[5.3] Allow reconnect in transaction by setting transactions to 0

[halaei]

I am not sure if I apply this to 5.1 as well. The tests and codes changed. Maybe need another PR.

Also here is a history you may check #5944 #5937

Thanks.

[taylorotwell]

So is this to replace all your other PRs? I'm confused by all these PRs.

[taylorotwell]

Can you provide description of initial problem and how this fixes it?

[halaei]

Code history check

In Laravel 4.2, this line were added to Connection::setPdo():

if ($this->transactions >= 1) throw new \RuntimeException("Attempt to change PDO inside running transaction");

As mentioned in #5937, the motivation was to prevent Connection::run() from breaking transaction logic by retrying part of a transaction in a lost connection inside another connection.

The initial proposed solutions in #5937 was this

(1) skip Connection::reconnectIfMissingConnection and Connection::tryAgainIfCausedByLostConnection calls if Connection::$transactions > 0
or
(2) modify them to throw exceptions and reset Connection::$transactions to 0 when they called.

Then (2) was implemented (without resetting $transactions to 0) in #5944, which I think was wrong, and some time later (1) is also done.

The behavior after this PR

I think by having solution (1) in the code, there is no need to throw exception in setPdo() when Connection::transactions > 0. Just reseting the value to 0 is enough.

The three new added tests shows the new behavior:

1. Connection::setPdo(), which may be called by DB::reconnect(), will not throw exception if a transaction is open in the current connection. Instead, it just resets Connection::transactions to 0.
2. Connection::run() attempts to reconnect(swap PDO) on lost connection event if and only if there is no open transaction in the connection. If there is an open transaction, it re-throws the exception so the client code decides what to do; It may give up and die, but only after this PR it can call DB::reconnect() and retry the transaction or pick the next job from the queue.

Thanks.