QoL (#13)

* Use the defer in the example
* Add magic comment in defer
* Add utility function to detect named return value
* Add caller information to metrics
* Register metrics globally only on nil argument

Author: gagbo

Diff for: .gitignore

@@ -1,2 +1,2 @@
 # Local build of autometrics generator
-autometrics
+/autometrics

Diff for: cmd/autometrics/main.go

@@ -9,8 +9,10 @@ import (
 
 func main() {
 	fileName := os.Getenv("GOFILE")
+	moduleName := os.Getenv("GOPACKAGE")
 	promGenerator := doc.NewPrometheusDoc()
-	if err := doc.TransformFile(fileName, promGenerator); err != nil {
+
+	if err := doc.TransformFile(fileName, moduleName, promGenerator); err != nil {
 		log.Fatalf("error transforming %s: %s", fileName, err)
 	}
 }

Diff for: examples/web/.gitignore

@@ -1,5 +1,5 @@
 # Local build of autometrics-dev/autometrics-go/cmd/autometrics
-autometrics
+/autometrics
 
 # Local build of the example server
-web-server
+/web-server

Diff for: examples/web/cmd/main.go

@@ -9,7 +9,6 @@ import (
 	"time"
 
 	"github.com/autometrics-dev/autometrics-go/pkg/autometrics"
-	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
@@ -20,14 +19,11 @@ import (
 func main() {
 	rand.Seed(time.Now().UnixNano())
 
-	reg := prometheus.NewRegistry()
-	autometrics.Init(reg)
+	autometrics.Init(nil)
 
 	http.HandleFunc("/", errorable(indexHandler))
 	http.HandleFunc("/random-error", errorable(randomErrorHandler))
-
-	// Expose /metrics HTTP endpoint using the created custom registry.
-	http.Handle("/metrics", promhttp.HandlerFor(reg, promhttp.HandlerOpts{Registry: reg}))
+	http.Handle("/metrics", promhttp.Handler())
 
 	log.Println("binding on http://localhost:62086")
 	log.Fatal(http.ListenAndServe(":62086", nil))
@@ -59,6 +55,7 @@ func main() {
 //
 //autometrics:doc
 func indexHandler(w http.ResponseWriter, _ *http.Request) (err error) {
+        defer autometrics.Instrument(autometrics.PreInstrument(), &err) //autometrics:defer-statement
 	_, err = fmt.Fprintf(w, "Hello, World!\n")
 	return
 }
@@ -91,6 +88,7 @@ var handlerError = errors.New("failed to handle request")
 //
 //autometrics:doc
 func randomErrorHandler(w http.ResponseWriter, _ *http.Request) (err error) {
+        defer autometrics.Instrument(autometrics.PreInstrument(), &err) //autometrics:defer-statement
 	isErr := rand.Intn(2) == 0
 
 	if isErr {

Diff for: internal/doc/generate.go

@@ -10,14 +10,14 @@ import (
 )
 
 type AutometricsLinkCommentGenerator interface {
-	GenerateAutometricsComment(funcName string) []string
+	GenerateAutometricsComment(funcName, moduleName string) []string
 }
 
 // TransformFile takes a file path and generates the documentation
 // for the `//autometrics:doc` functions.
 //
-// It also replaces the file in place
-func TransformFile(path string, generator AutometricsLinkCommentGenerator) error {
+// It also replaces the file in place.
+func TransformFile(path, moduleName string, generator AutometricsLinkCommentGenerator) error {
 	cwd, err := os.Getwd()
 	if err != nil {
 		return fmt.Errorf("error getting a working directory: %w", err)
@@ -27,6 +27,7 @@ func TransformFile(path string, generator AutometricsLinkCommentGenerator) error
 	if err != nil {
 		return fmt.Errorf("error reading file information from %s: %w", path, err)
 	}
+
 	permissions := info.Mode()
 
 	sourceBytes, err := os.ReadFile(path)
@@ -40,7 +41,8 @@ func TransformFile(path string, generator AutometricsLinkCommentGenerator) error
 	}
 
 	sourceCode := string(sourceBytes)
-	transformedSource, err := GenerateDocumentation(sourceCode, generator)
+
+	transformedSource, err := GenerateDocumentation(sourceCode, moduleName, generator)
 	if err != nil {
 		return fmt.Errorf("error generating documentation: %w", err)
 	}
@@ -57,7 +59,7 @@ func TransformFile(path string, generator AutometricsLinkCommentGenerator) error
 // the documentation for the `//autometrics:doc` functions.
 //
 // It returns the new source code with augmented documentation.
-func GenerateDocumentation(sourceCode string, generator AutometricsLinkCommentGenerator) (string, error) {
+func GenerateDocumentation(sourceCode, moduleName string, generator AutometricsLinkCommentGenerator) (string, error) {
 	fileTree, err := decorator.Parse(sourceCode)
 	if err != nil {
 		return "", fmt.Errorf("error parsing source code: %w", err)
@@ -83,7 +85,7 @@ func GenerateDocumentation(sourceCode string, generator AutometricsLinkCommentGe
 			// Insert new autometrics comment
 			listIndex := hasAutometricsDocDirective(docComments)
 			if listIndex >= 0 {
-				autometricsComment := generateAutometricsComment(x.Name.Name, generator)
+				autometricsComment := generateAutometricsComment(x.Name.Name, moduleName, generator)
 				x.Decorations().Start.Replace(insertComments(docComments, listIndex, autometricsComment)...)
 			}
 		}
@@ -131,17 +133,18 @@ func autometricsDocEndDirective(commentGroup []string) int {
 	return -1
 }
 
-func generateAutometricsComment(funcName string, generator AutometricsLinkCommentGenerator) []string {
+func generateAutometricsComment(funcName, moduleName string, generator AutometricsLinkCommentGenerator) []string {
 	var ret []string
 	ret = append(ret, "//")
 	ret = append(ret, "//   autometrics:doc-start DO NOT EDIT")
 	ret = append(ret, "//")
 	ret = append(ret, "// # Autometrics")
 	ret = append(ret, "//")
-	ret = append(ret, generator.GenerateAutometricsComment(funcName)...)
+	ret = append(ret, generator.GenerateAutometricsComment(funcName, moduleName)...)
 	ret = append(ret, "//")
 	ret = append(ret, "//   autometrics:doc-end DO NOT EDIT")
 	ret = append(ret, "//")
+
 	return ret
 }
 
@@ -161,3 +164,28 @@ func insertComments(inputArray []string, index int, values []string) []string {
 
 	return inputArray
 }
+
+// errorReturnValueName returns the name of the error return value if it exists.
+func errorReturnValueName(funcNode *dst.FuncDecl) (string, error) {
+	returnValues := funcNode.Type.Results
+	if returnValues == nil || returnValues.List == nil {
+		return "", nil
+	}
+
+	for _, field := range returnValues.List {
+		fieldType := field.Type
+		if spec, ok := fieldType.(*dst.Ident); ok {
+			if spec.Name == "error" {
+				// Assuming that the `error` type has 0 or 1 name before it.
+				if field.Names == nil {
+					return "", nil
+				} else if len(field.Names) > 1 {
+					return "", fmt.Errorf("expecting a single named `error` return value, got %d instead.", len(field.Names))
+				}
+				return field.Names[0].Name, nil
+			}
+		}
+	}
+
+	return "", nil
+}