[SR-6440] JSONDecoder behaves differently on macOS vs. Linux

[gwynne]

Previous ID	SR-6440
Radar	None
Original Reporter	@gwynne
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Foundation
Labels	Bug
Assignee	None
Priority	Medium

md5: ba2ce7916df7171b8a4d2917feb20190

Issue Description:

When decoding JSON, requesting an integer value which is too large for the requested integer type (e.g. 10000000000000000000000000000000000000) will throw the following exception on macOS:

dataCorrupted(Swift.DecodingError.Context(codingPath: [VaporTests.CodableTests.(TestModel in _3EDCAB1B5E5ADB4E80B35729410FF649).CodingKeys.bomb], debugDescription: "Parsed JSON number <10000000000000000000000000000000000000> does not fit in Int64.", underlyingError: nil))

However, on Linux, the value Int64.max is silently returned instead.

As far as I can determine, this corresponds to the documented behavior of strtol() on overflow, per https://github.com/apple/swift-corelibs-foundation/blob/master/Foundation/JSONSerialization.swift#L826.

The exception thrown on macOS appears at https://github.com/apple/swift-corelibs-foundation/blob/master/Foundation/JSONEncoder.swift#L1832, but the logic doesn't fire on Linux.

The test case which showed this behavior is seen in this commit (which removes the test due to the behavior difference): vapor/vapor@7ebe247

Credit for finding this issue goes to https://github.com/vzsg

[spevans]

Possible fix: #1655

[spevans]

This is now fixed in swift-DEVELOPMENT-SNAPSHOT-2018-11-13

$ cat sr_6440.swift 
import Foundation

let rawJSON = """
{
"foo": "abc",
"baz": 123,
"bomb": 10000000000000000000000000000000000000
}
""".data(using: .utf8)!

struct Foo: Decodable {
    let foo: String
    let baz: Int
    let bomb: Int64
}

let d = try JSONDecoder().decode(Foo.self, from: rawJSON)
print(d)

$ ~/swift-DEVELOPMENT-SNAPSHOT-2018-11-13-a-ubuntu18.04/usr/bin/swift sr_6440.swift 
Fatal error: Error raised at top level: Swift.DecodingError.dataCorrupted(Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "bomb", intValue: nil)], debugDescription: "Parsed JSON number <10000000000000000000000000000000000000> does not fit in Int64.", underlyingError: nil)): file /home/buildnode/jenkins/workspace/oss-swift-package-linux-ubuntu-18_04/swift/stdlib/public/core/ErrorType.swift, line 200

Changing the Int64 to Decimal now allows proper decoding

$ ~/swift-DEVELOPMENT-SNAPSHOT-2018-11-13-a-ubuntu18.04/usr/bin/swift sr_6440.swift 
Foo(foo: "abc", baz: 123, bomb: 10000000000000000000000000000000000000)