Ok, here's the deal...
First, I looked at the "documentation" (meaning the forum post that announced it) on this new feature. It stated the file would be downloaded to the "default directory" on the client. So I prepped my client by specifying a default directory for audio of X:\ since there is no X drive on the server.
Then I tried to reproduce dtc's results, which I successfully did. Here's what I did and what happened.
1. I started MC, which connected to the local library by default, and then connected to the remote library server.
2. I navigated view on the server, picked a file, and did a download from server.
3. Immediately MC opened up the little progress dialog and performed the download. The file appeared on my local file system at the expected location.
4. My view of the server through the MC client immediately showed the [Filename] field had been updated to X:\...
5. I went into the other room and looked at MC on the server. Nothing had changed. The file was still there on the fs, still there in the view and playlists, and [Filename] still showed the normal D:\... path for the file.
6. I tried playing the file on the server. It played.
7. I went back to the client PC and look around. Everything was still as it had been in step 4. So I thought I'll see if any local library changes are made. But first...
8. I went back into the other room to inspect the server a second time. The file was now GONE from the library, and removed from all playlists.
9. I disconnected the MC client from the remote server and reconnected to the local library.
10. No changes made to my local library on the "client" pc, but the file was still there where it had been downloaded; it was not added to the local library.
11. I went back to inspect the server yet again. Now, the file was BACK, in Album and Artist views. [Filename] field was as it originally was. The file could be played. However, the file was missing from all playlists. The file on the filesystem had not been modified, and was still in place.
Conclusions:
A. The server database is permanently modified by this function. The function is not safe to use.
B. The file reappeared in my server library because auto-import was running on the server. The [Date Imported] field confirmed this.
C. The changes to the server do not happen instantly. I think they are delayed somewhat because of the vagaries of "Sync changes with library server". I did not explicitly perform a sync during the test.
D. If auto-import is not running, the file will not reappear on the server. It must be re-imported, or a library backup restored.
E. The loss of playlist membership, and other database metadata not written to the file, is permanent.
F. The server change does not behave like a simple change of the [Filename] field; more below.
G. The consequences of the bug might be masked if the default path on the client is available to the server. For example, if the client default path is "C:\..." then the server will of course have access to an equivalent location. Likewise if the default path is a UNC path like \\NAS\Flac both servers could have access to it. If the path is the same on both machines, like D:\FLAC then perhaps the bug won't manifest at all. Perhaps this is why something this catastrophic escaped internal testing.
I'm not going to do further testing regarding point G, since I've done myself enough damage already. I have some playlists to fix and metadata to recover. JRiver should do that testing.
Point F is significant too. I will note that this server has "Fix Broken Links=OFF". If the file had been merely "renamed" in the database (see point F) by modifying the [Filename] field, the file would stay in the views, and would remain on the playlists, albeit as a broken link. But this did not happen on my server. The file was actually removed from the library, with the negative consequences that result from that. If it had been a simple corruption of the [Filename] field, that could have been corrected (on my server) with RMCF using the update database/no copy function. But the way the bug is, repair in that way is impossible.
So the good news DTC is that your files are still on the server. The bad news is that any playlist membership you had is destroyed, along with any metadata not saved in tags. If you have an appropriately timed library backup, that should fix you up.
Anyway, yes this is a bad one. Hopefully nobody uses the feature until this can be fixed.
-Will