Meta-query returns "SyntaxError: Unexpected end of JSON input"

[philrz]

Repro is with Zui commit 21abbf6.

Steps shown in the attached video:

1. Load some sample data into Zui
2. Click + and New Query Session
3. Run the meta-query from :branches

At Zui commit 21abbf6 (associated with the changes in #3085) the app shows the error "SyntaxError: Unexpected end of JSON input". At prior commit, ad5657d the meta-query result was successfully shown.

Repro.mp4

I poked through the changes in #3085 and I think I see what's going on, which is also visible in the attached packet captures
ad5657d.pcapng.gz and 21abbf6.pcapng.gz that I took when performing this operation at the respective commits. If we Follow TCP Stream in Wireshark on each, for the ad5657d case, we see that the API activity halts after the app gets a response to its query from :branches\n | count()\n | { i: count(), v: this}\n | i > 0\n | head 500\n | yield v.

Whereas at commit 21abbf6 that same first query result comes back much the same, but then the app marches on and issues a query from undefined | min(ts), max(ts) which comes back with an HTTP 400 Bad Request.

And based on my added console logging, it looks like this is getting handled as a DefaultError at this line:

zui/apps/zui/src/views/results-pane/error.tsx

Line 11 in 8cec81d

return <DefaultError {...props} />

[philrz]

Update: I've confirmed that the changes in @mattnibs's unpack-missing-dag-op branch from brimdata/super#5140 fixes most of this problem. By setting my Zed dependency in Zui to 57fb6ab9b4ca382b7927b5ced07bd0044595730d for that branch, now here's what we see in the app:

So, the meta query result is once again shown, but the error undefined: pool not found at line 1, column 6: from undefined | min(ts), max(ts) ~~~~~~~~~ does leak through into the stacked bar chart area. That effectively seems like another instance of the kinds of error messages we were silencing in #3085.

[philrz]

Verified in Zui commit 942b10f.

As shown in the attached video, now when running the meta-query from :branches the query result is shown and the stacked bar chart is hidden.

Verify.mp4

Thanks @mattnibs and @jameskerr!