fix: a division or modulo by zero is NaN, like Loki

CHANGELOG.md

@@ -7,6 +7,20 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- **A zero divisor of `/` and `%` is NaN, like Loki.** `sum(rate({app="api"}[5m])) / 0`,
+  `5 / <a series whose value is 0>`, `-1 / 0` and `1 / 0` answered `+Inf` or `-Inf`
+  (IEEE division), and the fallback sample arithmetic used when a query is not
+  parsed as a binary expression answered `0` for `/` and `%`. Loki routes every binary operation through
+  `syntax.MergeBinOp`, which returns NaN for a zero right operand of `/` and `%`
+  whatever the numerator is, for a literal fold (`1 / 0`), a vector against a
+  literal and two vectors alike. The proxy now returns NaN in every sample
+  arithmetic path; comparisons (with and without `bool`), `^`, NaN and infinite
+  operands already matched Loki and are pinned against a direct port of
+  `MergeBinOp` by an operator by operand edge table. Aggregations over a
+  division by zero are not changed here. (#745)
+
 ## [2.11.2] - 2026-10-12
 
 ### CI

conformance/registry/cases/semantics/binary-division-by-zero-nan.yaml

@@ -0,0 +1,59 @@
+id: semantics/binary-division-by-zero-nan
+title: A zero divisor of `/` and `%` is NaN, like Loki (fixed)
+proves:
+  - loki_api_v1_query_range
+  - loki_api_v1_query
+consumer: explore
+why: |
+  Loki guards a zero right operand of `/` and `%` with NaN in the one function
+  every binary operation goes through, syntax.MergeBinOp
+  (pkg/logql/syntax/ast.go:1978-2005): `if right.F == 0 { res.F = math.NaN() }`.
+  It applies to a literal fold (reduceBinOp, ast.go:1918-1935, so `1 / 0` is NaN),
+  to a vector against a literal (LiteralStepEvaluator.Next,
+  pkg/logql/evaluator.go:1179) and to two vectors (vectorBinop, evaluator.go:1020),
+  whatever the numerator is: `sum(rate(..)) / 0`, `-1 / 0` and `Inf / 0` are all
+  NaN, never +Inf or -Inf. The proxy computed IEEE a/b, so a nonzero numerator
+  over a zero divisor came back as "+Inf" or "-Inf" (only 0/0 was NaN), and the
+  legacy sample arithmetic used when a query is not parsed as a binary
+  expression answered 0 for both operators. `%` by zero was already NaN through
+  math.Mod on the main path. The comparison operators, `^`, NaN operands and
+  infinite operands already matched MergeBinOp; the edge table in
+  TestBinarySampleValueMatchesLokiMergeBinOp pins every operator against a
+  direct port of it, with and without bool.
+
+  min and max across series skip a NaN unless every value is NaN: Loki's
+  VectorAggEvaluator replaces a NaN accumulator (pkg/logql/evaluator.go:525-535),
+  so `max(x / 0)` over mixed values is the largest real value. Residual, not
+  fixed here: the one proxy fold that lets NaN win is withoutMerge.combine
+  (math.Min / math.Max in internal/proxy/vector_matching.go), fixed by #734
+  (semantics/non-sum-vector-aggregation-unreduced). With this change no other
+  min/max fold receives a NaN: on origin/main an aggregation over a binary
+  expression drops the binary operand (`max(v / 2)` and `max(v / 0)` send VL the
+  same query without the division), which #741 fixes; the folds #741 adds must
+  apply Loki's NaN rule.
+request:
+  method: GET
+  path: /loki/api/v1/query_range
+  params:
+    query: 'sum(count_over_time({app="payment-service"}[5m])) / 0'
+    start: 'now-1h'
+    end: 'now'
+    step: '60'
+  headers:
+    X-Scope-OrgID: "0"
+compare:
+  mode: observed
+  fields: [series, values]
+  expect: |
+    One unlabelled series whose every value is "NaN".
+gap:
+  status: fixed
+  fixed_by: "#745"
+  impact: api-only
+  effort: S
+  area: 'internal/proxy binary_matching.go (binarySampleValue), metric_binary.go (applyOp)'
+  loki: 'pkg/logql/syntax/ast.go:1944-2125 MergeBinOp (the / and % guards at :1978-2005), reduceBinOp :1918; pkg/logql/evaluator.go:1020 vectorBinop, :1179 LiteralStepEvaluator.Next'
+  proxy: 'a zero divisor of / and % is NaN in every sample-arithmetic path'
+  planned_test: 'TestCompat_BinaryDivisionByZeroIsNaN (test/e2e-compat, group core); TestBinarySampleValueMatchesLokiMergeBinOp, TestBinaryScalarFoldMatchesLokiReduceBinOp, TestBinaryMatchersMatchLokiMergeBinOp, TestApplyOpDivisionAndModuloByZeroAreNaN, TestBinaryVectorDivisionByZeroHandlers and TestBinaryConstantVectorDivisionByZeroInstant (internal/proxy)'
+  note: 'Measured on origin/main b20a550f. The e2e is written and not yet run. min/max NaN folding: residual in #734 (withoutMerge.combine) and #741 (aggregation over binary).'
+  clusters: []

