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Here's a tip for anyone who wants to downgrade to 10.15.5 and doesn't have a full installer:
1. Download MDS from twocanoes: https://twocanoes.com/products/mac/mac-deploy-stick/
2. Install and open MDS
3. Use Download macOS at bottom of left pane to download 10.15.5 - Make sure you download to a subfolder under /Users/Shared - if you don't you'll get a permissions error. No progress is reported in MDS but if you open the log viewer you'll see it downloading (Window | Show Log menu item in MDS).
4. This will create a sparse image in the folder you downloaded to. Double click the sparse image in Finder to mount it
5. Open the disk you just mounted in Finder and open the Applications Folder.
6. Double click “Install macOS Catalina“ and install. Crack open your favourite beverage and wait patiently for the install to complete.
I've just completed this - haven't run some VMs in anger much yet to confirm the rollback worked but macOS is reporting 10.15.5 is installed.
I tried running this MDS application (in a vm running macOS 10.14), and it only offers to download 10.15.3, 10.15.4, 10.15.6 from the 10.15.x series right now. :-/
If you're stuck you could downgrade to 10.15.4 then apply the 10.15.5 patch.
I wonder if the 10.15.5 is available for the MB Pro 16" because there was a supplemental update for 10.15.5 specifically for that model???
The build number for 10.15.5 in your screenshot doesn't match what I see elsewhere (it should be 19F96 or 19F101 for the supplemental update). Thanks for the tips anyways. I did manage to dig up a 10.15.5 installer elsewhere. Might still hold out for a quick fix from Apple.
I did file a report at https://feedbackassistant.apple.com/ because I've heard "duplicating radars" (filing duplicate bug reports that others have also filed) might count as another vote for getting things fixed.
19F2200 is specifically for the MB Pro 16" 9600M:
Apologies if it doesn't work for others... I was trying to help :-)
But at least you can downgrade to 10.15.4 then back up to 10.15.5 which is better than nothing.
You can't use this method to downgrade to 10.15.3 or 10.15.4 either, unless you have the right Mac model. Of the listed versions, 10.15.3 (19D2064) was the release system for the 2020 13-inch MacBook Air; 10.15.4 (19E2269) is probably for the 2020 13-inch MacBook Pro (according to MacTracker they were released with 19E2265 so this is likely to be a post-release minor fix). In general, if you see a four digit build number after the letter, then it is usually a model-specific installer. The general release has smaller numbers (10.15.3, 10.15.4 and 10.15.5 are 19D76, 19E266, and 19F96 or 19F101 respectively).
My 16-inch MacBook Pro (from late 2019) can't download 10.15.3, 10.15.4 or 10.15.5 using this method ("Install failed with error: Update not found"), but it can re-download 10.15.6.
For Macs introduced in 2019 or earlier, you can't download a 10.15.0 through 10.15.5 installer any more. To install 10.15.5 you would have needed to get the Catalina installer before 10.15.6 was released, and kept a copy of it (or moved it out of Applications) before proceeding with the install. If you downloaded Catalina but left installation for a few days, check your Time Machine backups - you might still have a copy sitting in an older version of the Applications folder.
I kept most of my older installers, so I can roll back to 10.15.5 if I decide it is worth the trouble. For now I've managed to dodge the problem by not using VMware Fusion on that computer - I can run it on another Mac which is still on Mojave.
Also worth noting that you don't need a third party utility to download the full installer. Catalina's sofwareupdate command line tool can download the available versions (which support your model) with something like this:
softwareupdate --fetch-full-installer --full-installer-version 10.15.6
This was a new feature for the softwareupdate tool in macOS Catalina so it won't work if you are running Mojave or earlier, but it can download the 10.13.6 and 10.14.6 installers (if your Mac is old enough to run them). These options aren't in the man page but are in the help (softwareupdate --help). If you just want the latest installer, you only need the first option.
So I ended up running the macOS 10.15.5 full installer (and then the 10.15.5 supplemental update afterwards) on top of my macOS 10.15.6 installation.
With just some quick testing, "sudo zprint -d" is now quiet when running a windows VM in VMWare Fusion. No more crazy "kalloc.32" leaks observed any more.
This is exactly what I found - it's just a front end for the softwareupdate tool, and Apple's pulled the installers for older machines.
What a mess. Thank you Apple (not).
I started with softwareupdate trying to fetch 10.15.5 and it downloaded something but failed after downloading.
Shift-Option-Command-R reinstalled 10.15.6 - not the original version that came on my MBPro
MDS worked - but as you suggest it looks like it will only work in some cases.
I just observed how this happens. Previously my Mac crashed while asleep. This time I noticed something strange - I saw a dialog window asking for keychain credentials on behalf of various macOS services. Sometimes I was able to switch between application windows, but they did not accept any input. OS honestly tried to fix this - some system applications randomly restarted from time to time, I saw desktop going black and then showing wallpaper again. This looked like complete madness. I had to long press power button to make it restart. So yeah, the kernel goes crazy. I do not think it's a memory leak, I think some part of the kernel becomes corrupted.
Everything you describe there is what I would expect to see from an OS suffering from resource starvation, which will trigger a lot of failures and program terminations, and will activate a lot of error-recovery and resilience routines which do not otherwise need to ever do any work.
Kernel memory corruption is much more likely to trigger a rapid kernel panic (from a page fault, a general protection fault, or an invalid opcode exception) than cause applications to misbehave for a period of time like you describe.
And based on your analysis, it's *clearly* a memory leak. Kernel's running out of that is just as bad as an engine running out of oil...smoke, grinding, and finally the whole thing seizes up.
I have a 2017 Macbook Pro and 10.15.5 appeared in the list for me with the same build number.
It installed fine and reverted me correctly as far as I can tell (issue has also disappeared - was impacting me several times a day with a Win10 VM).
I had a backup image if it all went wrong.
Thanks very much @rossco_au and dariusd for your help in this thread.
A very frustrating issue.
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Glad it helped you @savda - made me feel like it was worth the effort now . Interesting that the same build appeared for your '17 Macbook Pro - seems a bit random what works and what doesn't.