Fix context replacement for http.Request

The context of a HTTP request is not assignable, so we need to read from the existing context and create a new one

CHANGELOG.md

@@ -27,10 +27,11 @@ specification.
   tracking. If `autometrics` detects a `context.Context` it can use, it will shadow the
   context with the autometrics augmented one to reduce the changes to make in the code.
   Currently, `autometrics` can detect arguments of specific types in the function signatures
-  to replace contexts:
-  + `context.Context`
-  + `http.Request`
-  + `buffalo.Request`
+  to read from and replace contexts:
+  + `context.Context` (read and replace)
+  + `http.Request` (read only)
+  + `buffalo.Request` (read and replace)
+  + `gin.Context` (read only; _very_ experimental)
 - [Generator] The generator now tries to keep going with instrumentation even if some instrumentation
   fails. On error, still _No file will be modified_, and the generator will exit with an error code
   and output all the collected errors instead of only the first one.

examples/otel/cmd/main.go

@@ -98,14 +98,14 @@ func main() {
 //
 //autometrics:doc --slo "API" --latency-target 99 --latency-ms 5
 func indexHandler(w http.ResponseWriter, r *http.Request) error {
-	r.Context() = autometrics.PreInstrument(autometrics.NewContext(
+	amCtx := autometrics.PreInstrument(autometrics.NewContext(
 		r.Context(),
 		autometrics.WithConcurrentCalls(true),
 		autometrics.WithCallerName(true),
 		autometrics.WithSloName("API"),
 		autometrics.WithAlertLatency(5000000*time.Nanosecond, 99),
 	)) //autometrics:shadow-ctx
-	defer autometrics.Instrument(r.Context(), nil) //autometrics:defer
+	defer autometrics.Instrument(amCtx, nil) //autometrics:defer
 
 	msSleep := rand.Intn(200)
 	time.Sleep(time.Duration(msSleep) * time.Millisecond)
@@ -149,14 +149,14 @@ var handlerError = errors.New("failed to handle request")
 //
 //autometrics:doc --slo "API" --success-target 90
 func randomErrorHandler(w http.ResponseWriter, r *http.Request) (err error) {
-	r.Context() = autometrics.PreInstrument(autometrics.NewContext(
+	amCtx := autometrics.PreInstrument(autometrics.NewContext(
 		r.Context(),
 		autometrics.WithConcurrentCalls(true),
 		autometrics.WithCallerName(true),
 		autometrics.WithSloName("API"),
 		autometrics.WithAlertSuccess(90),
 	)) //autometrics:shadow-ctx
-	defer autometrics.Instrument(r.Context(), &err) //autometrics:defer
+	defer autometrics.Instrument(amCtx, &err) //autometrics:defer
 
 	isOk := rand.Intn(10) == 0
 

examples/web/cmd/main.go

@@ -107,14 +107,14 @@ func main() {
 //
 //autometrics:inst --slo "API" --latency-target 99 --latency-ms 5
 func indexHandler(w http.ResponseWriter, r *http.Request) error {
-	r.Context() = autometrics.PreInstrument(autometrics.NewContext(
+	amCtx := autometrics.PreInstrument(autometrics.NewContext(
 		r.Context(),
 		autometrics.WithConcurrentCalls(true),
 		autometrics.WithCallerName(true),
 		autometrics.WithSloName("API"),
 		autometrics.WithAlertLatency(5000000*time.Nanosecond, 99),
 	)) //autometrics:shadow-ctx
-	defer autometrics.Instrument(r.Context(), nil) //autometrics:defer
+	defer autometrics.Instrument(amCtx, nil) //autometrics:defer
 
 	msSleep := rand.Intn(200)
 	time.Sleep(time.Duration(msSleep) * time.Millisecond)

