Re: [PATCH] libstdc++: istreambuf_iterator keep attached streambuf

[Petr Ovtchenkov <ptr at void-ptr dot info>]

On Thu, 28 Sep 2017 13:38:06 +0100
Jonathan Wakely <jwakely@redhat.com> wrote:

> On 28/09/17 15:06 +0300, Petr Ovtchenkov wrote:
> >On Thu, 28 Sep 2017 11:34:25 +0100
> >Jonathan Wakely <jwakely@redhat.com> wrote:
> >
> >> On 23/09/17 09:54 +0300, Petr Ovtchenkov wrote:
> >> >istreambuf_iterator should not forget about attached
> >> >streambuf when it reach EOF.
> >> >
> >> >Checks in debug mode has no infuence more on character
> >> >extraction in istreambuf_iterator increment operators.
> >> >In this aspect behaviour in debug and non-debug mode
> >> >is similar now.
> >> >
> >> >Test for detached srteambuf in istreambuf_iterator:
> >> >When istreambuf_iterator reach EOF of istream, it should not
> >> >forget about attached streambuf.
> >> From fact "EOF in stream reached" not follow that
> >> >stream reach end of life and input operation impossible
> >> >more.
> >> >---
> >> > libstdc++-v3/include/bits/streambuf_iterator.h     | 41 +++++++--------
> >> > .../24_iterators/istreambuf_iterator/3.cc          | 61 ++++++++++++++++++++++
> >> > 2 files changed, 80 insertions(+), 22 deletions(-)
> >> > create mode 100644 libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/3.cc
> >> >
> >> >diff --git a/libstdc++-v3/include/bits/streambuf_iterator.h
> >> >b/libstdc++-v3/include/bits/streambuf_iterator.h index f0451b1..45c3d89 100644
> >> >--- a/libstdc++-v3/include/bits/streambuf_iterator.h
> >> >+++ b/libstdc++-v3/include/bits/streambuf_iterator.h
> >> >@@ -136,12 +136,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> >       istreambuf_iterator&
> >> >       operator++()
> >> >       {
> >> >-	__glibcxx_requires_cond(!_M_at_eof(),
> >> >+	__glibcxx_requires_cond(_M_sbuf,
> >> > 				_M_message(__gnu_debug::__msg_inc_istreambuf)
> >> > 				._M_iterator(*this));
> >> > 	if (_M_sbuf)
> >> > 	  {
> >> >+#ifdef _GLIBCXX_DEBUG_PEDANTIC
> >> >+	    int_type _tmp =
> >> >+#endif
> >> > 	    _M_sbuf->sbumpc();
> >> >+#ifdef _GLIBCXX_DEBUG_PEDANTIC
> >> >+	    __glibcxx_requires_cond(!traits_type::eq_int_type(_tmp,traits_type::eof()),
> >> >+				    _M_message(__gnu_debug::__msg_inc_istreambuf)
> >> >+				    ._M_iterator(*this));
> >> >+#endif
> >> > 	    _M_c = traits_type::eof();
> >> > 	  }
> >> > 	return *this;
> >> >@@ -151,14 +159,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> >       istreambuf_iterator
> >> >       operator++(int)
> >> >       {
> >> >-	__glibcxx_requires_cond(!_M_at_eof(),
> >> >+        _M_get();
> >> >+	__glibcxx_requires_cond(_M_sbuf
> >> >+				&& !traits_type::eq_int_type(_M_c,traits_type::eof()),
> >> > 				_M_message(__gnu_debug::__msg_inc_istreambuf)
> >> > 				._M_iterator(*this));
> >> >
> >> > 	istreambuf_iterator __old = *this;
> >> > 	if (_M_sbuf)
> >> > 	  {
> >> >-	    __old._M_c = _M_sbuf->sbumpc();
> >> >+	    _M_sbuf->sbumpc();
> >> > 	    _M_c = traits_type::eof();
> >> > 	  }
> >> > 	return __old;
> >> >@@ -177,18 +187,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> >       _M_get() const
> >> >       {
> >> > 	const int_type __eof = traits_type::eof();
> >> >-	int_type __ret = __eof;
> >> >-	if (_M_sbuf)
> >> >-	  {
> >> >-	    if (!traits_type::eq_int_type(_M_c, __eof))
> >> >-	      __ret = _M_c;
> >> >-	    else if (!traits_type::eq_int_type((__ret = _M_sbuf->sgetc()),
> >> >-					       __eof))
> >> >-	      _M_c = __ret;
> >> >-	    else
> >> >-	      _M_sbuf = 0;
> >> >-	  }
> >> >-	return __ret;
> >> >+	if (_M_sbuf && traits_type::eq_int_type(_M_c, __eof))
> >> >+          _M_c = _M_sbuf->sgetc();
> >> >+	return _M_c;
> >> >       }
> >> >
> >> >       bool
> >> >@@ -339,7 +340,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> >       typedef typename __is_iterator_type::streambuf_type  streambuf_type;
> >> >       typedef typename traits_type::int_type               int_type;
> >> >
> >> >-      if (__first._M_sbuf && !__last._M_sbuf)
> >> >+      if (__first._M_sbuf && (__last == istreambuf_iterator<_CharT>()))
> >> > 	{
> >> > 	  streambuf_type* __sb = __first._M_sbuf;
> >> > 	  int_type __c = __sb->sgetc();
> >> >@@ -374,7 +375,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> >       typedef typename __is_iterator_type::streambuf_type  streambuf_type;
> >> >       typedef typename traits_type::int_type               int_type;
> >> >
> >> >-      if (__first._M_sbuf && !__last._M_sbuf)
> >> >+      if (__first._M_sbuf && (__last == istreambuf_iterator<_CharT>()))
> >> > 	{
> >> > 	  const int_type __ival = traits_type::to_int_type(__val);
> >> > 	  streambuf_type* __sb = __first._M_sbuf;
> >> >@@ -395,11 +396,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >> > 	      else
> >> > 		__c = __sb->snextc();
> >> > 	    }
> >> >-
> >> >-	  if (!traits_type::eq_int_type(__c, traits_type::eof()))
> >> >-	    __first._M_c = __c;
> >> >-	  else
> >> >-	    __first._M_sbuf = 0;
> >> >+	  __first._M_c = __c;
> >> > 	}
> >> >       return __first;
> >> >     }
> >> >diff --git a/libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/3.cc
> >> >b/libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/3.cc new file mode 100644
> >> >index 0000000..803ede4
> >> >--- /dev/null
> >> >+++ b/libstdc++-v3/testsuite/24_iterators/istreambuf_iterator/3.cc
> >> >@@ -0,0 +1,61 @@
> >> >+// { dg-options "-std=gnu++17" }
> >> >+
> >> >+// Copyright (C) 2017 Free Software Foundation, Inc.
> >> >+//
> >> >+// This file is part of the GNU ISO C++ Library.  This library is free
> >> >+// software; you can redistribute it and/or modify it under the
> >> >+// terms of the GNU General Public License as published by the
> >> >+// Free Software Foundation; either version 3, or (at your option)
> >> >+// any later version.
> >> >+
> >> >+// This library is distributed in the hope that it will be useful,
> >> >+// but WITHOUT ANY WARRANTY; without even the implied warranty of
> >> >+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> >> >+// GNU General Public License for more details.
> >> >+
> >> >+// You should have received a copy of the GNU General Public License along
> >> >+// with this library; see the file COPYING3.  If not see
> >> >+// <http://www.gnu.org/licenses/>.
> >> >+
> >> >+#include <algorithm>
> >> >+#include <sstream>
> >> >+#include <iterator>
> >> >+#include <cstring>
> >> >+#include <testsuite_hooks.h>
> >> >+
> >> >+void test03()
> >> >+{
> >> >+  using namespace std;
> >> >+  bool test __attribute__((unused)) = true;
> 
> This variable should be removed, we don't need these variables any
> more.

