tree b2a7dcfe734bfe933a8abd940b8732e46e76d8e9
parent 6df407bc07cea62a53772325c60de06a3c869dd2
author Jason A. Donenfeld <Jason@zx2c4.com> 1559834861 +0200
committer Jason Donenfeld <Jason@zx2c4.com> 1560068736 +0000

windows: allow determining if manager is locked

The SCM can be locked by NT. While traditionally any process could lock
the SCM using "LockServiceDatabase", Microsoft removed this
functionality because it created so many bugs, and that function now
does nothing. However, the system itself, via the "NT Service Control
Manager", is still allowed to lock the SCM.

For example, at boot time on Windows 8.1, the SCM is locked after a
service is started until that service reports itself in a running state.
This poses a bit of a problem: it's useful to install device drivers
from inside services as part of their initialization, and mark the
service as having started only after the device has installed. But
device installation might potentially load new drivers, and drivers
themselves exist as a special type of service. This means that if a
driver is installed before marking the service as started, the entire
SCM will deadlock, and the OS will be partially unresponsive for a
minute or two.

Fortunately Microsoft supplies an API for exactly this purpose. The
solution is to mark the service as started before installing device
drivers, only under the circumstance that the SCM is locked. So, this
commit adds the proper API for determining this. It can be used like
this:

    if m, err := mgr.Connect(); err == nil {
        if lockStatus, err := m.LockStatus(); err == nil && lockStatus.IsLocked {
            log.Printf("SCM locked for %v by %s, marking service as started", lockStatus.Age, lockStatus.Owner)
            changes <- svc.Status{State: svc.Running}
        }
        m.Disconnect()
    }
    deviceDriver.Install()

This creates messages like the following, indicating that this API
works:

    SCM locked for 1s by .\NT Service Control Manager, marking service as started

Change-Id: Ic2f5b387e23efc3a287b2ab96ff84b357b712e36
Reviewed-on: https://go-review.googlesource.com/c/sys/+/180977
Run-TryBot: Jason Donenfeld <Jason@zx2c4.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>