Bug with nested path and variable-style parameters

[thaliaarchi]

I wanted to do performance and feature-coverage comparisons between jq and gojq using my wsjq Whitespace interpreter, which is considerably complex for a jq program (and overcomes the limitations of your jq Brainfuck interpreter). However, in the process, I uncovered a bug in gojq. Here's an example, reduced from various instructions in interpret_step:

def assert($cond; msg): if $cond then . else msg | halt_error end;
def at($n): assert(.stack|length > $n; "stack underflow") | .stack[-$n-1];
def top: at(0);

{ stack: [1, 2, 3] } |
top += top

$ jq -ncf minimal.jq
{"stack":[1,2,6]}
$ gojq -nf minimal.jq
gojq: invalid path against: object ({"stack":[1,2,3]})

If any of one these changes are made to the example program, it will run without error:

• Change the parameter from a filter to a variable: $cond -> cond
• Desugar the parameter filter: def assert(cond; msg): cond as $cond | … (the jq docs imply that this is equivalent)
• Inline the call to assert/2 in at/1
• Use bare array instead of in an object: { stack: [1, 2, 3] } and .stack -> [1, 2, 3] and .

So, the problem appears to be caused by a nested path generated by a filter that uses variable-style parameters.

[wader]

Spent some time trying to undersand what is going on. I think triggering the error can be minimize to:

{} | path(def f($_): .; f(.s)).

This might be a hint, def f($_) vs def f(c): c as $_. I would have expect them to be very similar? the expbegin difference looks suspicous?

$ GOJQ_DEBUG=1 go run -tags gojq_debug cmd/gojq/main.go -n '{} | path(def f($_): .; f(.s))'
	0	scope                    [1,2,0]
	1	store                    [1,0]	## $ARGS
	2	const                    {}	## {}
	3	pathbegin                null
	4	store                    [1,1]
	5	jump                     26
	6	scope                    [2,1,0]	## lambda:6
	7	jump                     17
	8	scope                    [3,3,1]	## f
	9	store                    [3,0]
	10	store                    [3,1]	## _
	11	load                     [3,0]
	12	load                     [3,1]
	13	callpc                   null
	14	store                    [3,2]	## $_
	15	load                     [3,0]
	16	ret                      null	## end of f
	17	store                    [2,0]
	18	jump                     22
	19	scope                    [4,0,0]	## lambda:19
	20	index                    "s"	## .s
	21	ret                      null	## end of lambda:19
	22	pushpc                   19
	23	load                     [2,0]
	24	call                     8	## call f
	25	ret                      null	## end of lambda:6
	26	load                     [1,1]
	27	pushpc                   6
	28	callpc                   null
	29	load                     [1,1]
	30	pathend                  null
	31	ret                      null
	----------------------------------------+
	0	scope                    [1,2,0]	|	null	{"named":{},"positional":[]}
	1	store                    [1,0]	|	null	{"named":{},"positional":[]}			## $ARGS
	2	const                    {}	|	null			## {}
	3	pathbegin                null	|	{}
	4	store                    [1,1]	|	{}
	5	jump                     26	|
	26	load                     [1,1]	|
	27	pushpc                   6	|	{}
	28	callpc                   null	|	{}	[6,0]
	6	scope                    [2,1,0]	|	{}			## lambda:6
	7	jump                     17	|	{}
	17	store                    [2,0]	|	{}
	18	jump                     22	|
	22	pushpc                   19	|
	23	load                     [2,0]	|	[19,1]
	24	call                     8	|	[19,1]	{}			## call f
	8	scope                    [3,3,1]	|	[19,1]	{}			## f
	9	store                    [3,0]	|	[19,1]	{}
	10	store                    [3,1]	|	[19,1]			## _
	11	load                     [3,0]	|
	12	load                     [3,1]	|	{}
	13	callpc                   null	|	{}	[19,1]
	19	scope                    [4,0,0]	|	{}			## lambda:19
	20	index                    "s"	|	{}			## .s
	21	ret                      null	|	null			## end of lambda:19
	14	store                    [3,2]	|	null			## $_
	15	load                     [3,0]	|
	16	ret                      null	|	{}			## end of f
	25	ret                      null	|	{}			## end of lambda:6
	29	load                     [1,1]	|	{}
	30	pathend                  null	|	{}	{}
gojq: invalid path against: object ({})
exit status 5

$ GOJQ_DEBUG=1 go run -tags gojq_debug cmd/gojq/main.go -n '{} | path(def f(c): c as $_ | .; f(.s))'
	0	scope                    [1,2,0]
	1	store                    [1,0]	## $ARGS
	2	const                    {}	## {}
	3	pathbegin                null
	4	store                    [1,1]
	5	jump                     28
	6	scope                    [2,1,0]	## lambda:6
	7	jump                     19
	8	scope                    [3,3,1]	## f
	9	store                    [3,0]
	10	store                    [3,1]	## c
	11	load                     [3,0]
	12	dup                      null
	13	expbegin                 null
	14	load                     [3,1]
	15	callpc                   null
	16	store                    [3,2]	## $_
	17	expend                   null
	18	ret                      null	## end of f
	19	store                    [2,0]
	20	jump                     24
	21	scope                    [4,0,0]	## lambda:21
	22	index                    "s"	## .s
	23	ret                      null	## end of lambda:21
	24	pushpc                   21
	25	load                     [2,0]
	26	call                     8	## call f
	27	ret                      null	## end of lambda:6
	28	load                     [1,1]
	29	pushpc                   6
	30	callpc                   null
	31	load                     [1,1]
	32	pathend                  null
	33	ret                      null
	----------------------------------------+
	0	scope                    [1,2,0]	|	null	{"named":{},"positional":[]}
	1	store                    [1,0]	|	null	{"named":{},"positional":[]}			## $ARGS
	2	const                    {}	|	null			## {}
	3	pathbegin                null	|	{}
	4	store                    [1,1]	|	{}
	5	jump                     28	|
	28	load                     [1,1]	|
	29	pushpc                   6	|	{}
	30	callpc                   null	|	{}	[6,0]
	6	scope                    [2,1,0]	|	{}			## lambda:6
	7	jump                     19	|	{}
	19	store                    [2,0]	|	{}
	20	jump                     24	|
	24	pushpc                   21	|
	25	load                     [2,0]	|	[21,1]
	26	call                     8	|	[21,1]	{}			## call f
	8	scope                    [3,3,1]	|	[21,1]	{}			## f
	9	store                    [3,0]	|	[21,1]	{}
	10	store                    [3,1]	|	[21,1]			## c
	11	load                     [3,0]	|
	12	dup                      null	|	{}
	13	expbegin                 null	|	{}	{}
	14	load                     [3,1]	|	{}	{}
	15	callpc                   null	|	{}	{}	[21,1]
	21	scope                    [4,0,0]	|	{}	{}			## lambda:21
	22	index                    "s"	|	{}	{}			## .s
	23	ret                      null	|	{}	null			## end of lambda:21
	16	store                    [3,2]	|	{}	null			## $_
	17	expend                   null	|	{}
	18	ret                      null	|	{}			## end of f
	27	ret                      null	|	{}			## end of lambda:6
	31	load                     [1,1]	|	{}
	32	pathend                  null	|	{}	{}
	33	ret                      null	|	[]
[]
	33	ret <backtrack>          null	|

[itchyny]

Thank you for reporting. I fixed the bug.