[SLIDER-1242] Review uses of double-checked locking

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: Slider 1.0.0
• Assignee: Billie Rinaldi
• Reporter: Billie Rinaldi
• Created: 25/Aug/17 14:17
• Updated: 06/Sep/17 20:42
• Resolved: 06/Sep/17 20:42

Description

There are several places where we perform double-checked locking. Even though the practice is discouraged, I believe it is technically correct when the check is performed on the presence of a ConcurrentHashMap key, which is how we are using it.

However, in two places, AgentProviderService#getCurrentExports and AgentProviderService#getAllocatedPorts, containsKey is used instead of get to perform the check. I am seeing some indication that containsKey is not sufficient, and that get must be used for double-checked locking to be correct. There is a comment in the ConcurrentHashMap#containsKey method that says "same as get() except no need for volatile value read" – and I think that volatile value read is what we need for correctness.

Also, in the ConcurrentHashMap api doc, it specifically mentions get and does not mention containsKey: "Any non-null result returned from get(key) and related access methods bears a happens-before relation with the associated insertion or update" and "an update operation for a given key bears a happens-before relation with any (non-null) retrieval for that key reporting the updated value."