tracer: make bytes truncation configurable (#60)

cmd/wasirun/main.go

@@ -103,6 +103,7 @@ var (
 	wasiHttpAddr     string
 	wasiHttpPath     string
 	trace            bool
+	tracerStringSize int
 	nonBlockingStdio bool
 	version          bool
 	maxOpenFiles     int
@@ -125,6 +126,7 @@ func main() {
 	flagSet.StringVar(&wasiHttpAddr, "http-server-addr", "", "")
 	flagSet.StringVar(&wasiHttpPath, "http-server-path", "/", "")
 	flagSet.BoolVar(&trace, "trace", false, "")
+	flagSet.IntVar(&tracerStringSize, "tracer-string-size", 32, "")
 	flagSet.BoolVar(&nonBlockingStdio, "non-blocking-stdio", false, "")
 	flagSet.BoolVar(&version, "version", false, "")
 	flagSet.BoolVar(&version, "v", false, "")
@@ -212,7 +214,7 @@ func run(wasmFile string, args []string) error {
 		WithDials(dials...).
 		WithNonBlockingStdio(nonBlockingStdio).
 		WithSocketsExtension(socketExt, wasmModule).
-		WithTracer(trace, os.Stderr).
+		WithTracer(trace, os.Stderr, wasi.WithTracerStringSize(tracerStringSize)).
 		WithMaxOpenFiles(maxOpenFiles).
 		WithMaxOpenDirs(maxOpenDirs)
 

imports/builder.go

@@ -36,6 +36,7 @@ type Builder struct {
 	pathOpenSockets    bool
 	nonBlockingStdio   bool
 	tracer             io.Writer
+	tracerOptions      []wasi.TracerOption
 	decorators         []wasi_snapshot_preview1.Decorator
 	wrappers           []func(wasi.System) wasi.System
 	errors             []error
@@ -198,11 +199,12 @@ func (b *Builder) WithNonBlockingStdio(enable bool) *Builder {
 
 // WithTracer enables the Tracer, and instructs it to write to the
 // specified io.Writer.
-func (b *Builder) WithTracer(enable bool, w io.Writer) *Builder {
+func (b *Builder) WithTracer(enable bool, w io.Writer, options ...wasi.TracerOption) *Builder {
 	if !enable {
 		w = nil
 	}
 	b.tracer = w
+	b.tracerOptions = options
 	return b
 }
 

imports/builder_unix.go

@@ -94,7 +94,7 @@ func (b *Builder) Instantiate(ctx context.Context, runtime wazero.Runtime) (ctxr
 		system = &unix.PathOpenSockets{System: unixSystem}
 	}
 	if b.tracer != nil {
-		system = wasi.Trace(b.tracer, system)
+		system = wasi.Trace(b.tracer, system, b.tracerOptions...)
 	}
 	for _, wrap := range b.wrappers {
 		system = wrap(system)

tracer.go

@@ -8,13 +8,35 @@ import (
 
 // Trace wraps a System to log all calls to its methods in a human-readable
 // format to the given io.Writer.
-func Trace(w io.Writer, s System) System {
-	return &tracer{writer: w, system: s}
+func Trace(w io.Writer, s System, options ...TracerOption) System {
+	t := &tracer{
+		writer:     w,
+		system:     s,
+		stringSize: 32,
+	}
+	for _, option := range options {
+		option(t)
+	}
+	return t
+}
+
+// TracerOption configures a tracer.
+type TracerOption func(*tracer)
+
+// WithTracerStringSize sets the number of bytes to print when
+// printing strings.
+//
+// To disable truncation of strings, use stringSize < 0.
+//
+// The default string size is 32.
+func WithTracerStringSize(stringSize int) TracerOption {
+	return func(t *tracer) { t.stringSize = stringSize }
 }
 
 type tracer struct {
-	writer io.Writer
-	system System
+	writer     io.Writer
+	system     System
+	stringSize int
 }
 
 func (t *tracer) ArgsSizesGet(ctx context.Context) (int, int, Errno) {
@@ -876,15 +898,13 @@ func (t *tracer) printAddressInfo(a AddressInfo) {
 	t.printf("}")
 }
 
-const maxBytes = 32
-
 func (t *tracer) printBytes(b []byte) {
 	t.printf("[%d]byte(\"", len(b))
 
 	if len(b) > 0 {
 		trunc := b
-		if len(b) > maxBytes {
-			trunc = trunc[:maxBytes]
+		if t.stringSize >= 0 && len(b) > t.stringSize {
+			trunc = trunc[:t.stringSize]
 		}
 		for _, c := range trunc {
 			if c < 32 || c >= 127 || c == '"' {
@@ -911,7 +931,7 @@ func (t *tracer) printBytes(b []byte) {
 		}
 	}
 	t.printf("\"")
-	if len(b) > maxBytes {
+	if t.stringSize >= 0 && len(b) > t.stringSize {
 		t.printf("...")
 	}
 	t.printf(")")