Feat[MQB]: Add authn plugin

[emelialei88]

This PR includes:

• Updated plugin interface in mqbplug
• Configurations for plugins in mqbcfg
• mqbauthn package that controls the plugin and includes built-in plugins

[678098]

Have some comments.
The most important ones are regarding breaking changes in plugins API (bslstl::StringRef -> bsl::string_view)
I cannot build everything on Solaris/Linux due to these issues

[678098]

This is a breaking change.
All other plugins still use bslstl::StringRef and cannot inherit from bsl::string_view one
Even if we update them, we will lose compatibility with older versions of the plugins. It might result in strange crashes due to incompatible API

[678098]

To deal with this, I propose to

1. Keep the original bslstl::StringRef declaration
2. Add a new one virtual virtual bsl::string_view name() const { return static_cast<bslstl::StringRef>(name()); }
   With the default implementation, it will be compatible with older plugins.
   We are free to gradually add bsl::string_view implementation in plugins and eventually remove bslstl::StringRef one.

Alternatively we can revert this change to remove possible risks
@hallfox what do you think?

[678098]

Updated in another PR #993

[678098]

If Authn Controller makes decision to print an alarm, should it also print it on other failures in this function?

[emelialei88]

Do you mean if we should ALARM on other errors in this function? I'm actually not fully sure what are the scenarios where we trigger an ALARM.

[678098]

Can move forward with this for now, can discuss the alarms separately.

[678098]

Have a few ideas about logs

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Suggested change

	bsl::string anonMechanism = bsl::string(AnonAuthenticator::k_MECHANISM,
	d_allocator_p);
	if (d_authenticators.find(anonMechanism) != d_authenticators.cend()) {
	if (d_authenticators.find(AnonAuthenticator::k_MECHANISM) != d_authenticators.cend()) {

Possible to avoid constructing tmp string

[emelialei88]

It would cause a compiler error since bsl::unordered_map<bsl::string, AuthenticatorMp> doesn't accept bsl::string_view as the parameter to find.

[678098]

Suggested change

	// If no authenticators are configured, use AnonAuthenticator for
	// anonymous pass authentication
	if (d_authenticators.empty()) {
	if (!d_authenticators.empty()) {
	// Have authenticators, nothing to do
	return 0;
	}
	// If no authenticators are configured, use AnonAuthenticator for
	// anonymous pass authentication

Not strictly necessary to do, but I would make an early return here to reduce indentation of the following code.
Your original option is also good: it only has 1 return, not 2.

If the body of if-then has more lines or if there are even more layers of indentation, early return looks better.

[678098]

How it might help the reader.
Consider the following code:

void foo(bool flag) {
    if (flag) {
        // 200 lines here
    }
    // nothing in the end
}

If a reader sees if like this, they must scroll down 200 lines to see the end of this function just to know if there is any more processing if flag == false.

Compared to this:

void foo(bool flag) {
    if (!flag) return;
    // 200 lines here
}

The reader knows that after the first line (with early return) there are no instructions for flag == false

Another case:

void foo(bool flag) {
    if (flag) {
        // 100 lines here
    }
    if (somethingElse()) {
        // 100 lines here
    }
    // nothing in the end
}

In this case, it is not even enough to scroll down 200 lines, the reader has to notice the end of the current if structure and start of another structure not related to flag. This might be solved by folding structures in IDE, however, these are extra mouse clicks

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Suggested change

	BALL_LOG_INFO << "Stopping AuthenticationController";

If we log starting, we might log stopping as well

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rc == e_SUCCESS here, might simplify:

Suggested change

	return (rc * 10 + rc_MECHANISM_NOT_SUPPORTED);
	return rc_MECHANISM_NOT_SUPPORTED;

We already have functions where we mix const * rc + enum with just rc or enum, this for example:

blazingmq/src/groups/mqb/mqbc/mqbc_clusterstateledgerutil.cpp

Line 512 in 1573133

return rc * 100 +

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Suggested change

	int rc = rc_SUCCESS;
	bmqu::MemOutStream errorStream(d_allocator_p);

Possible to move closer to authenticate below.
This is a "minimal scope" principle for definition of variables.
Minimal scope means that a reader has to keep in mind less variables at a point of time, making reading the code easier. Also, sometimes (rarely) defining variables in the least scope possible prevents others from misusing these variables later

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Suggested change

	rc = authenticator->authenticate(errorStream, result, input);
	bmqu::MemOutStream errorStream(d_allocator_p);
	const int rc = authenticator->authenticate(errorStream, result, input);

[678098]

IT failure is unrelated mqbu_exit.cpp:124 #EXIT Terminating BMQBRKR with error 'STORAGE_OUT_OF_SYNC'
Will be fixed here: #984