peer: panic early on queued nil messages

[jrick]

#3212 (running a commit from late June) appears to be a situation very similar to what was accidentally created by 3aaaf3f (Oct 30) and fixed by c1e4cdd (Oct 31). Since something else has been sending nil messages before, and we may not have found and fixed that caller too, let's go ahead and add this panic now to find any still-existing and future incorrect uses of this API.

[jrick]

There's another possible nil message panic that this check is not able to detect, which is if the interface variable is non-nil but contains a concrete wire message type with a nil value. I'm not sure of any great way to detect this, but it would be nice.

[ukane-philemon]

which is if the interface variable is non-nil but contains a concrete wire message type with a nil value. I'm not sure of any great way to detect this, but it would be nice.

We can cast and check for known types that implement the interface. msg, ok := interfaceVar.(type); if msg == nil { // do something? };

[jrick]

yes but you would need a check for every message type in the type switch.

[davecgh]

There's another possible nil message panic that this check is not able to detect, which is if the interface variable is non-nil but contains a concrete wire message type with a nil value. I'm not sure of any great way to detect this, but it would be nice.

It can be done with reflect, but that's pretty heavy. Another way would be to create a helper func that simply tries to type cast it to all wire types and checks against nil. That wouldn't be robust against new wire types, but it would still be quite an improvement over nothing I think.

[jrick]

I did a quick audit of the codebase and didn't spot any codepaths where it was obviously doing something that might cause that issue, i.e. nothing like

var tx *wire.MsgTx
if cond {
    // maybe assign tx
}
// queue tx without checking tx == nil
p.QueueMessage(tx, nil)

[davecgh]

I did a quick audit of the codebase and didn't spot any codepaths where it was obviously doing something that might cause that issue, i.e. nothing like

I was doing the same and didn't spot anything either.