Using both stdout and stderr of curl in a subsequent jq command - shell

Question

I want to do the following,
command2(stdout)
/ \
command1 command4
\ /
command3(stderr)
As it is covered in How can I split and re-join STDOUT from multiple processes?
Except, command1 outputs different text to both, stdout and stderr. So, it is a combination of the above question and Pipe stderr to another command
To have a context, what I am trying to achieve:
Execute curl
Capture raw output (stdout), base64, and embed it into json: curl https://someaddress.tld | base64 | jq --raw-input '{"curl_ret" : .}'
Output curl json (return code etc) and pass it to stderr: curl --write-out '%{stderr}%{json}' https://someaddress.tld
Given #2 and #3 is the same curl call, I want to merge the output of #2 and #3 and pass the merged result to jq: jq --slurp ...
All these in one piped command.
stdout and stderr separation is done to avoid parsing merged text, to avoid pitfalls, given curl output can be anything. Curl has --silent switch so no unexpected text in either output stream

Answer 1

In practice, for the use case at hand
You don't need to do this at all. --write-out '%{json}' will always be written after the body content, so it's always the last line of stdout. It's safe to have it in the same stream.
getExampleAndMetadata() {
curl --write-out '%{json}' https://example.com |
jq -RSs '
split("\n")
| {"body": .[:-1] | join("\n"),
"logs": .[-1] | fromjson}'
}
As an exercise
It's ugly, and I don't recommend it -- it'd be better to just parse the two sets of data out of stdout and leave stderr free so you have a way to log actual errors -- but, doing what you asked for:
getExampleAndMetadata() {
local tmpdir retval=0
# mkfifo is different between operating systems, adjust to fit
tmpdir=$(mktemp -d sa_query.XXXXXX) || return
mkfifo "$tmpdir/stdout" "$tmpdir/stderr" || { retval=$?; rm -rf "$tmpdir"; return "$retval"; }
curl --silent --write-out '%{stderr}%{json}' https://example.com \
>"$tmpdir/stdout" 2>"$tmpdir/stderr" & curl_pid=$!
# must read stdout _before_ stderr to avoid a deadlock here
# to stop caring what order of operations jq uses, we use process substitutions
# to buffer both stdout and stderr in-memory
jq -Rn \
--rawfile content <(out=$(cat "$tmpdir/stdout"); printf '%s\n' "$out") \
--slurpfile logs <(err=$(cat "$tmpdir/stderr"); printf '%s\n' "$err") \
'{"body": $content, "logs": $logs}'; (( retval |= $? ))
rm -rf -- "$tmpdir"; (( retval |= $? ))
wait "$curl_pid"; (( retval |= $? ))
return "$retval"
}
...gives you a simple command, getExampleAndMetadata. And of course, if you eliminate the comments and line continuations you can collapse the whole thing to one line by adding ;s appropriately.