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POLLIN and POLLHUP are not mutually exclusive. Currently the following seems possible: the child writes 3K to its stdout or stderr pipe, and immediately closes it. We get POLLIN|POLLHUP (I'm not sure that's possible on Linux, but SUSv4 seems to allow it). We read 1K and throw away the rest. When poll() returns and we're about to check the /revents/ member in a given array element, let's map all the revents bits to two (independent) ideas: "let's attempt to read()", and "let's attempt to write()". This should cover all errors, EOFs, and normal conditions; the read()/write() call should report any pending error. Under this approach, both POLLHUP and POLLERR are mapped to "needs read()" if we're otherwise prepared for POLLIN. POLLERR also maps to "needs write()" if we're otherwise prepared for POLLOUT. The rest of the mappings (POLLPRI etc.) would be easy, but probably useless for pipes. Additionally, SUSv4 doesn't appear to forbid POLLIN|POLLERR (or POLLOUT|POLLERR) set simultaneously. One could argue that the read() or write() call would return without blocking in these cases (with an error), so POLLIN / POLLOUT would be justified beside POLLERR. The code now penalizes POLLIN|POLLERR differently from plain POLLERR. The former (ie. read() returning -1) is terminal and we jump to cleanup, while plain POLLERR masks only the affected file descriptor for the future. Let's unify those. Signed-off-by: Laszlo Ersek <lersek@redhat.com>
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>