Right, I should have written a few lines about why there are two mczs.
The idea is to load Collections-ul.873 first then Collections-ul-874 
separated by an update map entry to ensure GoodPrimes gets initialized.
The cause of the error is that code loading is not atomic, so the new 
GoodPrimes will be accessed before it is initialized. That's what the 
"migration only" part works around.

Levente

On Wed, 5 Feb 2020, Chris Muller wrote:

> Be sure to load Collections-ul.873 first.  When I tried to load this one first directly, my image locked.
> 
> On Wed, Feb 5, 2020 at 9:47 AM <commits at source.squeak.org> wrote:
>       Levente Uzonyi uploaded a new version of Collections to project The Inbox:
>       http://source.squeak.org/inbox/Collections-ul.874.mcz
>
>       ==================== Summary ====================
>
>       Name: Collections-ul.874
>       Author: ul
>       Time: 5 February 2020, 4:40:30.240914 pm
>       UUID: de45f7ed-0176-48ed-b7d5-3811cbc0fcb9
>       Ancestors: Collections-ul.873
>
>       - remove cruft and migration code from HashedCollection class >> #goodPrimeAtLeast:
>
>       =============== Diff against Collections-ul.873 ===============
>
>       Item was changed:
>         ----- Method: HashedCollection class>>goodPrimeAtLeast: (in category 'sizing') -----
>         goodPrimeAtLeast: lowerLimit
>               "Answer the smallest good prime >= lowerlimit.
>               If lowerLimit is larger than the largest known good prime, just make it odd.
>               Use linear search, and exponential search to speed up cases when lowerLimit is small (<2500 and <100000, respectively).
>               Assume that there are goodPrimes greater than 100000."
>
>               | highIndex midIndex lowIndex prime |
>       -       lowerLimit <= 7 ifTrue: [
>       -               lowerLimit <= 3 ifTrue: [ ^3 ].
>       -               lowerLimit <= 5 ifTrue: [ ^5 ].
>       -               ^7 ].
>       -       GoodPrimes ifNil: [ "migration only"
>       -               self initializeGoodPrimes ].
>               lowerLimit < 2500 ifTrue: [
>                       "Use linear search when the limit is small. The boundary is based on measurements."
>       +               highIndex := 1.
>       -               highIndex := 4. "skip 3 5 and 7"
>                       [ (GoodPrimes at: highIndex) < lowerLimit ] whileTrue: [
>                               highIndex := highIndex + 1 ].
>                       ^GoodPrimes at: highIndex ].
>               lowerLimit < 100000
>                       ifTrue: [
>                               "Use exponential search when the limit is not too large. The boundary is based on measurements."
>                               highIndex := 1.
>                               [ (GoodPrimes at: highIndex) < lowerLimit ] whileTrue: [
>                                       highIndex := highIndex * 2 ].
>                               lowIndex := highIndex // 2 + 1. "highIndex // 2 was smaller than lowerLimit" ]
>                       ifFalse: [
>                               "Regular binary search."
>                               lowIndex := 1.
>                               highIndex := GoodPrimes size.
>                               "Check whether the largest prime would fit"
>                               (GoodPrimes at: highIndex) < lowerLimit ifTrue: [
>                                       ^lowerLimit bitOr: 1 ]. ].
>               [ highIndex - lowIndex <= 1 ] whileFalse: [
>                       midIndex := highIndex + lowIndex // 2.
>                       prime := GoodPrimes at: midIndex.
>                       lowerLimit < prime
>                               ifTrue: [ highIndex := midIndex ]
>                               ifFalse: [
>                                       lowerLimit > prime
>                                               ifTrue: [ lowIndex := midIndex ]
>                                               ifFalse: [ ^prime ] ] ].
>               (GoodPrimes at: lowIndex) >= lowerLimit ifTrue: [ ^GoodPrimes at: lowIndex ].
>               ^GoodPrimes at: highIndex!
> 
> 
> 
>