Re: [libvirt] [PATCH 1/8] Allow handshake with child process during startup
On 05/11/2011 03:33 AM, Daniel P. Berrange wrote:
Allow the parent process to perform a bi-directional handshake
with the child process during fork/exec. The child process
will fork and do its initial setup. Immediately prior to the
exec(), it will stop & wait for a handshake from the parent
process. The parent process will spawn the child and wait
until the child reaches the handshake point. It will do
whatever extra setup work is required, before signalling the
child to continue.
The implementation of this is done using two pairs of blocking
pipes. The first pair is used to block the parent, until the
child writes a single byte. Then the second pair pair is used
to block the child, until the parent confirms with another
single byte.
* src/util/command.c, src/util/command.h,
src/libvirt_private.syms: Add APIs to perform a handshake
---
src/libvirt_private.syms | 3 +
src/util/command.c | 161 +++++++++++++++++++++++++++++++++++++++++++++-
src/util/command.h | 22 ++++++
3 files changed, 185 insertions(+), 1 deletions(-)
Hopefully there aren't too many rebase issues if Cole's virCommand
cleanup series goes in first.
@@ -1255,6 +1315,10 @@ virCommandRunAsync(virCommandPtr cmd, pid_t
*pid)
FD_CLR(i, &cmd->transfer);
}
}
+ if (cmd->handshake) {
+ VIR_FORCE_CLOSE(cmd->handshakeWait[1]);
+ VIR_FORCE_CLOSE(cmd->handshakeNotify[0]);
+ }
You don't need this hunk if you use virCommandTransferFD below...
@@ -1395,6 +1459,94 @@ virCommandAbort(virCommandPtr cmd
ATTRIBUTE_UNUSED)
}
#endif
+
+void virCommandRequireHandshake(virCommandPtr cmd)
+{
+ if (!cmd || cmd->has_error)
+ return;
Avoid clobbering existing fds and causing an fd leak, by adding:
if (cmd->handshake) {
cmd->has_error = -1;
VIR_DEBUG("cannot require handshake twice");
return;
}
+
+ if (pipe(cmd->handshakeWait) < 0) {
+ cmd->has_error = errno;
+ return;
+ }
+ if (pipe(cmd->handshakeNotify) < 0) {
+ VIR_FORCE_CLOSE(cmd->handshakeWait[0]);
+ VIR_FORCE_CLOSE(cmd->handshakeWait[1]);
+ cmd->has_error = errno;
+ return;
+ }
+
+ VIR_DEBUG("Transfer handshake wait=%d notify=%d",
+ cmd->handshakeWait[1], cmd->handshakeNotify[0]);
+ virCommandPreserveFD(cmd, cmd->handshakeWait[1]);
+ virCommandPreserveFD(cmd, cmd->handshakeNotify[0]);
...here's where to use virCommandTransferFD, for slightly less bookkeeping.
+int virCommandHandshakeWait(virCommandPtr cmd)
+{
+ char c;
+ int rv;
+ if (!cmd ||cmd->has_error == ENOMEM) {
+ virReportOOMError();
+ return -1;
+ }
+ if (cmd->has_error) {
Change to this, to avoid calling saferead on an invalid fd if no one
called virCommandRequireHandshake:
if (cmd->has_error || !cmd->handshake)
Also, on completion you either need to set cmd->handshake = false or
call VIR_CLOSE the fd on completion; if you do the latter, then on entry
you should check that the fd is not -1, so that we ensure no one calls
this method twice for a single child process.
+ if (c != '1') {
+ char *msg;
+ ssize_t len;
+ if (VIR_ALLOC_N(msg, 1024) < 0) {
+ virReportOOMError();
Should we stack-allocate this, to minimize the chance of a malloc
failure while reporting the child's failure?
+
+int virCommandHandshakeNotify(virCommandPtr cmd)
+{
+ char c = '1';
+ if (!cmd ||cmd->has_error == ENOMEM) {
+ virReportOOMError();
+ return -1;
+ }
+ if (cmd->has_error) {
if (cmd->has_error || !cmd->handshake)
--
Eric Blake eblake(a)redhat.com +1-801-349-2682
Libvirt virtualization library http://libvirt.org
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