query: resolve all-map Bloblang field paths without gabs

[slachiewicz]

Field reads in a Bloblang mapping go through gabs.Wrap(target).S(path...).Data(), which builds and discards a *gabs.Container per path segment. Allocations therefore scale with the number of field reads in a mapping rather than staying flat. Walking map[string]any directly removes that.

Arrays are deliberately left on gabs. Container.S calls searchStrict with allowWildcard=true, so a * segment over an array is real behaviour; resolveFieldPath hands the whole path back to gabs the moment it meets anything that is not a map[string]any, rather than reimplementing it. Equivalence against gabs is asserted directly in TestResolveFieldPathMatchesGabs over missing keys, nil intermediates, scalars mid-path, negative and out-of-range indices, wildcards on both arrays and maps, and numeric-looking map keys.

The first commit adds benchmarks because there were none for this path — only the parser and iterator had any, so Executor.Query and Overlay were unmeasured.

benchstat, 10 runs per side, darwin/arm64 (M1):

                        │  before     │              after               │
                        │  allocs/op  │  allocs/op   vs base             │
ExecutorQuery/field_read    5.000        4.000       -20.00% (p=0.000)
ExecutorQuery/predicate     5.000        4.000       -20.00% (p=0.000)
ExecutorQuery/and2          6.000        4.000       -33.33% (p=0.000)
ExecutorQuery/and3          8.000        4.000       -50.00% (p=0.000)
ExecutorQuery/contains     13.00        12.00         -7.69% (p=0.000)
ExecutorQuery/exists        6.000        5.000       -16.67% (p=0.000)
ExecutorQuery/multi_assign 12.000        9.000       -25.00% (p=0.000)
ExecutorOverlay             8.000        7.000       -12.50% (p=0.000)

                        │  sec/op     │  sec/op      vs base             │
ExecutorQuery/field_read   147.0n       137.6n       ~ (p=0.118)
ExecutorQuery/predicate    178.6n       159.8n       -10.55% (p=0.000)
ExecutorQuery/and2         257.4n       229.4n       -10.90% (p=0.000)
ExecutorQuery/and3         370.1n       317.9n       -14.09% (p=0.000)
ExecutorQuery/contains     363.2n       353.4n        -2.70% (p=0.041)
ExecutorQuery/exists       172.0n       159.3n        -7.38% (p=0.000)
ExecutorQuery/multi_assign 433.2n       386.2n       -10.85% (p=0.000)
ExecutorOverlay            256.6n       236.5n        -7.82% (p=0.000)
geomean                    257.8n       234.5n        -9.01%   (B/op -14.10%)

and3 is the shape that shows the point: 8 allocations to 4, and constant from there regardless of how many fields the mapping reads. field_read's latency delta is not statistically significant at n=10 even though its allocation count drops, so I have not claimed it.

Two allocation sites remain on this path and are untouched here: the Vars: map[string]any{} built per call in public/bloblang/executor.go even when a mapping declares no variables, and FunctionContext.WithValue taking any by value and storing &value, which forces a heap escape per call. Both look addressable; I left them out to keep this reviewable, and am happy to follow up if the approach here is welcome.

Verified: make test and golangci-lint run internal/bloblang/query/... public/bloblang/... both clean (the two pre-existing govet inline hints in methods_strings.go are untouched).

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Disclosure: this change was developed with AI assistance (Claude Code) and reviewed by me before submission. I saw .github/ai-opt-out only after the work was done — I am raising the PR rather than discarding it because the measurements and the equivalence test stand on their own, but please close it without ceremony if AI-assisted contributions are not wanted here, and I will not follow up with the remaining two optimisations. No hard feelings either way.

I also skipped the issue-first step in CONTRIBUTING.md, which I should have followed; happy to move the discussion to an issue if you would prefer that.

[CLAassistant]

All committers have signed the CLA.

[slachiewicz]

Added the variables-map commit mentioned above, and closing out the third idea as a dead end.

Variables map. AssignsVariables walks the statements' existing AssignmentTargets for a TargetVariable; mappings without one share a single empty map. Two things make this narrower than it first looked: apply already replaces ctx.Vars before invoking a named map (the // ISOLATED VARIABLES line in methods.go), so named maps cannot write to their caller's variables and need no analysis; and the map has to be empty rather than nil, because a nil Vars reports variables were undefined where an empty one reports variable 'x' undefined. Tests cover both, plus a race-detector run and a case asserting a var-assigning mapping still gets a private map.

Cumulative against the branch point, allocations per Query:

mapping	before	after
this.a."b"."c" != "x"	5	3
3-term && over 4 field reads	8	3
several root.x = assignments	12	8
Overlay	8	6

B/op on the simple predicate goes 120 → 56.

The dead end, for anyone who tries it next: FunctionContext.WithValue takes any by value and stores &value, which looked like a free heap escape to remove. It is not. Adding a WithValuePtr(*any) and passing &val from Query moves the escape rather than removing it — -gcflags=-m then reports moved to heap: val, and the benchmark is unchanged to the nanosecond (3 allocs, 56 B/op both ways). Removing that one needs FunctionContext to hold the value directly instead of behind *any, which changes Value() and every caller — not worth it for 16 bytes, in my view, but flagging it so the option is on the record.

On wall-clock I am claiming nothing for this commit: this machine sat at load average 8 on 8 cores while I measured, and an interleaved A/B put the latency delta inside the noise (p=0.33–0.65 at ±22–40% variance). The allocation counts are exact and reproducible; the timing needs a quiet machine.