Severe Decimal error in Foundation on Linux needs workaround

(David Monagle) #1

There is a serious bug in Decimal which causes incorrect results when adding and subtracting. The bug can be seen here

It's been fixed and merged into the 4.2 branch, but that leaves many potential people running production code on 4.1.2 who may not be aware that this problem exists.

I wanted to post this here to generate a little awareness around this problem and encourage people to have a good look at their Decimal usage. One of Decimal's biggest use cases is currency - obviously not an ideal place to have a bug of this nature.

In the absence of a patch release for 4.1, it will be at least September before the release of 4.2 - a long time to have a bug of this magnitude in production. Today I started working through the process of creating a patched version of Swift in Docker for us to use in lieu of the offical 4.1.2 branch, but I've already hit several problems getting the build process to work and I think it's going to take a fair bit of my time to get it working.

It has occurred to me that others may have better ideas on how to get a running container up with a version of Foundation that contains the patch. So, if anybody has any input, it would be hugely appreciated!

(Thomas Krajacic) #2

:flushed: Now that's a pretty bug…

(Nicole Jacque) #3

I agree, this is a pretty serious bug. We have cherry-picked the fix to the 4.1 branch, and we're going to do a Swift 4.1.3 release with snapshots so that folks can pick this up. We are working on generating the snapshots today, but they may not be complete until Monday.

(David Monagle) #4

Thank you so much for getting this done Nicole! We are big advocates for Swift and use it extensively in production so this has been a bit of a headache for us. We will be building based on the new snapshot today and hopefully the days of incorrect invoices will be behind us.

1 Like
(Nicole Jacque) #5

Ok, snapshot is posted!

1 Like
(Dana Shadrick) #6

Thank you for this post and the fix in 4.1.3. There seems to be a related problem with NumberFormatter(). When converting the result of a Decimal calculation that results in 0.0. NumberFormatter().string(from:Decimal) returns the last element of the calc rather than 0.00. This happens in 4.1.3 and 4.2 Linux, but works fine in Mac OS. I'm trying to generate order acknowledgements and its important for calculation results to be displayed accurately and properly formatted. It seems that a zero calculated result is the only problem (but not sure and concerned). A workaround is to test for a zero result and then fix the string output. Please let me know if I'm doing something wrong. Thanks!

Here is a simple test script that can be run on Mac OS and Linux with different results:


import Foundation

print("Simple Test\nDecimal\t\tFormatted")

let nf = NumberFormatter()
nf.locale = Locale(identifier: "en_US")
nf.numberStyle = .decimal
nf.minimumFractionDigits = 2
nf.maximumFractionDigits = 2

let x = Decimal(10.5)
let y = Decimal(9.0)
let z = Decimal(1.5)

let a = x - y - z
let b = x - z - y

print("x=Decimal(10.5):Formatted \(nf.string(from: NSDecimalNumber(decimal:x)) ?? "")")
print("y=Decimal(9.0):\tFormatted \(nf.string(from: NSDecimalNumber(decimal:y)) ?? "")")
print("z=Decimal(1.5):\tFormatted \(nf.string(from: NSDecimalNumber(decimal:z)) ?? "")")
print("x - y - z = \(a):\tFormatted \(nf.string(from: NSDecimalNumber(decimal:a)) ?? "")")
print("x - z - y = \(b):\tFormatted \(nf.string(from: NSDecimalNumber(decimal:b)) ?? "")")

print("Possible Workaround?")
print("\\(b==Decimal(0.0) ? (nf.string(from: NSDecimalNumber(decimal:Decimal(0.0))) ?? \"\"):(nf.string(from: NSDecimalNumber(decimal:b)) ?? \"\"))")
print("Result \(b==Decimal(0.0) ? (nf.string(from: NSDecimalNumber(decimal:Decimal(0.0))) ?? ""):(nf.string(from: NSDecimalNumber(decimal:b)) ?? ""))")

Results - Docker Official - Swift 4.2 ubuntu:16.04

Simple Test
Decimal		Formatted
x=Decimal(10.5):Formatted 10.50
y=Decimal(9.0):	Formatted 9.00
z=Decimal(1.5):	Formatted 1.50
x - y - z = 0:	Formatted 1.50
x - z - y = 0:	Formatted 9.00
Possible Workaround?
\(b==Decimal(0.0) ? (nf.string(from: NSDecimalNumber(decimal:Decimal(0.0))) ?? ""):(nf.string(from: NSDecimalNumber(decimal:b)) ?? ""))
Result 0.00

Results Swift 4.1.2 MacOS 10.13.6

Simple Test
Decimal		Formatted
x=Decimal(10.5):Formatted 10.50
y=Decimal(9.0):	Formatted 9.00
z=Decimal(1.5):	Formatted 1.50
x - y - z = 0:	Formatted 0.00
x - z - y = 0:	Formatted 0.00
Possible Workaround?
\(b==Decimal(0.0) ? (nf.string(from: NSDecimalNumber(decimal:Decimal(0.0))) ?? ""):(nf.string(from: NSDecimalNumber(decimal:b)) ?? ""))
Result 0.00
(Simon Evans) #7

It looks to be a bug with the .doubleValue property in Decimal not generating the correct output whereas .description seems to be correct. Would you mind opening a JIRA for this? Thanks