Add Context Support for GraphQL Resolvers

[ThisaruGuruge]

Purpose

Add GraphQL context support that can be used to share metadata between the resolvers.

Fixes: #1906
Fixes: #1953

Examples

Developers can provide a function to initialize the context which must be an isolated function, and must have two input parameters: http:Request and http:RequestContext. This function must have the return type of graphql:Context|error.

Inside this function, the graphql:Context object can be created and the developer can add attributes to the context as key-value pairs. The key is always a string, the value can be a value:Clonable|isolated object {}. Simply put, this can be a primitive value, any readonly value, or an isolated function pointer, or an isolated object. The context object has three functions, add, get, and remove.
Then this function can be provided as a service config field. Using this function, a context will be created per request. When needed, the context can be defined as the first parameter of a resolver (resource/remote) function. Following is an example:

import ballerina/graphql;
import ballerina/http;

isolated function initContext(http:Request request, http:RequestContext requestContext) returns graphql:Context|error {
    graphql:Context context = new;
    check context.add("scope", request.getHeader("scope"));
    return context;
}

@graphql:ServiceConfig {
    contextInit: initContext
} 
service on new graphql:Listener(9000) {
    resource function get greting(graphql:Context context) returns string {
        if context.get("scope") is string && context.get("scope") == "admin" {
            return "Hello, admin!";
        } else {
            return "Hello, user!";
        }
    }
}

Checklist

• Linked to an issue
• Updated the changelog
• Added tests

[codecov]

Codecov Report

Merging #417 (393d4a8) into master (d2908bc) will decrease coverage by 0.21%.
The diff coverage is 86.31%.

@@             Coverage Diff              @@
##             master     #417      +/-   ##
============================================
- Coverage     94.84%   94.63%   -0.22%     
- Complexity      278      285       +7     
============================================
  Files            49       50       +1     
  Lines          2912     2983      +71     
  Branches       1347     1390      +43     
============================================
+ Hits           2762     2823      +61     
- Misses          132      138       +6     
- Partials         18       22       +4     

Impacted Files	Coverage Δ
...rina/stdlib/graphql/runtime/schema/TypeFinder.java	96.03% <0.00%> (ø)
.../ballerina/stdlib/graphql/runtime/utils/Utils.java	70.58% <33.33%> (-20.33%)	⬇️
...allerina/stdlib/graphql/runtime/engine/Engine.java	92.62% <71.42%> (-0.60%)	⬇️
ballerina/http_service.bal	88.88% <89.28%> (-11.12%)	⬇️
ballerina/context.bal	93.10% <93.10%> (ø)
ballerina/annotation_processor.bal	100.00% <100.00%> (ø)
ballerina/annotations.bal	100.00% <100.00%> (ø)
ballerina/engine.bal	97.01% <100.00%> (+1.24%)	⬆️
ballerina/executor_visitor.bal	91.83% <100.00%> (+0.17%)	⬆️
ballerina/listener.bal	80.48% <100.00%> (+0.48%)	⬆️
... and 6 more

---

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[shafreenAnfar]

What if we assign a default value for this ? Something like

isolated function initDefaultContext(http:Request request, http:RequestContext requestContext) returns graphql:Context|error {
    return graphql:Context();
}

That way it is explicit that we create a default one if nothing is given.

[ThisaruGuruge]

We already have a function for this:

module-ballerina-graphql/ballerina/context.bal

Line 68 in df9a8fa

isolated function initContext(http:Request request, http:RequestContext requestContext) returns Context|error {

I'll rename this as initDefaultContext

[ldclakmal]

@ThisaruGuruge @shafreenAnfar What do you think about the field name contextInit? I prefer this to be as Context context, and passing a function pointer which creates a Context for me. But I understand that it is not possible with Ballerina way of handling types. So, what do you think of having this as ContextInit context?

[ThisaruGuruge]

At first, I went through the same thought process. But then went with contextInit because this isn't actually a context, but a function that initializes a context.
IMHO, this makes it easy for the developers to understand what it is. They have to provide a ContextInit (which is a function pointer), not a Context (which is an object).

[ldclakmal]

Yeah. Agree with the clarification.