Go mutex copying: keep the lock identity with the state it protects

What it is

Go's sync documentation states that a Mutex must not be copied after first use. Its synchronization relation connects unlock and lock operations on that mutex. A structure containing a mutex therefore needs an ownership and passing convention that preserves the identity of the synchronization object throughout its active lifetime. Go sync Mutex

Why it matters

An agent may add a mutex field and locking calls without reviewing how the containing object is passed. A value receiver or assignment can undermine the intended single-lock discipline. The proposed review follows the object through constructors, methods, collections and helper functions before assessing individual critical sections.

How to apply

• Identify the state protected by each mutex and the exact object holding that mutex. Write the invariant that every access must share the same lock identity.
• Inspect method receivers, function parameters, return values and assignments involving the containing type. Check container operations and iteration variables that may introduce value copies.
• Choose an explicit ownership convention, commonly passing a pointer to the stable owner, and align the API with that convention. Do not expose a convenient copying operation that contradicts the lifecycle rule.
• If callers need a snapshot, construct a separate data-only result while holding the appropriate lock. Define which values are copied and whether nested mutable data remains shared.
• Propose a regression fixture using the actual helper and container paths that previously copied the owner. Verify shared-state behavior and run the relevant static and concurrency checks available in the project.

Pitfalls

A pointer receiver alone does not prevent an explicit copy elsewhere. Nor does preserving mutex identity establish a correct lock order or protect accesses that skip locking. Review copies before first use separately from prohibited active-state copies; avoid relying on an undocumented cloning contract. This is a proposed ownership audit, and no static-analysis or race-test result is claimed.