test/fuzz: add test to reproduce found fuzz errors (#6757)

This change does two things:
1. It fixes the json fuzzer to account for receiving array results. Arrays are returned by the rpc server when the input data is an array.
2. Adds a `fuzz_test.go` file and corresponding `testdata` directory containing the failing test case.

This seems like a reasonable way to add and track previous crash issues in our fuzz test cases. The upcoming stdlib go fuzz tool does effectively this automatically.

test/fuzz/rpc/jsonrpc/server/fuzz_test.go

@@ -0,0 +1,33 @@
+package server_test
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"github.com/tendermint/tendermint/test/fuzz/rpc/jsonrpc/server"
+)
+
+const testdataDir = "testdata"
+
+func TestServerOnTestData(t *testing.T) {
+	entries, err := os.ReadDir(testdataDir)
+	require.NoError(t, err)
+
+	for _, e := range entries {
+		entry := e
+		t.Run(entry.Name(), func(t *testing.T) {
+			defer func() {
+				r := recover()
+				require.Nilf(t, r, "testdata test panic")
+			}()
+			f, err := os.Open(filepath.Join(testdataDir, entry.Name()))
+			require.NoError(t, err)
+			input, err := ioutil.ReadAll(f)
+			require.NoError(t, err)
+			server.Fuzz(input)
+		})
+	}
+}

test/fuzz/rpc/jsonrpc/server/handler.go

@@ -1,4 +1,4 @@
-package handler
+package server
 
 import (
 	"bytes"
@@ -39,11 +39,26 @@ func Fuzz(data []byte) int {
 	if err := res.Body.Close(); err != nil {
 		panic(err)
 	}
-	if len(blob) > 0 {
-		recv := new(types.RPCResponse)
-		if err := json.Unmarshal(blob, recv); err != nil {
+	if len(blob) == 0 {
+		return 1
+	}
+
+	if inputJSONIsMultiElementSlice(data) {
+		recv := []types.RPCResponse{}
+		if err := json.Unmarshal(blob, &recv); err != nil {
 			panic(err)
 		}
+		return 1
+	}
+	recv := &types.RPCResponse{}
+	if err := json.Unmarshal(blob, recv); err != nil {
+		panic(err)
 	}
 	return 1
 }
+
+func inputJSONIsMultiElementSlice(input []byte) bool {
+	slice := []interface{}{}
+	err := json.Unmarshal(input, &slice)
+	return err == nil && len(slice) > 1
+}

test/fuzz/rpc/jsonrpc/server/testdata/1184f5b8d4b6dd08709cf1513f26744167065e0d

@@ -0,0 +1 @@
+[0]

test/fuzz/rpc/jsonrpc/server/testdata/clusterfuzz-testcase-minimized-fuzz_rpc_jsonrpc_server-4738572803506176

@@ -0,0 +1 @@
+[{"iD":7},{"iD":7}]