[pkg/ottl]: Add a new Function Getter to the OTTL package, to allow passing Converters as literal parameters (#24356)

**Description:** Add a new Function Getter to the OTTL package, to allow
passing Converters as literal parameters

**Link to tracking Issue:** [<Issue number if
applicable>](#22961)

**Testing:** Added a unit test and also tested the following
configuration

```
replace_pattern(attributes["message"], "device=*", attributes["device_name"], SHA256)
```

A tentative example of how the literal function gets invoked is
```
type ReplacePatternArguments[K any] struct {
	Target       ottl.GetSetter[K]      `ottlarg:"0"`
	RegexPattern string                 `ottlarg:"1"`
	Replacement  ottl.StringGetter[K]   `ottlarg:"2"`
	Function     ottl.FunctionGetter[K] `ottlarg:"3"`
}

func replacePattern[K any](target ottl.GetSetter[K], regexPattern string, replacement ottl.StringGetter[K], fn ottl.Expr[K]) (ottl.ExprFunc[K], error) {
	compiledPattern, err := regexp.Compile(regexPattern)
	if err != nil {
		return nil, fmt.Errorf("the regex pattern supplied to replace_pattern is not a valid pattern: %w", err)
	}
	return func(ctx context.Context, tCtx K) (interface{}, error) {
		originalVal, err := target.Get(ctx, tCtx)
		if err != nil {
			return nil, err
		}
		replacementVal, err := fn.Eval(ctx, tCtx)
		if err != nil {
			return nil, err
		}
          .......
          updatedStr := compiledPattern.ReplaceAllString(originalValStr, replacementValHash.(string))
}
```

Author: rnishtala-sumo

.chloggen/function-getter.yaml

@@ -0,0 +1,23 @@
+# Use this changelog template to create an entry for release notes.
+# If your change doesn't affect end users, such as a test fix or a tooling change,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: pkg/ottl
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Add a new Function Getter to the OTTL package, to allow passing Converters as literal parameters.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [22961]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext: |
+  Currently OTTL provides no way to use any defined Converter within another Editor/Converter.
+  Although Converters can be passed as a parameter, they are always executed and the result is what is actually passed as the parameter.
+  This allows OTTL to pass Converters themselves as a parameter so they can be executed within the function.

pkg/ottl/expression.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"encoding/hex"
 	"fmt"
+	"reflect"
 	"strconv"
 
 	jsoniter "github.com/json-iterator/go"
@@ -245,6 +246,56 @@ func (g StandardFloatGetter[K]) Get(ctx context.Context, tCtx K) (float64, error
 	}
 }
 
+// FunctionGetter uses a function factory to return an instantiated function as an Expr.
+type FunctionGetter[K any] interface {
+	Get(args Arguments) (Expr[K], error)
+}
+
+// StandardFunctionGetter is a basic implementation of FunctionGetter.
+type StandardFunctionGetter[K any] struct {
+	fCtx FunctionContext
+	fact Factory[K]
+}
+
+// Get takes an Arguments struct containing arguments the caller wants passed to the
+// function and instantiates the function with those arguments.
+// If there is a mismatch between the function's signature and the arguments the caller
+// wants to pass to the function, an error is returned.
+func (g StandardFunctionGetter[K]) Get(args Arguments) (Expr[K], error) {
+	if g.fact == nil {
+		return Expr[K]{}, fmt.Errorf("undefined function")
+	}
+	fArgs := g.fact.CreateDefaultArguments()
+	if reflect.TypeOf(fArgs).Kind() != reflect.Pointer {
+		return Expr[K]{}, fmt.Errorf("factory for %q must return a pointer to an Arguments value in its CreateDefaultArguments method", g.fact.Name())
+	}
+	if reflect.TypeOf(args).Kind() != reflect.Pointer {
+		return Expr[K]{}, fmt.Errorf("%q must be pointer to an Arguments value", reflect.TypeOf(args).Kind())
+	}
+	fArgsVal := reflect.ValueOf(fArgs).Elem()
+	argsVal := reflect.ValueOf(args).Elem()
+	if fArgsVal.NumField() != argsVal.NumField() {
+		return Expr[K]{}, fmt.Errorf("incorrect number of arguments. Expected: %d Received: %d", fArgsVal.NumField(), argsVal.NumField())
+	}
+	for i := 0; i < fArgsVal.NumField(); i++ {
+		field := argsVal.Field(i)
+		argIndex, err := getArgumentIndex(i, argsVal)
+		if err != nil {
+			return Expr[K]{}, err
+		}
+		fArgIndex, err := getArgumentIndex(argIndex, fArgsVal)
+		if err != nil {
+			return Expr[K]{}, err
+		}
+		fArgsVal.Field(fArgIndex).Set(field)
+	}
+	fn, err := g.fact.CreateFunction(g.fCtx, fArgs)
+	if err != nil {
+		return Expr[K]{}, fmt.Errorf("couldn't create function: %w", err)
+	}
+	return Expr[K]{exprFunc: fn}, nil
+}
+
 // PMapGetter is a Getter that must return a pcommon.Map.
 type PMapGetter[K any] interface {
 	// Get retrieves a pcommon.Map value.
@@ -399,7 +450,7 @@ func (g StandardFloatLikeGetter[K]) Get(ctx context.Context, tCtx K) (*float64,
 	return &result, nil
 }
 