Not a problem, if we will have consensus in principle.

> 
> >> >+  std::stringstream s;
> >> >+  char b[] = "c2ee3d09-43b3-466d-b490-db35999a22cf";
> >> >+  char r[] = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
> >> >+  char q[] = "3c4852d6-d47b-4f46-b05e-b5edc1aa440e";
> >> >+  //          012345678901234567890123456789012345
> >> >+  //          0         1         2         3
> >> >+  s << b;
> >> >+  VERIFY( !s.fail() );
> >> >+
> >> >+  istreambuf_iterator<char> i(s);
> >> >+  copy_n(i, 36, r);
> >> >+  ++i; // EOF reached
> >> >+  VERIFY(i == std::istreambuf_iterator<char>());
> >> >+
> >> >+  VERIFY(memcmp(b, r, 36) == 0);
> >> >+
> >> >+  s << q;
> >> >+  VERIFY(!s.fail());
> >> >+
> >> >+  copy_n(i, 36, r);
> >>
> >> This is undefined behaviour. The end-of-stream iterator value cannot
> >> be dereferenced.
> >
> >Within this test istreambuf_iterator in eof state never dereferenced.
> 
> That is quite implementation dependent.
> 
> The libc++ and VC++ implementations fail this test, because once an
> istreambuf_iterator has been detected to reach end-of-stream it
> doesn't get "reset" by changes to the streambuf.

If we will keep even "unspecified" behaviour same, then bug fix/drawback
removing become extremely hard: it should be identified as drawback
in all libs almost simultaneously.

> 
> The libc++ implementation crashes, because operator== on an
> end-of-stream iterator sets its streambuf* to null, and any further
> increment or dereference will segfault.
> 
> So this is testing something that other implementations don't support,
> and isn't justifiable from the standard.

I will use N4687 as reference.

27.2.3 par.2 Table 95:

++r

Requires: r is dereferenceable. Postconditions: r is dereferenceable or r is
past-the-end; any copies of the previous value of r are no longer required
either to be dereferenceable or to be in the domain of ==.

(void)r++ equivalent to (void)++r

*r++

{ T tmp = *r;
++r;
return tmp; }

[BTW, you see that r++ without dereference has no sense, and even more,

  copies of the previous
  value of r are no longer
  required either to be
  dereferenceable or to be in
  the domain of ==.

From this follow, that postfix increment operator shouldn't return
istreambuf_iterator.
]

> 
> >The test itself simulate "stop and go" istream usage.
> >stringstream is convenient for behaviuor illustration, but in "real life"
> >I can assume socket or tty on this place.
> 
> At the very minimum we should have a comment in the test explaining
> how it relies on non-standard, non-portable behaviour.
> 
> But I'd prefer to avoid introducing more divergence from other
> implementations.

Standard itself say nothting about "stop and go" scenario.
At least I don't see any words pro or contra.
But current implementation block usage of istreambuf_iterator
with underlying streams like socket or tty, so istreambuf_iterator become
almost useless structure for practice.

> 
> >debugmode-dependent behaviour is also issue in this patch;
> >I don't suggest it as a separate patch because solutions are intersect.

We have three issues with istreambuf_iterator:

  - debug-dependent behaviour
  - EOL of istreambuf_iterator when it reach EOF (but this not mean
    EOL of associated streambuf)
  - postfix increment operator return istreambuf_iterator, but here
    expected restricted type, that accept only dereference, if it possible.