internal/autometrics/ctx.go

@@ -47,8 +47,11 @@ type GeneratorContext struct {
 // only statically-known information is useful at this point.
 type RuntimeCtxInfo struct {
 	// Name of the variable to use as context.Context when building the autometrics.Context.
-	// The string will be empty if and only if 'nil' must be used as autometrics.NewContext() argument.
+	// The string will be empty if 'nil' must be used as autometrics.NewContext() argument.
 	ContextVariableName string
+	// Name of the variable to assign to the context returned by autometrics.PreInstrument
+	// The string will be empty if a new variable must be used as autometrics.PreInstrument() return value assignment.
+	NewContextVariableName string
 	// Verbatim code to use to fetch the TraceID.
 	// For example, if the instrumented function is detected to use Gin, like
 	// `func ginHandler(ginVarName *gin.Context)`
@@ -65,9 +68,10 @@ type RuntimeCtxInfo struct {
 
 func DefaultRuntimeCtxInfo() RuntimeCtxInfo {
 	return RuntimeCtxInfo{
-		TrackConcurrentCalls: true,
-		TrackCallerName:      true,
-		ContextVariableName:  "nil",
+		TrackConcurrentCalls:   true,
+		TrackCallerName:        true,
+		ContextVariableName:    "nil",
+		NewContextVariableName: "",
 	}
 }
 

internal/generate/context.go

@@ -148,8 +148,8 @@ func buildAutometricsContextNode(agc *internal.GeneratorContext) (newContextCall
 		}
 	}
 
-	contextShadowName = agc.RuntimeCtx.ContextVariableName
-	if contextShadowName == "nil" {
+	contextShadowName = agc.RuntimeCtx.NewContextVariableName
+	if contextShadowName == "nil" || contextShadowName == "" {
 		contextShadowName = amDefaultContextName
 	}
 
@@ -224,6 +224,7 @@ func detectContextIdentImpl(ctx *internal.GeneratorContext, argName string, iden
 
 		if canonical == vanillaContext && typeName == "Context" {
 			ctx.RuntimeCtx.ContextVariableName = argName
+			ctx.RuntimeCtx.NewContextVariableName = argName
 			ctx.RuntimeCtx.SpanIDGetter = ""
 			ctx.RuntimeCtx.TraceIDGetter = ""
 			return true, nil
@@ -236,6 +237,7 @@ func detectContextIdentImpl(ctx *internal.GeneratorContext, argName string, iden
 			} else {
 				ctx.RuntimeCtx.ContextVariableName = fmt.Sprintf("%s.Context()", argName)
 			}
+			ctx.RuntimeCtx.NewContextVariableName = ""
 			ctx.RuntimeCtx.SpanIDGetter = ""
 			ctx.RuntimeCtx.TraceIDGetter = ""
 			return true, nil
@@ -250,6 +252,7 @@ func detectContextIdentImpl(ctx *internal.GeneratorContext, argName string, iden
 		// Buffalo context embeds a context.Context so it can work like vanilla
 		if canonical == buffalo && typeName == "Context" {
 			ctx.RuntimeCtx.ContextVariableName = argName
+			ctx.RuntimeCtx.NewContextVariableName = argName
 			ctx.RuntimeCtx.SpanIDGetter = ""
 			ctx.RuntimeCtx.TraceIDGetter = ""
 			return true, nil
@@ -275,6 +278,7 @@ func detectContextSelectorImpl(ctx *internal.GeneratorContext, argName string, s
 		for alias, canonical := range ctx.ImportsMap {
 			if canonical == vanillaContext && parentName == alias && typeName == "Context" {
 				ctx.RuntimeCtx.ContextVariableName = argName
+				ctx.RuntimeCtx.NewContextVariableName = argName
 				ctx.RuntimeCtx.SpanIDGetter = ""
 				ctx.RuntimeCtx.TraceIDGetter = ""
 				return true, nil
@@ -288,6 +292,7 @@ func detectContextSelectorImpl(ctx *internal.GeneratorContext, argName string, s
 				} else {
 					ctx.RuntimeCtx.ContextVariableName = fmt.Sprintf("%s.Context()", argName)
 				}
+				ctx.RuntimeCtx.NewContextVariableName = ""
 				ctx.RuntimeCtx.SpanIDGetter = ""
 				ctx.RuntimeCtx.TraceIDGetter = ""
 				return true, nil
@@ -302,6 +307,7 @@ func detectContextSelectorImpl(ctx *internal.GeneratorContext, argName string, s
 			// Buffalo context embeds a context.Context so it can work like vanilla
 			if canonical == buffalo && parentName == alias && typeName == "Context" {
 				ctx.RuntimeCtx.ContextVariableName = argName
+				ctx.RuntimeCtx.NewContextVariableName = argName
 				ctx.RuntimeCtx.SpanIDGetter = ""
 				ctx.RuntimeCtx.TraceIDGetter = ""
 				return true, nil

