Introspect redirections for HTTP TPC xfers about davix

[mpatrascoiu]

Hello,

This is a great proposal and it targets a problem we hit very often in FTS/Gfal2 transfers.
The proposed solution needs to be adapted, as Davix cannot resolve the passive host. Instead, it will need to be told this value.

Some theory
In an HTTP-TPC, two parties are involved, named active and passive. You are correct, Davix resolves one of them before the copy operation starts, namely the active party. However, Davix has no way of resolving the passive party and in fact, it shouldn't. It's best to let the active party resolve the transfer location. Davix will simply pass the "passive party" location to the active party, which becomes responsible for doing the host resolution and then the file transfer.

Once the HTTP-TPC starts, the active party will continuously send transfer updates back to the initiating client, in this case Davix. These updates are called HTTP-TPC Performance Markers, and one such field is the RemoteConnections field (best documentation I know on this topic is here). The RemoteConnections field contains the IP address of the passive host.

Back to the problem
Knowing the actual hosts involved in the transfer is very valuable. For this reason, I've created the following issue DMC-1358. The change introduces two new getter methods in the Davix::Copy public API:

    std::string getTransferSourceHost() const;
    std::string getTransferDestinationHost() const;

After the transfer is done, they will return either the hostname or IP value, or empty, if no information is available.

The active party is resolved by Davix, just before initiating the COPY request. The passive party is extracted by Davix from the HTTP-TPC Performance Markers, via the RemoteConnections field.

Here is an example of how FTS/Gfal2 will leverage this information:

$ gfal-copy -v https://eospublic.cern.ch:443/eos/opstest/dteam/file.test https://eospps.cern.ch:443/eos/opstest/dteam/file.test

event: [1670855013815] BOTH   http_plugin	TRANSFER:EXIT	https://eospublic.cern.ch:443/eos/opstest/dteam/file.test (p05151113281066.cern.ch) => https://eospps.cern.ch:443/eos/opstest/dteam/file.test ([2001:1458:301:17::100:e])

All Davix changes have been implemented in commit 969b9a5.

Cheers,
Mihai