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As a result of different commits, ConnectionPool had become
of two minds about exceptions, sometimes using PoolFullError
and sometimes using ConnectionTimeoutError. In fact, it was
using ConnectionTimeoutError internally, but then recueing
and re-raising as a PoolFullError.

There's no reason for this bifurcation, standardize on
ConnectionTimeoutError, which is the rails2 name and still
accurately describes semantics at this point.

History

In Rails2, ConnectionPool raises a ConnectionTimeoutError if
it can't get a connection within timeout.

Originally in master/rails3, @tenderlove had planned on removing
wait/blocking in connectionpool entirely, at that point he changed
exception to PoolFullError.

But then later wait/blocking came back, but exception remained
PoolFullError.

Then in 02b23355 pmahoney introduced fair waiting logic, and
brought back ConnectionTimeoutError, introducing the weird bifurcation.

ConnectionTimeoutError accurately describes semantics as of this
point, and is backwards compat with rails2, there's no reason
for PoolFullError to be introduced, and no reason for two
different exception types to be used internally, no reason
to rescue one and re-raise as another.  Unify!

The core of this fix is a threadsafe, fair Queue class.  It is
very similar to Queue in stdlib except that it supports waiting
with a timeout.

The issue this solves is that if several threads are contending for
database connections, an unfair queue makes is possible that a thread
will timeout even while other threads successfully acquire and release
connections.  A fair queue means the thread that has been waiting the
longest will get the next available connection.

This includes a few test fixes to avoid test ordering issues that
cropped up during development of this patch.

An AR ConnectionSpec `wait_timeout` is pre-patch used for three
different things:

* mysql2 uses it for MySQL's own wait_timeout (how long MySQL
  should allow an idle connection before closing it), and
  defaults to 2592000 seconds.
* ConnectionPool uses it for "number of seconds to block and
  wait for a connection before giving up and raising a timeout error",
  default 5 seconds.
* ConnectionPool uses it for the Reaper, for deciding if a 'dead'
  connection can be reaped. Default 5 seconds.

Previously, if you want to change these from defaults, you need
to change them all together. This is problematic _especially_
for the mysql2/ConnectionPool conflict, you will generally _not_
want them to be the same, as evidenced by their wildly different
defaults. This has caused real problems for people #6441 #2894

But as long as we're changing this, forcing renaming the
ConnectionPool key to be more specific, it made sense
to seperate the two ConnectionPool uses too -- these two
types of ConnectionPool timeouts ought to be able to be
changed independently, you won't neccesarily want them
to be the same, even though the defaults are (currently)
the same.

This reverts commit d2901f0f, reversing
changes made to 525839fd.

Conflicts:
	activerecord/test/cases/connection_pool_test.rb

Reason: This change broke the build (http://travis-ci.org/#!/rails/rails/builds/1391490)
and we don't have any solution until now. I asked the author to try to
fix it and open a new pull request.

These variables were issuing some "not used" warnings.

make active_connection? return true only if there is an open connection in use for the current thread. fixes #5330

the following code is deprecated:

Thread.new { Post.find(1) }.join

It should be changed to close the database connection at the end of
the thread:

Thread.new {
  Post.find(1)
  Post.connection.close
}.join

Only people who spawn threads in their application code need to worry
about this change.

Make it the responsibility of the connection to hold onto an ARel visitor for generating SQL. This improves the code architecture generally, and solves some problems with marshalling. Adapter authors please take note: you now need to define an Adapter.visitor_for method, but it degrades gracefully with a deprecation warning for now.

Refactor Active Record test connection setup. Please see the RUNNING_UNIT_TESTS file for details, but essentially you can now configure things in test/config.yml. You can also run tests directly via the command line, e.g. ruby path/to/test.rb (no rake needed, uses default db connection from test/config.yml). This will help us fix the CI by enabling us to isolate the different Rails versions to different databases.

- Make sure ConnectionManagement clears connections after each call, except in tests
Signed-off-by: NJoshua Peek <josh@joshpeek.com>