internal/generate/defer_test.go

@@ -216,14 +216,14 @@ func main(w http.ResponseWriter, req *http.Request) {
 		"//\n" +
 		"//autometrics:inst --no-doc --slo \"Service Test\" --success-target 99\n" +
 		"func main(w http.ResponseWriter, req *http.Request) {\n" +
-		"\treq.Context() = prom.PreInstrument(prom.NewContext(\n" +
+		"\tamCtx := prom.PreInstrument(prom.NewContext(\n" +
 		"\t\treq.Context(),\n" +
 		"\t\tprom.WithConcurrentCalls(true),\n" +
 		"\t\tprom.WithCallerName(true),\n" +
 		"\t\tprom.WithSloName(\"Service Test\"),\n" +
 		"\t\tprom.WithAlertSuccess(99),\n" +
 		"\t)) //autometrics:shadow-ctx\n" +
-		"\tdefer prom.Instrument(req.Context(), nil) //autometrics:defer\n" +
+		"\tdefer prom.Instrument(amCtx, nil) //autometrics:defer\n" +
 		"\n" +
 		"	fmt.Println(hello) // line comment 3\n" +
 		"}\n"
@@ -274,14 +274,14 @@ func main(w vanilla.ResponseWriter, req *vanilla.Request) {
 		"//\n" +
 		"//autometrics:inst --no-doc --slo \"Service Test\" --success-target 99\n" +
 		"func main(w vanilla.ResponseWriter, req *vanilla.Request) {\n" +
-		"\treq.Context() = prom.PreInstrument(prom.NewContext(\n" +
+		"\tamCtx := prom.PreInstrument(prom.NewContext(\n" +
 		"\t\treq.Context(),\n" +
 		"\t\tprom.WithConcurrentCalls(true),\n" +
 		"\t\tprom.WithCallerName(true),\n" +
 		"\t\tprom.WithSloName(\"Service Test\"),\n" +
 		"\t\tprom.WithAlertSuccess(99),\n" +
 		"\t)) //autometrics:shadow-ctx\n" +
-		"\tdefer prom.Instrument(req.Context(), nil) //autometrics:defer\n" +
+		"\tdefer prom.Instrument(amCtx, nil) //autometrics:defer\n" +
 		"\n" +
 		"	fmt.Println(hello) // line comment 3\n" +
 		"}\n"
@@ -332,14 +332,14 @@ func main(w ResponseWriter, req *Request) {
 		"//\n" +
 		"//autometrics:inst --no-doc --slo \"Service Test\" --success-target 99\n" +
 		"func main(w ResponseWriter, req *Request) {\n" +
-		"\treq.Context() = prom.PreInstrument(prom.NewContext(\n" +
+		"\tamCtx := prom.PreInstrument(prom.NewContext(\n" +
 		"\t\treq.Context(),\n" +
 		"\t\tprom.WithConcurrentCalls(true),\n" +
 		"\t\tprom.WithCallerName(true),\n" +
 		"\t\tprom.WithSloName(\"Service Test\"),\n" +
 		"\t\tprom.WithAlertSuccess(99),\n" +
 		"\t)) //autometrics:shadow-ctx\n" +
-		"\tdefer prom.Instrument(req.Context(), nil) //autometrics:defer\n" +
+		"\tdefer prom.Instrument(amCtx, nil) //autometrics:defer\n" +
 		"\n" +
 		"	fmt.Println(hello) // line comment 3\n" +
 		"}\n"