Fix: mdsconnect of IDL API now always updates !MDS_SOCKET

[mwinkel-dev]

Fixes issue #2996.

The IDL API has a system variable (i.e., global), !MDS_SOCKET, used to store the most recent connection ID. It is now updated whenever mdsconnect establishes a mdsip connection.

Example output follows . . .

IDL> CD, '/usr/local/mdsplus/lib'
IDL> mdsconnect, 'localhost'
% Compiled module: MDSCONNECT.
% Compiled module: MDS_KEYWORD_SET.
% Compiled module: MDSDISCONNECT.
IDL> print, !MDS_SOCKET
       0
IDL> mdsconnect, 'localhost'
IDL> print, !MDS_SOCKET
       1
IDL> mdsconnect, 'localhost'
IDL> print, !MDS_SOCKET
       2
IDL> conn_id = -1
IDL> mdsconnect, 'localhost', socket=conn_id
IDL> print, !MDS_SOCKET, conn_id
       3           3
IDL> mdsconnect, 'localhost', socket=conn_id
IDL> print, !MDS_SOCKET, conn_id
       4           4
IDL> mdsdisconnect, socket=conn_id
IDL> print, !MDS_SOCKET, conn_id
       4           4
IDL> mdsconnect, 'localhost', socket=conn_id
IDL> print, !MDS_SOCKET, conn_id
       5           5
IDL> mdsdisconnect
IDL> print, !MDS_SOCKET, conn_id
      -1           5
IDL> mdsconnect, 'localhost', socket=conn_id
IDL> print, !MDS_SOCKET, conn_id
       6           6
IDL> mdsconnect, 'localhost'
IDL> print, !MDS_SOCKET, conn_id
       7           6
IDL> 

[mwinkel-dev]

This procedure is not used anywhere in the MDSplus source code. Plus it appears to be replaced by the mdsconnect procedure.

However, there is a chance that customers have old IDL programs that use this routine.

We should select one of these options:

• delete the procedure, or
• keep the procedure, or
• add a print statement to it indicating that it is deprecated and the user should instead use mdsconnect

[mwinkel-dev]

Delete, keep or deprecate?

[mwinkel-dev]

This draft PR will probably be cancelled after some additional investigation. For more information, refer to the post on Issue #2996.

#2996 (comment)

[mwinkel-dev]

Adding an optional database keyword to the MdsConnect procedure enables proper updating of !MDS_SOCKET and !MDSDB_SOCKET.

Even though this draft PR now passes the automated builds, it is debatable whether bug #2996 should be fixed or not.

[mwinkel-dev]

This draft PR and the original code both pass the unmodified automated test, which proves that they are functionally equivalent. However, any change to existing code brings with it the risk of unintended consequences; an innocuous change can inadvertently introduce a subtle bug.

An alternate approach is to add comments to the original code explaining that the optional socket keyword to the MdsConnect procedure is reserved for use by the MdsDbConnect procedure. The presence of the socket keyword causes !MDSDB_SOCKET to be updated; its absence causes !MDS_SOCKET to be updated. Thus, user programs must not use the socket keyword when using MdsConnect to access a remote MDSplus archive server.