Add UNIX domain socket support for multi-target scraping

[yosida95]

This pull request expands the multi-target scraping ability introduced by #504 and #651 to UNIX domain sockets.

If the target query starts with unix://, connect to the server using UNIX domain socket. Otherwise, use TCP.

This PR adds a new query parameter network to specify the network type either "tcp" or "unix". If no value is set, it defaults to "tcp".

Support for multiple UNIX domain sockets is useful if, for example, mysqld_exporter runs on Cloud Run configured with multiple Cloud SQL connections. Cloud Run mounts multiple UNIX domain sockets on the /cloudsql directory. https://cloud.google.com/sql/docs/mysql/connect-run?hl=en#connect_to

(Fixed in #714) By the way, I found that the /probe endpoint emits error messages to the client along with metrics in some error conditions. I don't know if Prometheus can handle it correctly, but at least prom2json couldn't handle it.

mysqld_exporter/probe.go

Lines 45 to 54 in 593b009

	cfg := c.GetConfig()
	cfgsection, ok := cfg.Sections[authModule]
	if !ok {
	level.Error(logger).Log("msg", fmt.Sprintf("Failed to parse section [%s] from config file", authModule), "err", err)
	http.Error(w, fmt.Sprintf("Error parsing config section [%s]", authModule), http.StatusBadRequest)
	}
	if dsn, err = cfgsection.FormDSN(target); err != nil {
	level.Error(logger).Log("msg", fmt.Sprintf("Failed to form dsn from section [%s]", authModule), "err", err)
	http.Error(w, fmt.Sprintf("Error forming dsn from config section [%s]", authModule), http.StatusBadRequest)
	}

In the same condition, the /metrics endpoint doesn't emit error messages.

mysqld_exporter/mysqld_exporter.go

Lines 140 to 147 in 593b009

	cfg := c.GetConfig()
	cfgsection, ok := cfg.Sections["client"]
	if !ok {
	level.Error(logger).Log("msg", "Failed to parse section [client] from config file", "err", err)
	}
	if dsn, err = cfgsection.FormDSN(""); err != nil {
	level.Error(logger).Log("msg", "Failed to form dsn from section [client]", "err", err)
	}

Is that OK or should be patched to emit only either error messages or metrics? I'd like to send a PR as needed.

[SuperQ]

Looks like there's a merge conflict.

[yosida95]

@SuperQ Resolved

[soara]

According to Go MySQL Driver documentation, UNIX domain socket is the absolute path.
So, I thought that target value is tcp or unix can be determined by whether it is the absolute path or not.

[SuperQ]

Thanks

[yosida95]

@soara Thanks for the suggestion.

Despite your suggestion, it seems the UNIX domain socket path is not necessary to be absolute. The driver uses the standard net.Dialer, which resolves relative paths based on the cwd. https://github.com/go-sql-driver/mysql/blob/v1.7.0/connector.go#L48-L49

Therefore checking whether the target value contains a slash (or starts with a slash) does not always distinguish the network.

[SuperQ]

Please make this handle the network with unix:// prefixing.

[yosida95]

@SuperQ Done.

[SuperQ]

Nice, super easy!