qemu-ga: guest-suspend: make the API synchronous
Currently, qemu-ga has a SIGCHLD handler that automatically reaps terminated children processes. The idea is to avoid having qemu-ga commands blocked waiting for children to terminate. That approach has two problems: 1. qemu-ga is unable to detect errors in the child, meaning that qemu-ga returns success even if the child fails to perform its task 2. if a command does depend on the child exit status, the command has to play tricks to bypass the automatic reaper Case 2 impacts the guest-suspend-* API, because it has to execute an external program to check for suspend support. Today, to bypass the automatic reaper, suspend code has to double fork and pass exit status information through a pipe. Besides being complex, this is prone to race condition bugs. Indeed, the current code does have such bugs. Making the guest-suspend-* API synchronous (ie. by dropping the SIGCHLD handler and calling waitpid() from commands) is a much simpler approach, which fixes current race conditions bugs and enables commands to detect errors in the child. This commit does just that. There's a side effect though, guest-shutdown will generate zombies if shutting down fails. This will be fixed by the next commit. Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com> Reviewed-by: NEric Blake <eblake@redhat.com> Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
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