printer: make sure output and errfile are always truncated (#2160)

Output and error files should be always truncated.

It is imperative that either the output file is truncated OR that the
cursor positioning (to EOF) is done before any write to the file
happens. If not, then we might have situations such as:

$ cat ebpf_events.json | wc -l
58010
$ cat ebpf_events.json | wc -l
58010
$ cat ebpf_events.json | wc -l
58011
$ cat ebpf_events.json | wc -l
58002
$ cat ebpf_events.json | wc -l
57989
$ cat ebpf_events.json | wc -l
57978

when the output file is truncated while being written to.

Author: rafaeldtinoco

cmd/tracee-ebpf/flags/output.go

@@ -110,17 +110,14 @@ func PrepareOutput(outputSlice []string) (tracee.OutputConfig, printer.Config, e
 	} else {
 		printcfg.OutPath = outPath
 		fileInfo, err := os.Stat(outPath)
-		if err == nil {
-			if fileInfo.IsDir() {
-				return outcfg, printcfg, fmt.Errorf("cannot use a path of existing directory %s", outPath)
-			}
-		} else {
-			dir := filepath.Dir(outPath)
-			os.MkdirAll(dir, 0755)
-			printcfg.OutFile, err = os.Create(outPath)
-			if err != nil {
-				return outcfg, printcfg, fmt.Errorf("failed to create output path: %v", err)
-			}
+		if err == nil && fileInfo.IsDir() {
+			return outcfg, printcfg, fmt.Errorf("cannot use a path of existing directory %s", outPath)
+		}
+		dir := filepath.Dir(outPath)
+		os.MkdirAll(dir, 0755)
+		printcfg.OutFile, err = os.Create(outPath)
+		if err != nil {
+			return outcfg, printcfg, fmt.Errorf("failed to create output path: %v", err)
 		}
 	}
 
@@ -129,17 +126,14 @@ func PrepareOutput(outputSlice []string) (tracee.OutputConfig, printer.Config, e
 	} else {
 		printcfg.ErrPath = errPath
 		fileInfo, err := os.Stat(errPath)
-		if err == nil {
-			if fileInfo.IsDir() {
-				return outcfg, printcfg, fmt.Errorf("cannot use a path of existing directory %s", errPath)
-			}
-		} else {
-			dir := filepath.Dir(errPath)
-			os.MkdirAll(dir, 0755)
-			printcfg.ErrFile, err = os.Create(errPath)
-			if err != nil {
-				return outcfg, printcfg, fmt.Errorf("failed to create output path: %v", err)
-			}
+		if err == nil && fileInfo.IsDir() {
+			return outcfg, printcfg, fmt.Errorf("cannot use a path of existing directory %s", errPath)
+		}
+		dir := filepath.Dir(errPath)
+		os.MkdirAll(dir, 0755)
+		printcfg.ErrFile, err = os.Create(errPath)
+		if err != nil {
+			return outcfg, printcfg, fmt.Errorf("failed to create output path: %v", err)
 		}
 	}
 

cmd/tracee-ebpf/internal/printer/printer.go

@@ -42,6 +42,14 @@ type Config struct {
 func New(config Config) (EventPrinter, error) {
 	var res EventPrinter
 	kind := config.Kind
+
+	if config.OutFile == nil {
+		return res, fmt.Errorf("out file is not set")
+	}
+	if config.ErrFile == nil {
+		return res, fmt.Errorf("err file is not set")
+	}
+
 	switch {
 	case kind == "ignore":
 		res = &ignoreEventPrinter{

cmd/tracee-ebpf/main.go

@@ -218,19 +218,6 @@ func main() {
 
 			}
 
-			if printerConfig.OutFile == nil {
-				printerConfig.OutFile, err = os.OpenFile(printerConfig.OutPath, os.O_WRONLY, 0755)
-				if err != nil {
-					return err
-				}
-			}
-			if printerConfig.ErrFile == nil {
-				printerConfig.ErrFile, err = os.OpenFile(printerConfig.ErrPath, os.O_WRONLY, 0755)
-				if err != nil {
-					return err
-				}
-			}
-
 			printer, err := printer.New(printerConfig)
 			if err != nil {
 				return err