ADB COMMUNICATION OVER QEMUD SERVICE

I. Overview:
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In certain cases ADB communication over TCP port forwarding might be unreliable.
For instance, connection manager in the guest system may decide to reset all
network connections, in which case connection between the ADB host, and ADB
daemon running in the guest will be broken, and will not be reestablish until ADB
host is killed, and then restarted. To address this issue ADB communication has
been moved to use QEMUD pipe to transfer data between the emulator, and the
ADB daemon running in the guest.


II. Implementation:
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There are two major components implemented in the emulator to support ADB
communication over QEMUD pipe:

1. ADB server.
2. ADB QEMUD service (or ADB client).

ADB server is implemented as a socket server that is bound to the ADB port (which
is the 'base port' + 1), and is responsible for accepting connections coming from
the ADB host, and associatiating accepted connections with an ADB client.

ADB client is implemented as QEMUD service (named 'adb'), and is responsible for
accepting connections coming from the ADB daemon running in the guest, and
associating accepted connections with ADB server.

Essentially, ADB server, and ADB client serve as a bridge between ADB running on
the host, and ADB running on the guest: ADB server receives data from ADB host,
and channels them through ADB client to the ADB guest. Likewise, ADB client
receives data from the ADB guest, and channels them through ADB server to the ADB
host. The only tricky part here is establishing the connection. Once connection
is established, everything is straight forward.

II.III Establishing the connection:
-----------------------------------

The connection between ADB host and ADB guest is considered to be established,
when there is an association between ADB socket connection in the ADB server, and
QEMUD pipe connection in the ADB client, and ADB guest is ready to receive data
from the ADB host.

Since there is no particular order, in which ADB server, and ADB client
connections are made, ADB server runs a list of pending host and guest
connections. Every time either connection is made, that connection goes into a
pending list on condition that there is no pending connection on the other side.
If, however, there is already a pending connection from the other side, that
pending connection is removed from the pending list, and gets associated with the
new connection.