"failed to decode PsaResult" when purs compile exits due to OOM

[cakekindel]

Spent several hours yesterday trying to figure out why my ci job running spago build / spago test was failing despite nothing visibly changing in the environment, and the purs compile step exiting at random points in the build with:

$ spago build --pure --strict --pedantic-packages && bun run scripts/fmt.js --check
Reading Spago workspace configuration...
✓ Selecting 13 packages to build:
    <snip>
Downloading dependencies...
Refreshing the Registry Index...
Building...
[  1 of 941] Compiling Unsafe.Coerce
[  2 of 941] Compiling Web.Event.Internal.Types
[  4 of 941] Compiling Type.Proxy
[  3 of 941] Compiling Web.DOM.Internal.Types
[  5 of 941] Compiling Type.Function
[  6 of 941] Compiling Type.Equality
[  7 of 941] Compiling Type.Row
<snip>
[xxx of 941] Compiling xxx
‼ Failed to decode PsaResult at index '0': Unexpected end of JSON input
Json was: 
✓ Build succeeded.

Eventually I had the thought "maybe the --memory=1G I added to the runner is making purs compile exit early" which turned out to be exactly the case. Not sure if this is necessarily a bug in spago, just documenting this as an issue here for people who run into this in the future.

[cakekindel]

Weird so we do catch non-ok exit cases
src/Spago/Cmd.purs#L110. Not sure what else we could do to trap this

[f-f]

Ah, it turns out we're not checking the exit status when trying to decode the compiler output in the psa machinery, see here:

spago/src/Spago/Psa.purs

Lines 43 to 50 in b7c0483

	result <- Purs.compile globs (Array.snoc pursArgs "--json-errors")
	let resultStdout = Cmd.getStdout result
	arrErrorsIsEmpty <- forWithIndex (Str.split (Str.Pattern "\n") resultStdout) \idx err ->
	case JSON.parse err >>= CJ.decode psaResultCodec >>> lmap CJ.DecodeError.print of
	Left decodeErrMsg -> do
	logWarn $ Array.intercalate "\n"
	[ "Failed to decode PsaResult at index '" <> show idx <> "': " <> decodeErrMsg
	, "Json was: " <> err

In line 44 we call getStdout (which I think is an antipattern and should only be used in tests) without first checking if result is a Left or a Right. I suspect that when the compiler fails we get the error on stderr and stdout is empty, but we still try to decode it.