libstdc++/8071: basic_ostream::operator<<(streambuf*) loops forever if streambuf::underflow() leaves gptr() NULL

[lennox@cs.columbia.edu]

>Number:         8071
>Category:       libstdc++
>Synopsis:       basic_ostream::operator<<(streambuf*) loops forever if streambuf::underflow() leaves gptr() NULL
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Sep 27 08:56:01 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     lennox@cs.columbia.edu
>Release:        gcc-3.1
>Organization:
>Environment:
sparc-sun-solaris2.8
>Description:
According to my reading of the C++ standard, a subclass of std::basic_streambuf<> is allowed to declare its uflow() and underflow() functions such that they don't fill their buffers.  This is useful, for instance, if the streambuf is a wrapper around another type that does its own buffering.

However, libstdc++'s basic_ostream::operator<<(streambuf*) operator loops forever when passed such a streambuf, repeatedly invoking underflow() and never actually extracting any data from the buffer.

I've attached some code written by Matt Austern, and published in the C++ Users Journal.  This code creates a streambuf and istream that wraps a C FILE*, so I/O can safely be intermixed on the two types.  Using operator<< on the syncbuf this code declares illustrates the problem.
>How-To-Repeat:
Compile the attached code.  (It looks like gnats won't allow me to attach more than one file in the initial bug report, so I've just attached the header.  I'll attach the body momentarily.)
>Fix:
I think the fix needs to change basic_streambuf::__copy_streambufs, but I'm not sure exactly what the fix should be.
>Release-Note:
>Audit-Trail:
>Unformatted:
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