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lib/logstorage: skip unsafe last-N optimization when pipes may overwrite _time #1549

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35 changes: 35 additions & 0 deletions apptest/tests/lastnoptimization_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -88,3 +88,38 @@ func TestVlsingleLastnOptimization(t *testing.T) {
}
assertLogsQLResponseEqual(t, got, wantResponse)
}

func TestVlsingleLastnOptimizationWithUnpackedInvalidTime(t *testing.T) {
fs.MustRemoveDir(t.Name())
tc := apptest.NewTestCase(t)
defer tc.Stop()
sut := tc.MustStartDefaultVlsingle()

sut.JSONLineWrite(t, []string{
`{"_msg":"{\"_time\":\"not-a-timestamp\",\"msg\":\"bad\"}","_time":"2025-01-01T01:00:00Z"}`,
}, apptest.IngestOpts{})
sut.ForceFlush(t)

query := `* | unpack_json | keep _msg, _time | sort by (_time desc) limit 1`

got := sut.LogsQLQuery(t, query, apptest.QueryOpts{
Start: "2025-01-01T01:00:00Z",
End: "2025-01-01T01:00:00.000000001Z",
})
wantResponse := &apptest.LogsQLQueryResponse{
LogLines: []string{
`{"_msg":"{\"_time\":\"not-a-timestamp\",\"msg\":\"bad\"}","_time":"not-a-timestamp"}`,
},
}
assertLogsQLResponseEqual(t, got, wantResponse)

res, statusCode := sut.LogsQLQueryRaw(t, query, apptest.QueryOpts{
Start: "2025-01-01T01:00:00Z",
End: "2025-01-01T01:00:03Z",
})
if statusCode != 200 {
t.Fatalf("unexpected response status code for optimized query: %d; response\n%s", statusCode, res)
}
got = apptest.NewLogsQLQueryResponse(t, res)
assertLogsQLResponseEqual(t, got, wantResponse)
}
14 changes: 10 additions & 4 deletions lib/logstorage/parser_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -4154,7 +4154,9 @@ func TestQueryCanReturnLastNResults(t *testing.T) {
f("* | delete foo, bar", true)
f("* | drop_empty_fields", true)
f("* | extract '<foo>bar<baz>'", true)
f("* | extract '<_time>bar<baz>'", false)
f("* | extract_regexp 'foo(?P<bar>baz)'", true)
f("* | extract_regexp 'foo(?P<_time>baz)'", false)
f("* | facets", false)
f("* | field_names", false)
f("* | fields foo, bar", false)
Expand Down Expand Up @@ -4201,8 +4203,12 @@ func TestQueryCanReturnLastNResults(t *testing.T) {
f("* | total_stats count()", false)
f("* | union (x)", false)
f("* | uniq (x)", false)
f("* | unpack_json x", true)
f("* | unpack_logfmt x", true)
f("* | unpack_json x", false)
f("* | unpack_json fields (_time)", false)
f("* | unpack_json result_prefix x_", true)
f("* | unpack_logfmt x", false)
f("* | unpack_logfmt fields (_time)", false)
f("* | unpack_logfmt result_prefix x_", true)
f("* | unpack_syslog x", true)
f("* | unpack_words x", true)
f("* | unroll by (x)", true)
Expand Down Expand Up @@ -4386,8 +4392,8 @@ func TestQueryGetStatsLabelsAddGroupingByTime_Success(t *testing.T) {
f("* | replace_regexp ('foo', 'bar') | count() x", nsecsPerDay, 0, []string{"_time"}, `* | replace_regexp (foo, bar) | stats by (_time:86400000000000) count(*) as x`)
f("* | split 'foo' | count() x", nsecsPerDay, 0, []string{"_time"}, `* | split foo | stats by (_time:86400000000000) count(*) as x`)
f("* | time_add 1h | count() x", nsecsPerDay, 0, []string{"_time"}, `* | time_add 1h | stats by (_time:86400000000000) count(*) as x`)
f("* | unpack_json x | count() x", nsecsPerDay, 0, []string{"_time"}, `* | unpack_json from x | stats by (_time:86400000000000) count(*) as x`)
f("* | unpack_logfmt x | count() x", nsecsPerDay, 0, []string{"_time"}, `* | unpack_logfmt from x | stats by (_time:86400000000000) count(*) as x`)
f("* | unpack_json x result_prefix x_ | count() x", nsecsPerDay, 0, []string{"_time"}, `* | unpack_json from x result_prefix x_ | stats by (_time:86400000000000) count(*) as x`)
f("* | unpack_logfmt x result_prefix x_ | count() x", nsecsPerDay, 0, []string{"_time"}, `* | unpack_logfmt from x result_prefix x_ | stats by (_time:86400000000000) count(*) as x`)
f("* | unpack_syslog x | count() x", nsecsPerDay, 0, []string{"_time"}, `* | unpack_syslog from x | stats by (_time:86400000000000) count(*) as x`)
f("* | unpack_words x | count() x", nsecsPerDay, 0, []string{"_time"}, `* | unpack_words from x | stats by (_time:86400000000000) count(*) as x`)
f("* | unroll by (x) | count() x", nsecsPerDay, 0, []string{"_time"}, `* | unroll by (x) | stats by (_time:86400000000000) count(*) as x`)
Expand Down
7 changes: 5 additions & 2 deletions lib/logstorage/pipe_extract.go
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Original file line number Diff line number Diff line change
Expand Up @@ -52,8 +52,11 @@ func (pe *pipeExtract) canLiveTail() bool {
}

