enconter panic when calling unmarshal()

[PearsonCHN]

panic: runtime error: invalid memory address or nil pointer dereference [signal SIGSEGV: segmentation violation code=0x1 addr=0x88 pc=0xa0cc93]
--
goroutine 698 [running]:
github.com/goccy/go-json/internal/decoder.(*structDecoder).Decode(0x0, 0xc0019a8100, 0x1e0?, 0xc000faa000?, 0xc000fca000) 	/builds/worker/vendor/github.com/goccy/go-json/internal/decoder/struct.go:760 +0x3f3
github.com/goccy/go-json.unmarshal({0xc000faa000?, 0xc0016ad428?, 0xb287a6?}, {0x1e42420, 0xc000fca000}, {0x0, 0x0, 0x59?}) 	/builds/worker/vendor/github.com/goccy/go-json/decode.go:47 +0x186
github.com/goccy/go-json.Unmarshal(...) 	/builds/worker/vendor/github.com/goccy/go-json/json.go:276

go-json version in go.mod is v0.9.10

[3AceShowHand]

Do you have reproduce scripts? I think I also meet this problem, but don't know how to reproduce since lack of data

[Yu-HuanWu]

Looks like an issue i've encountered as well. The problem seems to be that the structDecoder, d, in the Decode func is nil.

I have a PR for this bandaid fix: #557

The actual root cause is most likely due to race condition, where the shared cachedDecoder map was being written to and read from by multiple goroutines without any synchronization.

To reproduce this panic, I wrote this test utilizing t.Parallel:

import (
	"sync"
	"testing"

	gojson "github.com/goccy/go-json"
)

// simple, valid struct that should never fail.
type ValidStruct struct {
	SID string `json:"sid"`
}

// this struct contains a function field, which is an unsupported type that should trigger the silent (nil, nil) return from the decoder's compiler.
type WillFailStruct struct {
	ID   int
	Func func()
}

func TestGojsonUnmarshal(t *testing.T) {
	t.Run("Will Fail", func(t *testing.T) {
		t.Parallel() 

		willFailJSON := []byte(`{"ID": 1}`)
		var f WillFailStruct
		for i := 0; i < 1000; i++ {
			_ = gojson.Unmarshal(willFailJSON, &f)
		}
	})

	t.Run("Valid Struct", func(t *testing.T) {
		t.Parallel() 

		validJSON := []byte(`{"sid":"12345"}`)
		var v ValidStruct

		var wg sync.WaitGroup
		numGoroutines := 100
		wg.Add(numGoroutines)

		for i := 0; i < numGoroutines; i++ {
			go func() {
				defer wg.Done()
				for j := 0; j < 100; j++ {
					func() {
						defer func() {
							if r := recover(); r != nil {
								t.Errorf("PANIC DETECTED while unmarshaling a valid struct. This proves the cache was polluted. Panic: %v", r)
							}
						}()
						_ = gojson.Unmarshal(validJSON, &v)
					}()
				}
			}()
		}
		wg.Wait()
	})
}

When i ran go test -race -v, I was able to see WARNING: DATA RACE

Hope this helps.

[TelpeNight]

Similar bug: #566
Potential fix: #567