perf(libs/json): Lower heap overhead of JSON encoding (backport #2846) (

#2876)

---

Many RPC methods require JSON marshalled responses. We saw this taking a
notable amount of heap allocation in query serving full nodes. This PR
removes some extra heap allocations that were being done. We avoided
using the more efficient encoder.Encode before, because it added a
newline. This PR changes the function signature for these private
methods to be using *bytes.Buffer, and then uses the in-buffer methods
(rather than a second copy). We then just truncate the final byte after
each such call, which does not waste any allocations.

I added a benchmark for the most complex test case. 

OLD:
```
BenchmarkJsonMarshalStruct-12              78992             15542 ns/op            4487 B/op        191 allocs/op
```
New:
```
BenchmarkJsonMarshalStruct-12              93346             11132 ns/op            3245 B/op         58 allocs/op
```

Roughly a 3-4x reduction in the number of allocations, and 20% speedup.

#### PR checklist

- [x] Tests written/updated - Existing tests cover this
- [x] Changelog entry added in `.changelog` (we use
[unclog](https://github.com/informalsystems/unclog) to manage our
changelog)
- [x] Updated relevant documentation (`docs/` or `spec/`) and code
comments
- [x] Title follows the [Conventional
Commits](https://www.conventionalcommits.org/en/v1.0.0/) spec
<hr>This is an automatic backport of pull request #2846 done by
[Mergify](https://mergify.com).

---------

Co-authored-by: Dev Ojha <ValarDragon@users.noreply.github.com>
Co-authored-by: Andy Nogueira <me@andynogueira.dev>

.changelog/v0.37.5/improvements/2846-speedup-json-encoding.md

@@ -0,0 +1,2 @@
+- `[libs/json]` Lower the memory overhead of JSON encoding by using JSON encoders internally
+  ([\#2846](https://github.com/cometbft/cometbft/pull/2846)).

libs/json/encoder.go

@@ -42,7 +42,7 @@ func MarshalIndent(v interface{}, prefix, indent string) ([]byte, error) {
 	return buf.Bytes(), nil
 }
 
-func encode(w io.Writer, v interface{}) error {
+func encode(w *bytes.Buffer, v any) error {
 	// Bare nil values can't be reflected, so we must handle them here.
 	if v == nil {
 		return writeStr(w, "null")
@@ -60,7 +60,7 @@ func encode(w io.Writer, v interface{}) error {
 	return encodeReflect(w, rv)
 }
 
-func encodeReflect(w io.Writer, rv reflect.Value) error {
+func encodeReflect(w *bytes.Buffer, rv reflect.Value) error {
 	if !rv.IsValid() {
 		return errors.New("invalid reflect value")
 	}
@@ -115,7 +115,7 @@ func encodeReflect(w io.Writer, rv reflect.Value) error {
 	}
 }
 
-func encodeReflectList(w io.Writer, rv reflect.Value) error {
+func encodeReflectList(w *bytes.Buffer, rv reflect.Value) error {
 	// Emit nil slices as null.
 	if rv.Kind() == reflect.Slice && rv.IsNil() {
 		return writeStr(w, "null")
@@ -150,7 +150,7 @@ func encodeReflectList(w io.Writer, rv reflect.Value) error {
 	return writeStr(w, "]")
 }
 
-func encodeReflectMap(w io.Writer, rv reflect.Value) error {
+func encodeReflectMap(w *bytes.Buffer, rv reflect.Value) error {
 	if rv.Type().Key().Kind() != reflect.String {
 		return errors.New("map key must be string")
 	}
@@ -181,7 +181,7 @@ func encodeReflectMap(w io.Writer, rv reflect.Value) error {
 	return writeStr(w, "}")
 }
 
-func encodeReflectStruct(w io.Writer, rv reflect.Value) error {
+func encodeReflectStruct(w *bytes.Buffer, rv reflect.Value) error {
 	sInfo := makeStructInfo(rv.Type())
 	if err := writeStr(w, "{"); err != nil {
 		return err
@@ -212,7 +212,7 @@ func encodeReflectStruct(w io.Writer, rv reflect.Value) error {
 	return writeStr(w, "}")
 }
 
-func encodeReflectInterface(w io.Writer, rv reflect.Value) error {
+func encodeReflectInterface(w *bytes.Buffer, rv reflect.Value) error {
 	// Get concrete value and dereference pointers.
 	for rv.Kind() == reflect.Ptr || rv.Kind() == reflect.Interface {
 		if rv.IsNil() {
@@ -237,14 +237,17 @@ func encodeReflectInterface(w io.Writer, rv reflect.Value) error {
 	return writeStr(w, "}")
 }
 
-func encodeStdlib(w io.Writer, v interface{}) error {
-	// Doesn't stream the output because that adds a newline, as per:
-	// https://golang.org/pkg/encoding/json/#Encoder.Encode
-	blob, err := json.Marshal(v)
+func encodeStdlib(w *bytes.Buffer, v any) error {
+	// Stream the output of the JSON marshaling directly into the buffer.
+	// The stdlib encoder will write a newline, so we must truncate it,
+	// which is why we pass in a bytes.Buffer throughout, not io.Writer.
+	enc := json.NewEncoder(w)
+	err := enc.Encode(v)
 	if err != nil {
 		return err
 	}
-	_, err = w.Write(blob)
+	// Remove the last byte from the buffer
+	w.Truncate(w.Len() - 1)
 	return err
 }
 

libs/json/encoder_test.go

@@ -102,3 +102,20 @@ func TestMarshal(t *testing.T) {
 		})
 	}
 }
+
+func BenchmarkJsonMarshalStruct(b *testing.B) {
+	s := "string"
+	sPtr := &s
+	i64 := int64(64)
+	ti := time.Date(2020, 6, 2, 18, 5, 13, 4346374, time.FixedZone("UTC+2", 2*60*60))
+	car := &Car{Wheels: 4}
+	boat := Boat{Sail: true}
+	for i := 0; i < b.N; i++ {
+		_, _ = json.Marshal(Struct{
+			Bool: true, Float64: 3.14, Int32: 32, Int64: 64, Int64Ptr: &i64,
+			String: "foo", StringPtrPtr: &sPtr, Bytes: []byte{1, 2, 3},
+			Time: ti, Car: car, Boat: boat, Vehicles: []Vehicle{car, boat},
+			Child: &Struct{Bool: false, String: "child"}, private: "private",
+		})
+	}
+}