eth: fix some review comments

eth/api_tracer.go

@@ -62,7 +62,7 @@ type TraceConfig struct {
 
 // txTraceResult is the result of a single transaction trace.
 type txTraceResult struct {
-	Result interface{} `json:"result,omitempty"` // Trace results procuded by the tracer
+	Result interface{} `json:"result,omitempty"` // Trace results produced by the tracer
 	Error  string      `json:"error,omitempty"`  // Trace failure produced by the tracer
 }
 
@@ -79,7 +79,7 @@ type blockTraceTask struct {
 type blockTraceResult struct {
 	Block  hexutil.Uint64   `json:"block"`  // Block number corresponding to this trace
 	Hash   common.Hash      `json:"hash"`   // Block hash corresponding to this trace
-	Traces []*txTraceResult `json:"traces"` // Trace results procuded by the task
+	Traces []*txTraceResult `json:"traces"` // Trace results produced by the task
 }
 
 // txTraceTask represents a single transaction trace task when an entire block
@@ -118,6 +118,8 @@ func (db *ephemeralDatabase) Get(key []byte) ([]byte, error) {
 }
 
 // Prune does a state sync into a new memory write layer and replaces the old one.
+// This allows us to discard entries that are no longer referenced from the current
+// state.
 func (db *ephemeralDatabase) Prune(root common.Hash) {
 	// Pull the still relevant state data into memory
 	sync := state.NewStateSync(root, db.diskdb)

eth/tracers/internal/tracers/4byte_tracer.js

@@ -79,7 +79,7 @@
 	result: function(ctx) {
 		// Save the outer calldata also
 		if (ctx.input.length > 4) {
-			this.store(ctx.input.slice(0, 4), ctx.input.length-4)
+			this.store(slice(ctx.input, 0, 4), ctx.input.length-4)
 		}
 		return this.ids;
 	},

eth/tracers/tracer.go

@@ -370,6 +370,23 @@ func New(code string) (*Tracer, error) {
 		ctx.PushBoolean(ok)
 		return 1
 	})
+	tracer.vm.PushGlobalGoFunction("slice", func(ctx *duktape.Context) int {
+		start, end := ctx.GetInt(-2), ctx.GetInt(-1)
+		ctx.Pop2()
+
+		blob := popSlice(ctx)
+		size := end - start
+
+		if start < 0 || start > end || end > len(blob) {
+			// TODO(karalabe): We can't js-throw from Go inside duktape inside Go. The Go
+			// runtime goes belly up https://github.com/golang/go/issues/15639.
+			log.Warn("Tracer accessed out of bound memory", "available", len(blob), "offset", start, "size", size)
+			ctx.PushFixedBuffer(0)
+			return 1
+		}
+		copy(makeSlice(ctx.PushFixedBuffer(size), uint(size)), blob[start:end])
+		return 1
+	})
 	// Push the JavaScript tracer as object #0 onto the JSVM stack and validate it
 	if err := tracer.vm.PevalString("(" + code + ")"); err != nil {
 		log.Warn("Failed to compile tracer", "err", err)
@@ -559,25 +576,11 @@ func (jst *Tracer) CaptureEnd(output []byte, gasUsed uint64, t time.Duration, er
 
 // GetResult calls the Javascript 'result' function and returns its value, or any accumulated error
 func (jst *Tracer) GetResult() (json.RawMessage, error) {
-	// Push all the data buffers into the stack first
-	keys := make([]string, 0, len(jst.ctx))
-	for key := range jst.ctx {
-		keys = append(keys, key)
-	}
-	buffers := 0
-	for _, key := range keys {
-		switch val := jst.ctx[key].(type) {
-		case []byte:
-			ptr := jst.vm.PushFixedBuffer(len(val))
-			copy(makeSlice(ptr, uint(len(val))), val[:])
-			buffers++
-		}
-	}
 	// Transform the context into a JavaScript object and inject into the state
 	obj := jst.vm.PushObject()
 
-	for _, key := range keys {
-		switch val := jst.ctx[key].(type) {
+	for key, val := range jst.ctx {
+		switch val := val.(type) {
 		case uint64:
 			jst.vm.PushUint(uint(val))
 
@@ -585,8 +588,8 @@ func (jst *Tracer) GetResult() (json.RawMessage, error) {
 			jst.vm.PushString(val)
 
 		case []byte:
-			jst.vm.PushBufferObject(-(buffers + 1), len(val), len(val), duktape.BufobjArraybuffer)
-			buffers--
+			ptr := jst.vm.PushFixedBuffer(len(val))
+			copy(makeSlice(ptr, uint(len(val))), val[:])
 
 		case common.Address:
 			ptr := jst.vm.PushFixedBuffer(20)