It seems to be related to ghc-up. I think installing ghc-up changed my path. Perhaps it's bundled with an old version of stack. Changing the paths back fixed the problem. So I must be running the version of stack now that I've been using all this time, that's been working all this time. So.. ghcup seems to have an old version of stack, or puts an old version on the path? Just thinking out loud here.
Dennis

On Sun, Oct 25, 2020 at 12:21 AM Viktor Dukhovni <ietf-dane@dukhovni.org> wrote:
On Sat, Oct 24, 2020 at 06:03:59PM -0700, Dennis Raddle wrote:

> I get this error when running stack:
>
> Downloading lts- xxxx building plan
> aesonException "Error in $['system-info']: key \"os\" not present"

The files in question are the build plan files, e.g.

   ~/.stack/build-plan/lts-13.10.yaml

Older versions of stack used to look for an "os" field
under "system-info" in these files:

    ...
    system-info:
      core-packages:
        ghc: 8.6.3
        bytestring: 0.10.8.2
        unix: 2.7.2.2
        base: 4.12.0.0
        time: 1.8.0.2
        hpc: 0.6.0.3
        filepath: 1.4.2.1
        process: 1.6.3.0
        array: 0.5.3.0
        integer-gmp: 1.0.2.0
        containers: 0.6.0.1
        ghc-boot: 8.6.3
        binary: 0.8.6.0
        ghc-prim: 0.5.3
        ghc-heap: 8.6.3
        ghci: 8.6.3
        rts: '1.0'
        terminfo: 0.4.1.2
        transformers: 0.5.5.0
        deepseq: 1.4.4.0
        ghc-boot-th: 8.6.3
        pretty: 1.1.3.6
        template-haskell: 2.14.0.0
        directory: 1.3.3.0
      ghc-version: 8.6.3
      os: linux
    ...

That code was dropped from stack some time back (between v1.5.1 and
v1.6.1), and new build plans likely don't include the no longer needed
information.  Perhaps you're trying to use a very old stack executable
(older than 1.6.1?) to process a new build plan (LTS snapshot).

    commit 624165387b77df09108888a8c4ec3b0b512e5096
    Author: Michael Snoyman <michael@snoyman.com>
    Date:   Mon Jun 26 08:35:31 2017 +0300

        Remove a bunch of unneeded info from BuildPlan

> running "stack update" didn't help.
> I'm very confused because I don't remember what I did that provoked this
> error.

Simplest is probably to reinstall stack from scratch, perhaps you're
missing too many intermediate versions to perform a direct upgrade.

--
    Viktor.
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