Add a PoolAvailableRule to easily add backup pools

[RobinGeuze]

Short description

This adds the PoolAvailableRule. Can be used to apply different pools based on availability.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified unit test(s)

[RobinGeuze]

Still need to add tests and documentation, will work on that now.

[RobinGeuze]

I've added documentation and did some basic testing using this to check correct behavior in case of up:

addLocal("127.0.0.2", { doTCP=true, reusePort=true })

newServer({address="8.8.8.8", name="googledns", mustResolve=true, checkName="transip.nl", pool="upserver"})

newServer({address="8.8.8.9", name="randomip", mustResolve=true, checkName="google.nl", pool="downserver"})

addAction(PoolAvailableRule("upserver"), PoolAction("downserver"))

addAction(AllRule(), PoolAction("upserver"))

And this to see if it works properly when a pool is down:

addLocal("127.0.0.2", { doTCP=true, reusePort=true })

newServer({address="8.8.8.8", name="googledns", mustResolve=true, checkName="transip.nl", pool="upserver"})

newServer({address="8.8.8.9", name="randomip", mustResolve=true, checkName="google.nl", pool="downserver"})

addAction(PoolAvailableRule("upserver"), PoolAction("downserver"))

addAction(AllRule(), PoolAction("upserver"))

[pavel-odintsov]

Nice! I like it!

[rgacogne]

LGTM!

[pavel-odintsov]

Yes! It looks great! :) I think I can test it.

[pavel-odintsov]

Hello!

I tested it in following configuration:

newServer({address="8.8.8.8:53", name="first", retries=1, pool="" })
newServer({address="1.1.1.1:53", name="second", retries=1, pool="" })
newServer({address="1.0.0.1:53", name="third", retries=1, pool=""})

newServer({address="208.67.222.222:53", weight=1, name="backup", retries=1,  pool="backup" })

-- Use default pool only of we have alive servers in it
addAction(PoolAvailableRule(""), PoolAction(""))

-- Otherwise use backup pool
addAction(AllRule(), PoolAction("backup"))

Then I manually disabled all three servers in default pool:

getServer(0):setDown()
getServer(1):setDown()
getServer(2):setDown()

And all requests were redirected to backup one:

Got query for yandex.ru.|A from 127.0.0.1:52001, relayed to third
Got answer from 1.0.0.1:53, relayed to 127.0.0.1:52001, took 2644.98 usec
Got query for yandex.ru.|A from 127.0.0.1:40043, relayed to third
Got answer from 1.0.0.1:53, relayed to 127.0.0.1:40043, took 2458.87 usec
Got control connection from 127.0.0.1:54372
Got query for yandex.ru.|A from 127.0.0.1:50675, relayed to backup
Got answer from 208.67.222.222:53, relayed to 127.0.0.1:50675, took 3688.15 usec

Then I returned one of the servers to active status and it worked well tool:

Got query for yandex.ru.|A from 127.0.0.1:55171, relayed to second
Got answer from 1.1.1.1:53, relayed to 127.0.0.1:55171, took 2601.94 usec

[pavel-odintsov]

I closed my PR about "backup pool for pool". This PR implement it much better way.