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.

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);

libstdc++-v3/testsuite/27_io/basic_istream/ignore/char/93672.cc

@@ -0,0 +1,101 @@
+// { dg-do run }
+
+#include <sstream>
+#include <limits>
+#include <testsuite_hooks.h>
+
+void
+test_pr93672() // std::basic_istream::ignore hangs if delim MSB is set
+{
+  std::istringstream in(".\xfc..\xfd...\xfe.");
+
+  // This should find '\xfd' even on platforms where char is signed,
+  // because the delimiter is correctly converted to the stream's int_type.
+  in.ignore(100, std::char_traits<char>::to_int_type('\xfc'));
+  VERIFY( in.gcount() == 2 );
+  VERIFY( ! in.eof() );
+
+  // This should work equivalently to traits_type::to_int_type
+  in.ignore(100, (unsigned char)'\xfd');
+  VERIFY( in.gcount() == 3 );
+  VERIFY( ! in.eof() );
+
+  // This only works if char is unsigned.
+  in.ignore(100, '\xfe');
+  if (std::numeric_limits<char>::is_signed)
+  {
+    // When char is signed, '\xfe' != traits_type::to_int_type('\xfe')
+    // so the delimiter does not match the character in the input sequence,
+    // and ignore consumes all input until EOF.
+    VERIFY( in.gcount() == 5 );
+    VERIFY( in.eof() );
+  }
+  else
+  {
+    // When char is unsigned, '\xfe' == to_int_type('\xfe') so the delimiter
+    // matches the character in the input sequence, and doesn't reach EOF.
+    VERIFY( in.gcount() == 4 );
+    VERIFY( ! in.eof() );
+  }
+
+  in.clear();
+  in.str(".a.");
+  in.ignore(100, 'a' + 256); // Should not match 'a'
+  VERIFY( in.gcount() == 3 );
+  VERIFY( in.eof() );
+}
+
+// Custom traits type that inherits all behaviour from std::char_traits<char>.
+struct traits : std::char_traits<char> { };
+
+void
+test_primary_template()
+{
+  // Check that the primary template for std::basic_istream::ignore
+  // works the same as the std::istream::ignore specialization.
+  // The infinite loop bug was never present in the primary template,
+  // because it doesn't use traits_type::find to search the input sequence.
+
+  std::basic_istringstream<char, traits> in(".\xfc..\xfd...\xfe.");
+
+  // This should find '\xfd' even on platforms where char is signed,
+  // because the delimiter is correctly converted to the stream's int_type.
+  in.ignore(100, std::char_traits<char>::to_int_type('\xfc'));
+  VERIFY( in.gcount() == 2 );
+  VERIFY( ! in.eof() );
+
+  // This should work equivalently to traits_type::to_int_type
+  in.ignore(100, (unsigned char)'\xfd');
+  VERIFY( in.gcount() == 3 );
+  VERIFY( ! in.eof() );
+
+  // This only works if char is unsigned.
+  in.ignore(100, '\xfe');
+  if (std::numeric_limits<char>::is_signed)
+  {
+    // When char is signed, '\xfe' != traits_type::to_int_type('\xfe')
+    // so the delimiter does not match the character in the input sequence,
+    // and ignore consumes all input until EOF.
+    VERIFY( in.gcount() == 5 );
+    VERIFY( in.eof() );
+  }
+  else
+  {
+    // When char is unsigned, '\xfe' == to_int_type('\xfe') so the delimiter
+    // matches the character in the input sequence, and doesn't reach EOF.
+    VERIFY( in.gcount() == 4 );
+    VERIFY( ! in.eof() );
+  }
+
+  in.clear();
+  in.str(".a.");
+  in.ignore(100, 'a' + 256); // Should not match 'a'
+  VERIFY( in.gcount() == 3 );
+  VERIFY( in.eof() );
+}
+
+int main()
+{
+  test_pr93672();
+  test_primary_template();
+}

libstdc++-v3/testsuite/27_io/basic_istream/ignore/wchar_t/93672.cc

@@ -0,0 +1,34 @@
+// { dg-do run }
+
+#include <sstream>
+#include <limits>
+#include <climits>
+#include <testsuite_hooks.h>
+
+// PR 93672 was a bug in std::istream that never affected std::wistream.
+// This test ensures that the bug doesn't get introduced to std::wistream.
+void
+test_pr93672()
+{
+  std::wstring str = L".x..x.";
+  str[1] = (wchar_t)-2;
+  str[4] = (wchar_t)-3;
+  std::wistringstream in(str);
+
+  // This should find the character even on platforms where wchar_t is signed,
+  // because the delimiter is correctly converted to the stream's int_type.
+  in.ignore(100, std::char_traits<wchar_t>::to_int_type((wchar_t)-2));
+  VERIFY( in.gcount() == 2 );
+  VERIFY( ! in.eof() );
+
+  // This also works, because std::char_traits<wchar_t>::to_int_type(wc) is
+  // equivalent to (int_type)wc so using to_int_type isn't needed.
+  in.ignore(100, (wchar_t)-3);
+  VERIFY( in.gcount() == 3 );
+  VERIFY( ! in.eof() );
+}
+
+int main()
+{
+  test_pr93672();
+}