Access already allocated connections in a thread safe manner when che…

…cking out connections in the sharded threaded connection pool

I'm not positive the previous code is thread unsafe, but it
appears to be.

While here, add some documentation to the threaded connection
pool and use local variable instead of instance variable.

CHANGELOG

@@ -1,5 +1,7 @@
 === master
 
+* Access already allocated connections in a thread safe manner when checking out connections in the sharded threaded connection pool (jeremyevans)
+
 * Automatically support datasets using qualified tables in the class_table_inheritance plugin without having to use the :alias option (benalavi) (#1565)
 
 * Support rename_column without emulation on SQLite 3.25+ (jeremyevans)

lib/sequel/connection_pool/sharded_threaded.rb

@@ -223,10 +223,11 @@ def acquire(thread, server)
   # Assign a connection to the thread, or return nil if one cannot be assigned.
   # The caller should NOT have the mutex before calling this.
   def assign_connection(thread, server)
-    alloc = allocated(server)
+    alloc = nil
 
     do_make_new = false
     sync do
+      alloc = allocated(server)
       if conn = next_available(server)
         alloc[thread] = conn
         return conn

lib/sequel/connection_pool/threaded.rb

@@ -172,6 +172,8 @@ def acquire(thread)
   # Assign a connection to the thread, or return nil if one cannot be assigned.
   # The caller should NOT have the mutex before calling this.
   def assign_connection(thread)
+    # Thread safe as instance variable is only assigned to local variable
+    # and not operated on outside mutex.
     allocated = @allocated
     do_make_new = false
     to_disconnect = nil
@@ -184,7 +186,7 @@ def assign_connection(thread)
       if (n = _size) >= (max = @max_size)
         allocated.keys.each do |t|
           unless t.alive?
-            (to_disconnect ||= []) << @allocated.delete(t)
+            (to_disconnect ||= []) << allocated.delete(t)
           end
         end
         n = nil