fix: use cap to check slice emptiness. Fixes #27. (#28)

decoder/assembler_amd64.go

@@ -1176,7 +1176,7 @@ func (self *_Assembler) _asm_OP_array_clear_p(p *_Instr) {
 func (self *_Assembler) _asm_OP_slice_init(p *_Instr) {
     self.Emit("XORL" , _AX, _AX)                    // XORL    AX, AX
     self.Emit("MOVQ" , _AX, jit.Ptr(_VP, 8))        // MOVQ    AX, 8(VP)
-    self.Emit("MOVQ" , jit.Ptr(_VP, 0), _AX)        // MOVQ    (VP), AX
+    self.Emit("MOVQ" , jit.Ptr(_VP, 16), _AX)       // MOVQ    16(VP), AX
     self.Emit("TESTQ", _AX, _AX)                    // TESTQ   AX, AX
     self.Sjmp("JNZ"  , "_done_{n}")                 // JNZ     _done_{n}
     self.Emit("MOVQ" , jit.Imm(_MinSlice), _CX)     // MOVQ    ${_MinSlice}, CX
@@ -1189,6 +1189,8 @@ func (self *_Assembler) _asm_OP_slice_init(p *_Instr) {
     self.Emit("MOVQ" , jit.Ptr(_SP, 24), _AX)       // MOVQ    24(SP), AX
     self.Emit("MOVQ" , _AX, jit.Ptr(_VP, 0))        // MOVQ    AX, (VP)
     self.Link("_done_{n}")                          // _done_{n}:
+    self.Emit("XORL" , _AX, _AX)                    // XORL    AX, AX
+    self.Emit("MOVQ" , _AX, jit.Ptr(_VP, 8))        // MOVQ    AX, 8(VP)
 }
 
 func (self *_Assembler) _asm_OP_slice_append(p *_Instr) {

issue27_test.go

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2021 ByteDance Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package sonic
+
+import (
+    `reflect`
+    `sync`
+    `testing`
+
+    `github.com/stretchr/testify/require`
+)
+
+func TestIssue27_EmptySlice(t *testing.T) {
+    _ = Pretouch(reflect.TypeOf([]*int{}))
+    wg := sync.WaitGroup{}
+    for i := 0; i < 1000; i++ {
+        go func() {
+            defer wg.Done()
+            v := make([]*int, 16)
+            err := UnmarshalString(`[null, null]`, &v)
+            require.NoError(t, err)
+            require.Equal(t, []*int{nil, nil}, v)
+        }()
+        wg.Add(1)
+    }
+    wg.Wait()
+}