conformance/registry/generated/wiring.json

@@ -186,6 +186,7 @@
       "internal/proxy/tenant_query_limits_test.go",
       "internal/proxy/tumbling_axis_parity_test.go",
       "internal/proxy/unwrap_conversion_error_test.go",
+      "test/e2e-compat/binary_division_zero_test.go",
       "test/e2e-compat/extracted_suffix_collision_test.go",
       "test/e2e-compat/parenthesized_range_or_filters_test.go",
       "test/e2e-compat/parser_capture_identity_test.go",
@@ -210,6 +211,7 @@
       "internal/proxy/unwrap_conversion_error_test.go",
       "internal/translator/extracted_suffix_test.go",
       "internal/translator/regexp_escaping_test.go",
+      "test/e2e-compat/binary_division_zero_test.go",
       "test/e2e-compat/drilldown_field_breakdown_test.go",
       "test/e2e-compat/extracted_suffix_collision_test.go",
       "test/e2e-compat/heavy_query_protection_test.go",
@@ -361,6 +363,10 @@
     "semantics/backend-timeout-argument": [
       "internal/proxy/backend_heavy_test.go"
     ],
+    "semantics/binary-division-by-zero-nan": [
+      "internal/proxy/binary_division_zero_test.go",
+      "test/e2e-compat/binary_division_zero_test.go"
+    ],
     "semantics/client-cancellation-propagates": [
       "internal/proxy/backend_heavy_test.go"
     ],
@@ -541,7 +547,7 @@
       "test/e2e-compat/unwrap_ungrouped_sum_test.go"
     ]
   },
-  "registry_items": 299,
+  "registry_items": 300,
   "routes_without_registry_item": [],
   "test_to_items": {
     "cmd/proxy/main_test.go": [
@@ -592,6 +598,9 @@
       "operator-configurable-limits",
       "semantics/backend-aborted-response"
     ],
+    "internal/proxy/binary_division_zero_test.go": [
+      "semantics/binary-division-by-zero-nan"
+    ],
     "internal/proxy/compat_cache_freshness_test.go": [
       "semantics/metadata-answers-include-last-24h-like-loki"
     ],
@@ -895,6 +904,11 @@
     "test/e2e-compat/bad_request_status_parity_test.go": [
       "loki-compatible-profile"
     ],
+    "test/e2e-compat/binary_division_zero_test.go": [
+      "loki_api_v1_query",
+      "loki_api_v1_query_range",
+      "semantics/binary-division-by-zero-nan"
+    ],
     "test/e2e-compat/compat_option_matrix_test.go": [
       "loki-compatible-profile",
       "profiles/detected-fields-dotted-json-keys",
@@ -1064,7 +1078,7 @@
       "profiles/structured-metadata-keys-per-profile"
     ]
   },
-  "tests_with_markers": 96,
+  "tests_with_markers": 98,
   "unknown_ids": {},
   "unwired_items": [
     "api_prom_label",
@@ -1172,5 +1186,5 @@
     "versions/capabilities-floor-vs-matrix",
     "versions/rejected-stats-query-wording-vl-1.47"
   ],
-  "wired_items": 106
+  "wired_items": 107
 }

conformance/reports/parity-gaps.md

@@ -114,4 +114,5 @@ Last run: 2026-10-05T20:16:15Z on `fcaccfcc`, window [1791220800, 1791224400].
 | `semantics/unwrap-rate-ungrouped-sum-per-stream` — An ungrouped sum over rate with unwrap is one series on query_range, like Loki (fixed) | #716 | 0 | 0 |
 | `severity/otlp-severity-before-vl-1.50` — OTLP rows stored by VictoriaLogs before v1.50 (one `severity` field, no severity_number) get detected_level "Unspecified" / "Info2" (fixed) | #666 | 0 | 0 |
 | `versions/rejected-stats-query-wording-vl-1.47` — The proxy did not recognise VictoriaLogs v1.47 parse errors, so a rejected query was not Loki's 400 (fixed) | #659 | 0 | 0 |
+| `semantics/binary-division-by-zero-nan` — A zero divisor of `/` and `%` is NaN, like Loki (fixed) | #745 | 0 | 0 |
 | `versions/capabilities-floor-vs-matrix` — logsql.Capabilities treated VictoriaLogs below v1.40 as unsupported while the matrix claimed v1.30 (fixed) | #659 | 0 | 0 |