Go binding: add GetClientStatus method to Database

[gm42]

Add GetClientStatus method to Database.

Allow fetching client status JSON information for any database.
Added a test to make sure that it works with an open database.

By looking in fdbclient/MultiVersionTransaction.actor.cpp I found out that fdb_database_get_client_status() will return an empty string when the db is invalid; I will document this in this same PR.

Related: #11621

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[johscheuer]

Changes LGTM, do you think it make sense to have a go struct that can be used to deserialize the result?

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[gm42]

Changes LGTM, do you think it make sense to have a go struct that can be used to deserialize the result?

I was going to say that if we add it, should also be added for the machine-readable JSON status (from key \xFF\xFF/status/json), but I just realized that there is no dedicated Go API method to fetch it, so that's not a concern.

Pros:

• will be gladly welcome downstream

Cons:

• needs a commitment to be kept up to date every time there is a change
• if there is a breaking change in such JSON format, the Go binding will need an immediate update because otherwise nobody will be able to read any field from it (even if it's not the one affected by the breaking change)
• in case of multiple JSON formats for multiple versions, the Go binding will have to forward-port the older versions of the struct to the new version

I can give this a try, and point out fields whose type I am not sure about; what do you prefer?

[gm42]

Another take at this would be: if we want to support as many versions as possible, we will return the raw values from FoundationDB for all of the special keys (coordinators etc) and never act as a middle-man/translator between client and server. This would extend to the client status information as well.

[gm42]

@johscheuer any idea why in some cases an empty string and no error can be returned? I am trying to reproduce this, but without success so far.

[gm42]

Re-based and added a commit which updates the GoDoc for ReadTransact().

[johscheuer]

LGTM 👍

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[gm42]

Thanks for triggering the CI; it is still failing, I think I begin to understand what's happening.

The C client library has logic to seed an internal versionMonitorDb with the first working database connection which supports it, otherwise it falls back on creating one locally.

The get client status function will perform some magic and in one case can return empty string:

ThreadFuture<Standalone<StringRef>> MultiVersionDatabase::getClientStatus() {
	auto stateRef = dbState;
	auto db = stateRef->dbVar->get();
	if (!db.value.isValid()) {
		db.value = stateRef->versionMonitorDb;
	}
	if (!db.value.isValid()) {
		return onMainThread([stateRef] { return Future<Standalone<StringRef>>(stateRef->getClientStatus(""_sr)); });
	} else {

So I believe that for some reason neither databases are considered valid.

[gm42]

So I believe that for some reason neither databases are considered valid.

The reason turned out to be: multi-version client must be enabled in order for this function call to return non-empty string.

I have updated the PR accordingly; what's left to figure out is whether I can cover both scenarios in tests (MVC enabled/disabled).

[johscheuer]

LGTM 👍