I have noticed that the 20.x (stable) and 21.x (beta) releases for Windows have introduced a behaviour which is causing an issue with Windows power management. After upgrading to 20.4.19, I noticed that my PC was no longer automatically going to sleep on idle.
After investigating, it seems that in these releases, the client is making a network IPC request to read the local machine's Event Log every 120 seconds. These requests go over the loopback interface via the Windows SMB server (RPC/DCE over SMB). As a result, this causes the Windows power manager to block the PC from entering sleep on idle due to a perceived "remote connection" to this PC:
>> powercfg /requests
DISPLAY:
None.
SYSTEM:
[DRIVER] \FileSystem\srvnet
An active remote client has recently sent requests to this machine.
Running a netstat -n -o -b, the only connection seen on port 445 was on the IPv6 loopback interface, and was associated with the System process (PID 4), indicating it is probably the result of a kernel function call.
Pinning down what the connections were required a packet capture on the loopback interface (using npcap rather than WinPCap). The requests are reads of the System Event Log and are likely the result of a call to OpenEventLog specifying "localhost" as the first parameter (computer name) rather than NULL (or in PyWin32, "None") which should read the Event Log via internal IPC rather than DCE/RPC over the network. The packet capture clearly shows the Event Log being read exactly every 120 seconds.
After eliminating a few other possible culprits (I had upgraded/installed a few pieces of software between when I knew sleep was last working and when I noticed it had stopped), it turned out to be being caused by the DB client.
When the desktop client is exited, the polling requests cease (and after about 3 minutes of no requests being received by the SMB server, the Power Manager releases the lock request and the client is able to sleep on idle again). As soon as the desktop client is restarted though, the polling resumes and continues every 120 seconds, effectively completely blocking the PC from being able to sleep on idle.
Checking the behaviour of older clients (e.g. 19.4.13, which I was running prior to updating to 20.4.19), I only see a single request to read the Event Log immediately after the client starts (or at least if it is polling, it is doing so much, much less frequently and so does not prevent the client from sleeping).
I have reported this via the bug report form also, but as I am only a basic user, there's no interaction or way to update the ticket. I've also notified @DropboxSupport via Twitter who have replied asking for a bit more info, so I will point them to this post.
