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internalGet: truncate capacity on return value #199

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@thempatel

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@thempatel

Using ArrayEach in concert with Set leads to some weird behavior. Specifically that ArrayEach provides slices over the original json object, such that the capacity of the slice is jsonEnd-(startOffsetOfSlice). This interacts poorly with Set when a particular key does not exist in the object being mutated. Given this testcase, you can see the output shows the second JSON object becoming malformed upon setting a non-existent key in the first object.

func TestWhatHappens(t *testing.T) {
	jsonObject := `[{"key1":"Foo","key2":"Bar","key3":"baz"},{"key1":"boo","key2":"dog","key3":"cat","key4":"mouse"}]`
	var allSlices [][]byte
	_, err := jsonparser.ArrayEach([]byte(jsonObject), func(value []byte, _ jsonparser.ValueType, _ int, err error) {
		if err != nil {
			t.Fatalf("failed: %s", err)
		}
		allSlices = append(allSlices, value)
	})

	if err != nil {
		t.Fatalf("failed: %s", err)
	}

	newVal, err := jsonparser.Set(allSlices[0], []byte(strconv.Quote(`mouse`)), "key4")
	if err != nil {
		t.Fatalf("failed %s: ", err)
	}
	fmt.Println(string(newVal))
	fmt.Println(string(allSlices[1]))
}

The suggested fix is to change this line:

value = append(data[:startOffset], append(createInsertComponent(keys[depth:], setValue, comma, object), data[depthOffset:]...)...)

to

value = append(data[:startOffset:startOffset], append(createInsertComponent(keys[depth:], setValue, comma, object), data[depthOffset:]...)...)

which truncates the capacity causing append to allocate a new slice. Alternatively, the pattern in the path currently exists flow can be used, this just happened to be a nifty one-liner

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