libstdc++: Fix infinite loop in std::istream::ignore(n, delim) [PR93672]

A negative delim value passed to std::istream::ignore can never match
any character in the stream, because the comparison is done using
traits_type::eq_int_type(sb->sgetc(), delim) and sgetc() never returns
negative values (except at EOF). The optimized version of ignore for the
std::istream specialization uses traits_type::find to locate the delim
character in the streambuf, which _can_ match a negative delim on
platforms where char is signed, but then we do another comparison using
eq_int_type which fails. The code then keeps looping forever, with
traits_type::find locating the character and traits_type::eq_int_type
saying it's not a match, so traits_type::find is used again and finds
the same character again.

A possible fix would be to check with eq_int_type after a successful
find, to see whether we really have a match. However, that would be
suboptimal since we know that a negative delimiter will never match
using eq_int_type. So a better fix is to adjust the check at the top of
the function that handles delim==eof(), so that we treat all negative
delim values as equivalent to EOF. That way we don't bother using find
to search for something that will never match with eq_int_type.

The version of ignore in the primary template doesn't need a change,
because it doesn't use traits_type::find, instead characters are
extracted one-by-one and always matched using eq_int_type. That avoids
the inconsistency between find and eq_int_type. The specialization for
std::wistream does use traits_type::find, but traits_type::to_int_type
is equivalent to an implicit conversion from wchar_t to wint_t, so
passing a wchar_t directly to ignore without using to_int_type works.

libstdc++-v3/ChangeLog:

	PR libstdc++/93672
	* src/c++98/istream.cc (istream::ignore(streamsize, int_type)):
	Treat all negative delimiter values as eof().
	* testsuite/27_io/basic_istream/ignore/char/93672.cc: New test.
	* testsuite/27_io/basic_istream/ignore/wchar_t/93672.cc: New
	test.

1 files changed, 11 insertions, 2 deletions

diff --git a/libstdc++-v3/src/c++98/istream.cc b/libstdc++-v3/src/c++98/istream.cc
index 07ac739..d1b4444 100644
--- a/libstdc++-v3/src/c++98/istream.cc
+++ b/libstdc++-v3/src/c++98/istream.cc
@@ -112,8 +112,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     basic_istream<char>::
     ignore(streamsize __n, int_type __delim)
     {
-      if (traits_type::eq_int_type(__delim, traits_type::eof()))
-	return ignore(__n);
+      {
+	// If conversion to int_type changes the value then __delim does not
+	// correspond to a value of type char_type, and so will never match
+	// a character extracted from the input sequence. Just use ignore(n).
+	const int_type chk_delim = traits_type::to_int_type(__delim);
+	const bool matchable = traits_type::eq_int_type(chk_delim, __delim);
+	if (__builtin_expect(!matchable, 0))
+	  return ignore(__n);
+	// Now we know that __delim is a valid char_type value, so it's safe
+	// for the code below to use traits_type::find to search for it.
+      }
 
       _M_gcount = 0;
       sentry __cerb(*this, true);