spanner: JSON number decode precision error

[harshachinta]

Issue
When JSON data contains a number, then decoding it to NullJson leads to loss of precision.

Test case to reproduce

func TestDecodeJsonValue(t *testing.T) {
	nulljson := NullJSON{}
	err := decodeValue(intProto(145688415796432520), jsonType(), &nulljson)
	if err != nil {
		fmt.Println(err)
	}
	fmt.Println(nulljson.Value)
	fmt.Println(nulljson)
}

The output of the above test case is

=== RUN   TestDecodeJsonValue
1.456884157964325e+17
145688415796432500

The expected output is 145688415796432520, but the returned output is 145688415796432500.

Code causing the issue
In decodeValue() function in value.go, looking at the switch case when interface is *NullJson. The code uses,

err := json.Unmarshal([]byte(x), &y)

The json package in golang by default considers the numbers in JSON as float64 values. The documentation is attached below,

Attaching the debugger output where the number is converted to float by the json package.

References
https://stackoverflow.com/questions/16946306/preserve-int64-values-when-parsing-json-in-go
https://stackoverflow.com/questions/24480835/decoding-json-int-into-string
golang/go#42246

[noahdietz]

On rotation, I took the liberty of "triaging" this issue. Update as you see fit.