What's the problem?
autoload is thread unsafe. When we define a constant to be
autoloaded, we expect the constant construction is invariant. But
current autoload implementation allows other threads to access the
constant while the first thread is loading a file.
What's happening inside?
The current implementation uses Qundef as a marker of autoload in
Constant table. Once the first thread find Qundef as a value at
constant lookup, it starts loading a defined feature. Generally a
loaded file overrides the Qundef in Constant table by module/class
declaration at very beginning lines of the file, so other threads
can see the new Module/Class object before feature loading is
finished. It breaks invariant construction.
How to solve?
To ensure invariant constant construction, we need to override
Qundef with defined Object after the feature loading. For keeping
Qundef in Constant table, I expanded autoload_data struct in
Module to have a slot for keeping the defined object while feature
loading. And changed Module's constant lookup/update logic a
little so that the slot is only visible from the thread which
invokes feature loading. (== the first thread which accessed the
autoload constant)
Evaluation?
All test passes (bootstrap test, test-all and RubySpec) and added
8 tests for threading behavior. Extra logics are executed only
when Qundef is found, so no perf drop should happen except
autoloading.
* variable.c (rb_autoload): Prepare new autoload_data struct.
* variable.c (rb_autoload_load): Load feature and update Constant
table after feature loading is finished.
* variable.c (rb_const_get_0): When the fetched constant is under
autoloading, it returns the object only for the thread which starts
autoloading.
* variable.c (rb_const_defined_0): Ditto.
* variable.c (rb_const_set): When the specified constant is under
autoloading, it sets the object only for the thread which starts
autoloading. Otherwise, simply overrides Qundef with constant
override warning.
* vm_insnhelper.c (vm_get_ev_const): Apply same change as
rb_const_get_0 in variable.c.
* test/ruby/test_autoload.rb: Added tests for threading behavior.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@33078 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
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