Sharing values by reference between kernels should be opt-in

Reference sharing isn’t the common case. Most kernels don’t share a process (e.g. JavaScript versus C#) and/or have no values that can helpfully be shared by reference without assuming a specific runtime and language in order to bind data to specific types (e.g. SQL, KQL, HTTP). And even in cases where reference sharing is possible (e.g. between C#, F#, and PowerShell in the default kernel configuration in dotnet-interactive), if the necessary imports and assembly references aren’t in place ahead of time, then the share will fail.

Sharing by reference is useful for some scenarios though, including certain optimizations (reducing serialization and chattiness) and cases where developers want to use kernels directly in process.

For these reasons, #!share, #!set, and other magic commands that dispatch SendValue and RequestValue commands should be made to opt into reference sharing, for example by specifying an option like --by-ref. When this is specified for out-of-proc kernels or kernels that don’t wish to share internal types, the magic command should fail.

[AbhitejJohn]

I like the idea of a warning/diagnostic in the product to help with the transition 👍

[jonsequitur]

Changing this behavior for #!share would most likely be a larger breaking change for more users, and for our samples docs, than is worthwhile.

After some time spent working on #2765, it seems to me more prudent to use the shift to the new #!set command as an opportunity to fix this there. So #!share will continue to share by reference by default, with that being disabled when the --mime-type option is used. #!set on the other hand will require the use of a --byref option to perform reference sharing, with the following resulting in an error:

• Attempting to use both --mime-type and --byref together.
• Attempting to share by reference from a proxy kernel.
• Attempting to share by reference to a proxy kernel.