[New Feature] Run strands safely on a separate thread

[lasinicl]

$title will be achieved by

1. defining a start-action to be isolated
2. extending isolated concept to named workers

Defining a start-action to be isolated

A start-action will be defined isolated, if

• the method or function being called is isolated
• the expression for every argument is an isolated expression
• for a method-call or client-remote-method-call-action the expression for the object is an isolated expression

A start-action is modeled as an action invocation in the compiler. ISOLATED flag will be added for the invocation at the IsolationAnayzer if it satisfies the above conditions, so that can be identified as an isolated start-action.

The strand created by an isolated start-action can run in a different thread from the current strand - @strand{thread:"any"} annotation will be added internally for the isolated start-action at the desugar phase.

An isolated function can contain an isolated start-action - According to the current implementation, it is not allowed to have start-action in the body of an isolated function. But with this change, it will be allowed to have an isolated start-action in an isolated function and the compilation errors will be updated to reflect the invalid invocations.

Extending isolated concept to named workers

An isolated function can contain a named-worker-decl - According to the current implementation, it is not allowed to have a named-worker-decl in the body of an isolated function. But with this change, it will be allowed to have an isolated worker in an isolated function and the compilation errors will be updated to reflect the invalid worker declarations.

When a named worker is isolated its body will have similar restrictions to an isolated function (https://ballerina.io/spec/lang/master/#isolated_functions). Within a named-worker-decl in an isolated function, a reference to a local variable outside the named-worker-decl should satisfy the same requirements as a captured variable in an isolated function/method.

Captured variables are restricted as follows: if the body is part of an anonymous-function-expr, then any captured variable references must refer to variables that are both final (either explicitly or implicitly) and have a static type that is a subtype of readonly|isolated object {}.

A worker is modeled as a lambda function in the compiler. ISOLATED flag will be added for the lambda function at the IsolationAnayzer if it satisfies the above conditions, so that can be identified as an isolated worker.

The named worker's strand will run on a separate thread from the function's strand - @strand{thread:"any"} annotation will be added internally for the isolated worker at the desugar phase.

Isolated inference for start-action and named workers
Should also support inferring the isolatedness of a start-action and a named worker.

Highly appreciate your input on this discussion.

[warunalakshitha]

@lasinicl If the code contains @strand{thread:"any"} to a non isolated start action or named worker will that be a compilation error?
And also if code has @strand{thread:"parent"} in an isolated start action or named worker should we still run them on separate thread?

[lasinicl]

If the code contains @strand{thread:"any"} to a non isolated start action or named worker will that be a compilation error?

An isolated function can not contain a non-isolated start action or a named worker. Hence it will be a compilation error.
A non-isolated function can contain a non-isolated start action or a named worker. We can use the annotations as well.

If code has @strand{thread:"parent"} in an isolated start action or named worker should we still run them on a separate thread

If so, we will be ignoring the @strand{thread: "parent"} annotation and add the @strand{thread: "any"} annotation internally so that it runs on a separate thread. This was decided since it's currently in discussion to deprecate the @strand{} annotation. We will also be logging a warning if @strand{thread: "parent"} has been used in such cases.

[lasinicl]

Update: It will be an error to use @strand on a start-action or named worker that occurs in a function explicitly declared as isolated. (ballerina-platform/ballerina-spec#1068 (comment))

[johana-star]

/unsubscribe 😝

[lasinicl]

When @strand is used on a start action or a named worker,

• if the containing function is explicitly declared as isolated, it will be a compile-time error
• otherwise
  - the containing function will not be inferred to be isolated
  - the existing @strand runtime semantics will continue as now
  - there will be a deprecation warning saying not to use @strand, but instead use the safe features introduced by this issue