mirror of https://gitee.com/openkylin/libvirt.git
25f87817ab
The old semantics of virFork() violates the priciple of good usability: it requires the caller to check the pid argument after use, *even when virFork returned -1*, in order to properly abort a child process that failed setup done immediately after fork() - that is, the caller must call _exit() in the child. While uses in virfile.c did this correctly, uses in 'virsh lxc-enter-namespace' and 'virt-login-shell' would happily return from the calling function in both the child and the parent, leading to very confusing results. [Thankfully, I found the problem by inspection, and can't actually trigger the double return on error without an LD_PRELOAD library.] It is much better if the semantics of virFork are impossible to abuse. Looking at virFork(), the parent could only ever return -1 with a non-negative pid if it misused pthread_sigmask, but this never happens. Up until this patch series, the child could return -1 with non-negative pid if it fails to set up signals correctly, but we recently fixed that to make the child call _exit() at that point instead of forcing the caller to do it. Thus, the return value and contents of the pid argument are now redundant (a -1 return now happens only for failure to fork, a child 0 return only happens for a successful 0 pid, and a parent 0 return only happens for a successful non-zero pid), so we might as well return the pid directly rather than an integer of whether it succeeded or failed; this is also good from the interface design perspective as users are already familiar with fork() semantics. One last change in this patch: before returning the pid directly, I found cases where using virProcessWait unconditionally on a cleanup path of a virFork's -1 pid return would be nicer if there were a way to avoid it overwriting an earlier message. While such paths are a bit harder to come by with my change to a direct pid return, I decided to keep the virProcessWait change in this patch. * src/util/vircommand.h (virFork): Change signature. * src/util/vircommand.c (virFork): Guarantee that child will only return on success, to simplify callers. Return pid rather than status, now that the situations are always the same. (virExec): Adjust caller, also avoid open-coding process death. * src/util/virprocess.c (virProcessWait): Tweak semantics when pid is -1. (virProcessRunInMountNamespace): Adjust caller. * src/util/virfile.c (virFileAccessibleAs, virFileOpenForked) (virDirCreate): Likewise. * tools/virt-login-shell.c (main): Likewise. * tools/virsh-domain.c (cmdLxcEnterNamespace): Likewise. * tests/commandtest.c (test23): Likewise. Signed-off-by: Eric Blake <eblake@redhat.com> |
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README
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>