unlimited) {
-        offset_ = restart;
+        if (height_ > 1 && GetNextChar(context) != ')') {
+          ReportBadFormat(context,
+              "Unlimited repetition in FORMAT may not be followed by more "
+              "items",
+              restart);
+          return 0;
+        }
         if (offset_ == unlimitedLoopCheck) {
           ReportBadFormat(context,
               "Unlimited repetition in FORMAT lacks data edit descriptors",
               restart);
+          return 0;
         }
+        offset_ = restart;
       } else if (stack_[height_ - 1].remaining-- > 0) {
         offset_ = restart;
       } else {
         --height_;
       }
     } else if (ch == '\'' || ch == '"') {
       // Quoted 'character literal'
       CharType quote{ch};

flang/unittests/Runtime/Format.cpp

static const std::vector<ParamsTy> params{
       {1, "('PI=',F9.7)", ResultsTy{"'PI='", "F9.7"}, 1},
       {1, "(3HPI=F9.7)", ResultsTy{"'PI='", "F9.7"}, 1},
       {1, "(3HPI=/F9.7)", ResultsTy{"'PI='", "/", "F9.7"}, 1},
       {2, "('PI=',F9.7)", ResultsTy{"'PI='", "F9.7", "/", "'PI='", "F9.7"}, 1},
       {2, "(2('PI=',F9.7),'done')",
           ResultsTy{"'PI='", "F9.7", "'PI='", "F9.7", "'done'"}, 1},
       {2, "(3('PI=',F9.7,:),'tooFar')",
           ResultsTy{"'PI='", "F9.7", "'PI='", "F9.7"}, 1},
-      {2, "(*('PI=',F9.7,:),'tooFar')",
-          ResultsTy{"'PI='", "F9.7", "'PI='", "F9.7"}, 1},
+      {2, "(*('PI=',F9.7,:))", ResultsTy{"'PI='", "F9.7", "'PI='", "F9.7"}, 1},
       {1, "(3F9.7)", ResultsTy{"2*F9.7"}, 2},
   };
 
   for (const auto &[n, format, expect, repeat] : params) {
     TestFormatContext context;
     FormatControl<decltype(context)> control{
         context, format, std::strlen(format)};