Implement inlining content of external file to policy

[Mielek]

Proposal

It would be beneficial to be able to inline external files in policy document. It would allow maintaining those files separately from policy.

Example of C# code:

context.SetBody(File.ReadAllText("body.liquid"), new SetBodyConfig() { Template = "liquid" });

Compiled to

<set-body template="liquid">
  {% if user %}
    Hello {{ user.name }}!
  {% endif %}
</set-body>

Component

Authoring library, Compiler

[damiennnnn]

I've somewhat built this functionality into my fork of the policy toolkit (commit here), as my team wants to be able to configure the policy compilation outside of the code itself (e.g. in CI through environment variables)

This comes with a new EvaluateExpression attribute to be used on static methods, which will allow methods to be evaluated using Roslyn's scripting API when compiling the policy document.

E.g.

            public void Inbound(IInboundContext context) {
                context.SetVariable("Inbound", CreateString());
            }
            
            [EvaluatedExpression]
            public static string CreateString(){
                StringBuilder sb = new StringBuilder(); 
                
                for (int i =0; i < 10; i++)
                    sb.Append(i);
                
                return sb.ToString();
            }

becomes

        <policies>
            <inbound>
                <set-variable name="Inbound" value="0123456789" />
            </inbound>
        </policies>

This does also work for file reading, envvars, etc.. as long as it is using features in the core libraries (currently).

There's also a primitive implementation of argument passing, which only currently supports args that evaluate to literal expressions

[damiennnnn]

I've reconsidered my approach to this issue. My recent commit adds specific handling for invocations of Environment.GetEnvironmentVariable and File.ReadAllText where the compiler will invoke these methods on compile-time using the provided parameter. I've still kept in the scripting functionality for future use.

e.g. with File.ReadAllText

public void Inbound(IInboundContext context) {
    context.SetVariable("Inbound", File.ReadAllText("test.txt"));
}

Here, the compiler will identify File.ReadAllText and call it with the "test.txt" parameter, returning the contents of "test.txt" as the value.

<policies>
    <inbound>
        <set-variable name="Inbound" value="{contents of test.txt here!!!}" />
    </inbound>
</policies>

This also works the same way with getting environment variables, and could be expanded beyond these two