Support logging in starlark (influxdata#8408)

plugins/processors/starlark/README.md

@@ -96,6 +96,7 @@ While Starlark is similar to Python, there are important differences to note:
 The ability to load external scripts other than your own is pretty limited. The following libraries are available for loading:
 
 * json: `load("json.star", "json")` provides the following functions: `json.encode()`, `json.decode()`, `json.indent()`. See [json.star](/plugins/processors/starlark/testdata/json.star) for an example.
+* log: `load("logging.star", "log")` provides the following functions: `log.debug()`, `log.info()`, `log.warn()`, `log.error()`. See [logging.star](/plugins/processors/starlark/testdata/logging.star) for an example.
 
 If you would like to see support for something else here, please open an issue.
 
@@ -185,6 +186,7 @@ def failing(metric):
 - [rename](/plugins/processors/starlark/testdata/rename.star) - Rename tags or fields using a name mapping.
 - [scale](/plugins/processors/starlark/testdata/scale.star) - Multiply any field by a number
 - [value filter](/plugins/processors/starlark/testdata/value_filter.star) - remove a metric based on a field value.
+- [logging](/plugins/processors/starlark/testdata/logging.star) - Log messages with the logger of Telegraf
 
 [All examples](/plugins/processors/starlark/testdata) are in the testdata folder.
 

plugins/processors/starlark/logging.go

@@ -0,0 +1,47 @@
+package starlark
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/influxdata/telegraf"
+	"go.starlark.net/starlark"
+	"go.starlark.net/starlarkstruct"
+)
+
+// Builds a module that defines all the supported logging functions which will log using the provided logger
+func LogModule(logger telegraf.Logger) *starlarkstruct.Module {
+	var logFunc = func(t *starlark.Thread, b *starlark.Builtin, args starlark.Tuple, kwargs []starlark.Tuple) (starlark.Value, error) {
+		return log(t, b, args, kwargs, logger)
+	}
+	return &starlarkstruct.Module{
+		Name: "log",
+		Members: starlark.StringDict{
+			"debug": starlark.NewBuiltin("log.debug", logFunc),
+			"info":  starlark.NewBuiltin("log.info", logFunc),
+			"warn":  starlark.NewBuiltin("log.warn", logFunc),
+			"error": starlark.NewBuiltin("log.error", logFunc),
+		},
+	}
+}
+
+// Logs the provided message according to the level chosen
+func log(t *starlark.Thread, b *starlark.Builtin, args starlark.Tuple, kwargs []starlark.Tuple, logger telegraf.Logger) (starlark.Value, error) {
+	var msg starlark.String
+	if err := starlark.UnpackPositionalArgs(b.Name(), args, kwargs, 1, &msg); err != nil {
+		return starlark.None, fmt.Errorf("%s: %v", b.Name(), err)
+	}
+	switch b.Name() {
+	case "log.debug":
+		logger.Debug(string(msg))
+	case "log.info":
+		logger.Info(string(msg))
+	case "log.warn":
+		logger.Warn(string(msg))
+	case "log.error":
+		logger.Error(string(msg))
+	default:
+		return nil, errors.New("method " + b.Name() + " is unknown")
+	}
+	return starlark.None, nil
+}

plugins/processors/starlark/starlark.go

@@ -52,7 +52,9 @@ func (s *Starlark) Init() error {
 
 	s.thread = &starlark.Thread{
 		Print: func(_ *starlark.Thread, msg string) { s.Log.Debug(msg) },
-		Load:  loadFunc,
+		Load: func(thread *starlark.Thread, module string) (starlark.StringDict, error) {
+			return loadFunc(thread, module, s.Log)
+		},
 	}
 
 	builtins := starlark.StringDict{}
@@ -217,12 +219,16 @@ func init() {
 	})
 }
 
-func loadFunc(thread *starlark.Thread, module string) (starlark.StringDict, error) {
+func loadFunc(thread *starlark.Thread, module string, logger telegraf.Logger) (starlark.StringDict, error) {
 	switch module {
 	case "json.star":
 		return starlark.StringDict{
 			"json": starlarkjson.Module,
 		}, nil
+	case "logging.star":
+		return starlark.StringDict{
+			"log": LogModule(logger),
+		}, nil
 	default:
 		return nil, errors.New("module " + module + " is not available")
 	}

plugins/processors/starlark/starlark_test.go

@@ -2535,6 +2535,31 @@ func TestScript(t *testing.T) {
 				),
 			},
 		},
+		{
+			name: "logging",
+			plugin: &Starlark{
+				Script: "testdata/logging.star",
+				Log:    testutil.Logger{},
+			},
+			input: []telegraf.Metric{
+				testutil.MustMetric("log",
+					map[string]string{},
+					map[string]interface{}{
+						"debug": "a debug message",
+					},
+					time.Unix(0, 0),
+				),
+			},
+			expected: []telegraf.Metric{
+				testutil.MustMetric("log",
+					map[string]string{},
+					map[string]interface{}{
+						"debug": "a debug message",
+					},
+					time.Unix(0, 0),
+				),
+			},
+		},
 	}
 
 	for _, tt := range tests {

plugins/processors/starlark/testdata/logging.star

@@ -0,0 +1,19 @@
+# Example of the way to log a message with all the supported levels
+# using the logger of Telegraf.
+#
+# Example Input:
+# log debug="a debug message" 1465839830100400201
+#
+# Example Output:
+# log debug="a debug message" 1465839830100400201
+
+load("logging.star", "log")
+# loads log.debug(), log.info(), log.warn(), log.error()
+
+def apply(metric):
+    log.debug("debug: {}".format(metric.fields["debug"]))
+    log.info("an info message")
+    log.warn("a warning message")
+    log.error("an error message")
+    return metric
+