We should eventually do something about it. The problem is that you remove the
device and after plugging it in it is considered by the OS as a new device.
This also depends on the Os used and whether PC/SC, ctAPI or the internalCCID
driver is used.

We should eventually do soemthing about it. In the meantime you may use

gpgconf --reload scdaemon

or the Assuan command

SCD KILLSCD

don't do it automagically, better ask the user.

More info:

• scd killscd works when in the replug-without-hibernate case, for hibernate, a restart of pcscd is needed (probably not a scd problem).
• when I don't issue any commands to scd while the reader is unplugged, I can re-plug it anywhere (same or different usb port) and scd recognises this as a USABLE->ACTIVE->NOCARD->USABLE transition chain
• when I issue a SERIALNO command while the reader is unplugged, I get "Card Error" back (sitenote: shouldn't that be a ERR_NO_CARD_READER?) and that one sticks until a KILLSCD.
• this is not the case for other commands (tested: GETINFO version).

So it seems that the new usb address is not what's disturbing scd (otherwise
re-plugging itself would cause the sticky error). Rather, it seems like the
SERIALNO error code on unplugged reader is somehow sticking with the scd
instance until killed.

FWIW: I consider this an honest-to-$GOD bug, not a missing feature. Esp. in the
light of the above new data.

This looks pretty much like a pc/sc problem. Please shutdown pcscd and use the
internal CCID driver. With PC/SC we don't have any control over the USB - we
don't even know what this is.

Let me confirm. Does this bus still exist in recent version of gpg 1.4 and/or
2.0, 2.1?