feat: include function signatures in rpc cli autocompletions (#2026)

Adds function signatures (excluding input context field, and output
error field) to the RPC CLI autocompletions. Now the autocompletions for
the methods looks like this:


![image](https://user-images.githubusercontent.com/16523232/230006576-a7263d3f-5360-4277-adaa-e500eed3fed6.png)

cmd/celestia/cmd_test.go

@@ -4,11 +4,39 @@ import (
 	"bytes"
 	"context"
 	"os"
+	"reflect"
 	"testing"
 
 	"github.com/stretchr/testify/require"
+
+	"github.com/celestiaorg/celestia-node/header"
 )
 
+func TestCompletionHelpString(t *testing.T) {
+	type TestFields struct {
+		NoInputOneOutput        func(context.Context) (*header.ExtendedHeader, error)
+		TwoInputsOneOutputArray func(
+			context.Context,
+			*header.ExtendedHeader,
+			uint64,
+		) ([]*header.ExtendedHeader, error)
+		OneInputOneOutput  func(context.Context, uint64) (*header.ExtendedHeader, error)
+		NoInputsNoOutputs  func(ctx context.Context) error
+		NoInputsChanOutput func(ctx context.Context) (<-chan *header.ExtendedHeader, error)
+	}
+	testOutputs := []string{
+		"() -> (*header.ExtendedHeader)",
+		"(*header.ExtendedHeader, uint64) -> ([]*header.ExtendedHeader)",
+		"(uint64) -> (*header.ExtendedHeader)",
+		"() -> ()",
+		"() -> (<-chan *header.ExtendedHeader)",
+	}
+	methods := reflect.VisibleFields(reflect.TypeOf(TestFields{}))
+	for i, method := range methods {
+		require.Equal(t, testOutputs[i], parseSignatureForHelpstring(method))
+	}
+}
+
 func TestLight(t *testing.T) {
 	// Run the tests in a temporary directory
 	tmpDir := t.TempDir()

cmd/celestia/rpc.go

@@ -71,7 +71,7 @@ var rpcCmd = &cobra.Command{
 			methods := reflect.VisibleFields(reflect.TypeOf(module).Elem())
 			var methodNames []string
 			for _, m := range methods {
-				methodNames = append(methodNames, m.Name)
+				methodNames = append(methodNames, m.Name+"\t"+parseSignatureForHelpstring(m))
 			}
 			return methodNames, cobra.ShellCompDirectiveNoFileComp
 		}
@@ -297,3 +297,23 @@ func parseAddressFromString(addrStr string) (state.Address, error) {
 
 	return addr, nil
 }
+
+func parseSignatureForHelpstring(methodSig reflect.StructField) string {
+	simplifiedSignature := "("
+	in, out := methodSig.Type.NumIn(), methodSig.Type.NumOut()
+	for i := 1; i < in; i++ {
+		simplifiedSignature += methodSig.Type.In(i).String()
+		if i != in-1 {
+			simplifiedSignature += ", "
+		}
+	}
+	simplifiedSignature += ") -> ("
+	for i := 0; i < out-1; i++ {
+		simplifiedSignature += methodSig.Type.Out(i).String()
+		if i != out-2 {
+			simplifiedSignature += ", "
+		}
+	}
+	simplifiedSignature += ")"
+	return simplifiedSignature
+}