-// IntLikeGetter is a Getter that returns an int by converting the underlying value to an int if necessary.
+// IntLikeGetter is a Getter that returns an int by converting the underlying value to an int if necessary
 type IntLikeGetter[K any] interface {
 	// Get retrieves an int value.
 	// Unlike `IntGetter`, the expectation is that the underlying value is converted to an int if possible.

pkg/ottl/expression_test.go

@@ -676,6 +676,126 @@ func Test_StandardStringGetter(t *testing.T) {
 	}
 }
 
+func Test_FunctionGetter(t *testing.T) {
+	functions := CreateFactoryMap(
+		createFactory[any](
+			"SHA256",
+			&stringGetterArguments{},
+			functionWithStringGetter,
+		),
+		createFactory[any](
+			"test_arg_mismatch",
+			&multipleArgsArguments{},
+			functionWithStringGetter,
+		),
+		createFactory[any](
+			"no_struct_tag",
+			&noStructTagFunctionArguments{},
+			functionWithStringGetter,
+		),
+		NewFactory(
+			"cannot_create_function",
+			&stringGetterArguments{},
+			func(FunctionContext, Arguments) (ExprFunc[any], error) {
+				return functionWithErr()
+			},
+		),
+	)
+	type EditorArguments struct {
+		Replacement StringGetter[any]
+		Function    FunctionGetter[any]
+	}
+	type FuncArgs struct {
+		Input StringGetter[any] `ottlarg:"0"`
+	}
+	tests := []struct {
+		name             string
+		getter           StringGetter[any]
+		function         FunctionGetter[any]
+		want             interface{}
+		valid            bool
+		expectedErrorMsg string
+	}{
+		{
+			name: "function getter",
+			getter: StandardStringGetter[interface{}]{
+				Getter: func(ctx context.Context, tCtx interface{}) (interface{}, error) {
+					return "str", nil
+				},
+			},
+			function: StandardFunctionGetter[any]{fCtx: FunctionContext{Set: componenttest.NewNopTelemetrySettings()}, fact: functions["SHA256"]},
+			want:     "anything",
+			valid:    true,
+		},
+		{
+			name: "function getter nil",
+			getter: StandardStringGetter[interface{}]{
+				Getter: func(ctx context.Context, tCtx interface{}) (interface{}, error) {
+					return nil, nil
+				},
+			},
+			function:         StandardFunctionGetter[any]{fCtx: FunctionContext{Set: componenttest.NewNopTelemetrySettings()}, fact: functions["SHA250"]},
+			want:             "anything",
+			valid:            false,
+			expectedErrorMsg: "undefined function",
+		},
+		{
+			name: "function arg mismatch",
+			getter: StandardStringGetter[interface{}]{
+				Getter: func(ctx context.Context, tCtx interface{}) (interface{}, error) {
+					return nil, nil
+				},
+			},
+			function:         StandardFunctionGetter[any]{fCtx: FunctionContext{Set: componenttest.NewNopTelemetrySettings()}, fact: functions["test_arg_mismatch"]},
+			want:             "anything",
+			valid:            false,
+			expectedErrorMsg: "incorrect number of arguments. Expected: 4 Received: 1",
+		},
+		{
+			name: "Invalid Arguments struct tag",
+			getter: StandardStringGetter[interface{}]{
+				Getter: func(ctx context.Context, tCtx interface{}) (interface{}, error) {
+					return nil, nil
+				},
+			},
+			function:         StandardFunctionGetter[any]{fCtx: FunctionContext{Set: componenttest.NewNopTelemetrySettings()}, fact: functions["no_struct_tag"]},
+			want:             "anything",
+			valid:            false,
+			expectedErrorMsg: "no `ottlarg` struct tag on Arguments field \"StringArg\"",
+		},
+		{
+			name: "Cannot create function",
+			getter: StandardStringGetter[interface{}]{
+				Getter: func(ctx context.Context, tCtx interface{}) (interface{}, error) {
+					return nil, nil
+				},
+			},
+			function:         StandardFunctionGetter[any]{fCtx: FunctionContext{Set: componenttest.NewNopTelemetrySettings()}, fact: functions["cannot_create_function"]},
+			want:             "anything",
+			valid:            false,
+			expectedErrorMsg: "couldn't create function: error",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			editorArgs := EditorArguments{
+				Replacement: tt.getter,
+				Function:    tt.function,
+			}
+			fn, err := editorArgs.Function.Get(&FuncArgs{Input: editorArgs.Replacement})
+			if tt.valid {
+				var result interface{}
+				result, err = fn.Eval(context.Background(), nil)
+				assert.NoError(t, err)
+				assert.Equal(t, tt.want, result.(string))
+			} else {
+				assert.EqualError(t, err, tt.expectedErrorMsg)
+			}
+		})
+	}
+}
+
 // nolint:errorlint
 func Test_StandardStringGetter_WrappedError(t *testing.T) {
 	getter := StandardStringGetter[interface{}]{

