error ignores null input

[odnoletkov]

Describe the bug
Both error/0 and error/1 do nothing for null (not empty!) input

To Reproduce

$ jq -n error
$ echo $?
0

$ jq -n 'error(null)'
$ echo $?
0
$

Expected behavior

Errors should never be ignored, should write stderr message and cause non-zero exit code

$ jq -n error
jq: error (at <unknown>) (not a string): null
$ echo $?
5

Environment (please complete the following information):

$ jq --version
jq-1.6
$ uname
Darwin
$ sw_vers -productVersion
11.0.1

[itchyny]

The behavior was changed by the following two commits before 1.5.

• a68958e fixing error(x) should not tostring its inputs #466.
• b963eba changing the behavior of jv_invalid_with_msg(jv_null()).

 % ./jq-1.4 -n 'error(null)'
jq: error: null
 % ./jq-1.5 -n 'error(null)'

This seems to behave like empty (backtracking).

 % ./jq-1.4 -n '[1,error(null),2]'
jq: error: null
[
  1,
  2
]
 % ./jq-1.5 -n '[1,error(null),2]'
[
  1,
  2
]

I am not sure we can change the behavior from now whether it is intended or not.

[dmiraj]

This works on my side. Perhaps putting the message between a double quote will return the message you desired:
$ jq -n 'error("null")'
jq: error (at ): null

[itchyny]

This issue is about the input is null value, not just want a error message of null. Dirty workaround is to use if . == null then tostring else . end | error.

[odnoletkov]

I am not sure we can change the behavior from now whether it is intended or not.

Do you mean we can't change it because of backwards compatibility? I.e. someone might be relying on this behavior of error ignored for null input? This seemed like a clear bug to me because:

• null and empty behave very differently almost everywhere in jq. The only exception I can remember is the // operator
• Ignoring errors is major source of issues in practice

@nicowilliams did you intend to change this behavior in above commits? Do you think this should be fixed or maybe just documented?

[itchyny]

This is an expected behavior of error filters. We should add some notes to the manual. I also noticed that error/0 is not documented yet.

[nicowilliams]

@nicowilliams did you intend to change this behavior in above commits? Do you think this should be fixed or maybe just documented?

The change to error to make it like empty when its input is null was unintentional. I can see how that change can both: break backwards-compatibility with earlier versions of jq, and also be useful. It's been like this in master for 9 years now, so I think we'll leave it as it is as changing it now would also be a backwards-incompatible change. @itchyny has updated the documentation of error to match it's behavior.

[itchyny]

#2823 changed the behavior to throw an error this case.