What will go wrong if one stores a Context in a struct?
I've done so for a specific use case, and did not notice any issues.
If you store contexts on your structs it’s very likely you won’t thread them correctly, leading to errors like database code not properly handling transactions.
Actually super fragile and you should avoid doing this as much as is possible. It’s never a good idea!
The specific use case I had was where the context represented the lifetime of a (shared) TCP connection. I then wanted to use its cancellation to drive the destruction of various dynamic graph elements hanging off that shared connection.
Think a graph of muxes/demuxes in a dynamic message graph while the whole program is a CSP style thing.
I needed something to drive destruction, the context provided what I needed.
Contexts are about the dynamic contour, i.e. the dynamic call stack. Storing the current context in a struct and then referring to it in some other dynamic … context … is going to lead to all sorts of pain: timeouts or deadlines which have already expired and/or values which are no longer pertinent.
While there are some limited circumstances in which it may be appropriate, in general it is a very strong code smell. Any code which passes a context should receive a context. And any code which may pass a context in the future should receive one now, to preserve API compatibility. So any exported function really should have a context as its first argument for forwards-compatibility.
The documented semantics of the facility (and the implementation) seemed perfect for destroying a dynamic graph of CSP elements, which were exchanging messages. Where the cancellation causes the goroutines within the (think Actor-like) CSP element to clean up, tear down, and exit.
The warning about not storing it in a struct seems to assume it can only be used for one specific type of purpose, say web server processes and/or related database requests.
If I hadn't used it, I would have had to create something almost identical - but without the ability to store a data element.
