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

1 files changed, 5 insertions, 3 deletions

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",
                 )