fn: Add new Req type to abstract the pattern of remote processing.

It is common throughout the codebase to send data to a remote
goroutine for processing. Typically, along with the data we are
processing, we also send a one-shot channel where we intend to
listen for the response. This type encapsulates that pattern.

fn/req.go

@@ -0,0 +1,34 @@
+package fn
+
+// Req is a type to encapsulate RPC-like calls wherein we send some data
+// structure as a request, as well as a channel to receive the response on where
+// the remote goroutine will send the result.
+//
+// NOTE: This construct should only be used for request/response patterns for
+// which there is only a single response for the request.
+type Req[Input any, Output any] struct {
+	// Request is the data we are sending to the remote goroutine for
+	// processing.
+	Request Input
+
+	// Response is the channel on which we will receive the result of the
+	// remote computation.
+	Response chan<- Output
+}
+
+// NewReq is the base constructor of the Req type. It returns both the packaged
+// Req object as well as the receive side of the response channel that we will
+// listen on for the response.
+func NewReq[Input, Output any](input Input) (
+	Req[Input, Output], <-chan Output) {
+
+	// Always buffer the response channel so that the goroutine doing the
+	// processing job does not block if the original requesting routine
+	// takes an unreasonably long time to read the response.
+	responseChan := make(chan Output, 1)
+
+	return Req[Input, Output]{
+		Request:  input,
+		Response: responseChan,
+	}, responseChan
+}