pkg/ottl/functions.go

@@ -47,42 +47,57 @@ func (p *Parser[K]) newFunctionCall(ed editor) (Expr[K], error) {
 	return Expr[K]{exprFunc: fn}, err
 }
 
+func getArgumentIndex(index int, args reflect.Value) (int, error) {
+	argsType := args.Type()
+	fieldTag, ok := argsType.Field(index).Tag.Lookup("ottlarg")
+	if !ok {
+		return 0, fmt.Errorf("no `ottlarg` struct tag on Arguments field %q", argsType.Field(index).Name)
+	}
+	argNum, err := strconv.Atoi(fieldTag)
+	if err != nil {
+		return 0, fmt.Errorf("ottlarg struct tag on field %q is not a valid integer: %w", argsType.Field(index).Name, err)
+	}
+	if argNum < 0 || argNum >= args.NumField() {
+		return 0, fmt.Errorf("ottlarg struct tag on field %q has value %d, but must be between 0 and %d", argsType.Field(index).Name, argNum, args.NumField())
+	}
+	return argNum, nil
+}
+
 func (p *Parser[K]) buildArgs(ed editor, argsVal reflect.Value) error {
 	if len(ed.Arguments) != argsVal.NumField() {
 		return fmt.Errorf("incorrect number of arguments. Expected: %d Received: %d", argsVal.NumField(), len(ed.Arguments))
 	}
 
-	argsType := argsVal.Type()
-
 	for i := 0; i < argsVal.NumField(); i++ {
 		field := argsVal.Field(i)
 		fieldType := field.Type()
-
-		fieldTag, ok := argsType.Field(i).Tag.Lookup("ottlarg")
-
-		if !ok {
-			return fmt.Errorf("no `ottlarg` struct tag on Arguments field %q", argsType.Field(i).Name)
-		}
-
-		argNum, err := strconv.Atoi(fieldTag)
-
+		argNum, err := getArgumentIndex(i, argsVal)
 		if err != nil {
-			return fmt.Errorf("ottlarg struct tag on field %q is not a valid integer: %w", argsType.Field(i).Name, err)
+			return err
 		}
-
-		if argNum < 0 || argNum >= len(ed.Arguments) {
-			return fmt.Errorf("ottlarg struct tag on field %q has value %d, but must be between 0 and %d", argsType.Field(i).Name, argNum, len(ed.Arguments))
-		}
-
 		argVal := ed.Arguments[argNum]
-
 		var val any
-		if fieldType.Kind() == reflect.Slice {
+		switch {
+		case strings.HasPrefix(fieldType.Name(), "FunctionGetter"):
+			var name string
+			switch {
+			case argVal.Enum != nil:
+				name = string(*argVal.Enum)
+			case argVal.FunctionName != nil:
+				name = *argVal.FunctionName
+			default:
+				return fmt.Errorf("invalid function name given")
+			}
+			f, ok := p.functions[name]
+			if !ok {
+				return fmt.Errorf("undefined function %s", name)
+			}
+			val = StandardFunctionGetter[K]{fCtx: FunctionContext{Set: p.telemetrySettings}, fact: f}
+		case fieldType.Kind() == reflect.Slice:
 			val, err = p.buildSliceArg(argVal, fieldType)
-		} else {
+		default:
 			val, err = p.buildArg(argVal, fieldType)
 		}
-
 		if err != nil {
 			return fmt.Errorf("invalid argument at position %v: %w", i, err)
 		}