fix: apply patches in ascending order (#24)

internal/testdata/basic.go

@@ -128,3 +128,16 @@ func Closure(ctx context.Context) (int, error) {
 	a := func(x int) (int, error) { return x + 1, nil }
 	return a(5)
 }
+
+func FunctionCallingAnonymousFunc(ctx context.Context) error {
+	if err := Exec(ctx, func(ctx context.Context) error {
+		return nil
+	}); err != nil {
+		return err
+	}
+	return nil
+}
+
+func Exec(ctx context.Context, fn func(ctx context.Context) error) error {
+	return fn(ctx)
+}

internal/testdata/instrumented/basic.go.exp

@@ -219,3 +219,25 @@ func Closure(ctx context.Context) (int, error) {
 	a := func(x int) (int, error) { return x + 1, nil }
 	return a(5)
 }
+
+func FunctionCallingAnonymousFunc(ctx context.Context) error {
+	ctx, span := otel.Tracer("app").Start(ctx, "FunctionCallingAnonymousFunc")
+	defer span.End()
+
+	if err := Exec(ctx, func(ctx context.Context) error {
+		ctx, span := otel.Tracer("app").Start(ctx, "anonymous")
+		defer span.End()
+
+		return nil
+	}); err != nil {
+		return err
+	}
+	return nil
+}
+
+func Exec(ctx context.Context, fn func(ctx context.Context) error) error {
+	ctx, span := otel.Tracer("app").Start(ctx, "Exec")
+	defer span.End()
+
+	return fn(ctx)
+}

processor/processor.go

@@ -4,6 +4,7 @@ import (
 	"go/ast"
 	"go/token"
 	"go/types"
+	"sort"
 
 	"golang.org/x/tools/go/ast/astutil"
 )
@@ -185,6 +186,10 @@ func (p *Processor) Process(fset *token.FileSet, file *ast.File) error {
 	})
 
 	if len(patches) > 0 {
+		// Patches must be applied in the ascending order, otherwise the
+		// modified source file will become corrupted.
+		sort.Slice(patches, func(i, j int) bool { return patches[i].pos < patches[j].pos })
+
 		if err := patchFile(fset, file, patches...); err != nil {
 			return err
 		}