QueuePool: support subsecond timeout

Fixes: #5719

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### Description
Make it explicit in the documentation and in the default value for the 'timeout'
parameter that `timeout` can be a float. Because Python timing is not
very accurate, warn about the precision.

### Checklist
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This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [x] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #5710
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5710
Pull-request-sha: 5f4eef8b4aba756d32e14ea41f71ef2919c26b84

Change-Id: I462524b1624ca5cc76d083a1d58e5dc89501c1a9

4 files changed, 27 insertions, 5 deletions

diff --git a/doc/build/changelog/unreleased_14/5719.rst b/doc/build/changelog/unreleased_14/5719.rst
new file mode 100644
index 000000000..3f0d73e5d
--- /dev/null
+++ b/doc/build/changelog/unreleased_14/5719.rst
@@ -0,0 +1,6 @@
+.. change::
+    :tags: pool, tests, usecase
+    :tickets: 5582
+
+    Improve documentation and add test for sub-second pool timeouts.
+    Pull request courtesy Jordan Pittier.
diff --git a/lib/sqlalchemy/engine/create.py b/lib/sqlalchemy/engine/create.py
index 786f8f5d6..b48cead79 100644
--- a/lib/sqlalchemy/engine/create.py
+++ b/lib/sqlalchemy/engine/create.py
@@ -440,9 +440,11 @@ def create_engine(url, **kwargs):
 
             :paramref:`_pool.Pool.reset_on_return`
 
-    :param pool_timeout=30: number of seconds to wait before giving
+    :param pool_timeout=30.0: number of seconds to wait before giving
         up on getting a connection from the pool. This is only used
-        with :class:`~sqlalchemy.pool.QueuePool`.
+        with :class:`~sqlalchemy.pool.QueuePool`. This can be a float but is
+        subject to the limitations of Python time functions which may not be
+        reliable in the tens of milliseconds.
 
     :param pool_use_lifo=False: use LIFO (last-in-first-out) when retrieving
         connections from :class:`.QueuePool` instead of FIFO
diff --git a/lib/sqlalchemy/pool/impl.py b/lib/sqlalchemy/pool/impl.py
index fc543053d..38afbc7a1 100644
--- a/lib/sqlalchemy/pool/impl.py
+++ b/lib/sqlalchemy/pool/impl.py
@@ -40,7 +40,7 @@ class QueuePool(Pool):
         creator,
         pool_size=5,
         max_overflow=10,
-        timeout=30,
+        timeout=30.0,
         use_lifo=False,
         **kw
     ):
@@ -73,7 +73,9 @@ class QueuePool(Pool):
           connections. Defaults to 10.
 
         :param timeout: The number of seconds to wait before giving up
-          on returning a connection. Defaults to 30.
+          on returning a connection. Defaults to 30.0. This can be a float
+          but is subject to the limitations of Python time functions which
+          may not be reliable in the tens of milliseconds.
 
         :param use_lifo: use LIFO (last-in-first-out) when retrieving
           connections instead of FIFO (first-in-first-out). Using LIFO, a
@@ -129,7 +131,7 @@ class QueuePool(Pool):
             else:
                 raise exc.TimeoutError(
                     "QueuePool limit of size %d overflow %d reached, "
-                    "connection timed out, timeout %d"
+                    "connection timed out, timeout %0.2f"
                     % (self.size(), self.overflow(), self._timeout),
                     code="3o7r",
                 )
diff --git a/test/engine/test_pool.py b/test/engine/test_pool.py
index 9ea3065b0..1a49cf4b9 100644
--- a/test/engine/test_pool.py
+++ b/test/engine/test_pool.py
@@ -14,6 +14,7 @@ from sqlalchemy.testing import assert_raises
 from sqlalchemy.testing import assert_raises_context_ok
 from sqlalchemy.testing import assert_raises_message
 from sqlalchemy.testing import eq_
+from sqlalchemy.testing import expect_raises
 from sqlalchemy.testing import fixtures
 from sqlalchemy.testing import is_
 from sqlalchemy.testing import is_not
@@ -886,6 +887,17 @@ class QueuePoolTest(PoolTestBase):
         assert_raises(tsa.exc.TimeoutError, p.connect)
         assert int(time.time() - now) == 2
 
+    @testing.requires.timing_intensive
+    def test_timeout_subsecond_precision(self):
+        p = self._queuepool_fixture(pool_size=1, max_overflow=0, timeout=0.5)
+        c1 = p.connect()  # noqa
+        with expect_raises(tsa.exc.TimeoutError):
+            now = time.time()
+            c2 = p.connect()  # noqa
+        # Python timing is not very accurate, the time diff should be very
+        # close to 0.5s but we give 200ms of slack.
+        assert 0.3 <= time.time() - now <= 0.7, "Pool timeout not respected"
+
     @testing.requires.threading_with_mock
     @testing.requires.timing_intensive
     def test_timeout_race(self):