func (pe *pipeExtract) canReturnLastNResults() bool {
// TODO: properly verify that the extracted fields do not overwrite the _time field with non-timestamp values.

for _, f := range pe.ptn.fields {
if f.name == "_time" {
return false
}
}
return true
}

Expand Down
7 changes: 5 additions & 2 deletions lib/logstorage/pipe_extract_regexp.go
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Original file line number Diff line number Diff line change
Expand Up @@ -59,8 +59,11 @@ func (pe *pipeExtractRegexp) canLiveTail() bool {
}

func (pe *pipeExtractRegexp) canReturnLastNResults() bool {
// TODO: properly verify that the extracted fields do not overwrite the _time field with non-timestamp values.

for _, f := range pe.reFields {
if f == "_time" {
return false
}
}
return true
}

Expand Down
4 changes: 1 addition & 3 deletions lib/logstorage/pipe_unpack_json.go
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Expand Up @@ -67,9 +67,7 @@ func (pu *pipeUnpackJSON) canLiveTail() bool {
}

func (pu *pipeUnpackJSON) canReturnLastNResults() bool {
// TODO: verify that the unpacked fields do not overwrite _time with non-timestamp values.

return true
return pu.resultPrefix != "" || !prefixfilter.MatchFilters(pu.fieldFilters, "_time")
}

func (pu *pipeUnpackJSON) isFixedOutputFieldsOrder() bool {
Expand Down
4 changes: 1 addition & 3 deletions lib/logstorage/pipe_unpack_logfmt.go
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Original file line number Diff line number Diff line change
Expand Up @@ -58,9 +58,7 @@ func (pu *pipeUnpackLogfmt) canLiveTail() bool {
}

func (pu *pipeUnpackLogfmt) canReturnLastNResults() bool {
// TODO: verify that the unpacked fields do not overwrite _time with non-timestamp values.

return true
return pu.resultPrefix != "" || !prefixfilter.MatchFilters(pu.fieldFilters, "_time")

@cubic-dev-ai cubic-dev-ai Bot Jun 26, 2026

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P2: resultPrefix != "" is not a sufficient safety guarantee: fieldsUnpackerContext.addField concatenates fieldPrefix + name directly, so a prefix like "_" with a field named "time" produces _time. The canReturnLastNResults guard should account for whether any combination of resultPrefix and a matched field name could resolve to _time.

Prompt for AI agents 
Check if this issue is valid — if so, understand the root cause and fix it. At lib/logstorage/pipe_unpack_logfmt.go, line 61:

<comment>`resultPrefix != ""` is not a sufficient safety guarantee: `fieldsUnpackerContext.addField` concatenates `fieldPrefix + name` directly, so a prefix like `"_"` with a field named `"time"` produces `_time`. The `canReturnLastNResults` guard should account for whether any combination of `resultPrefix` and a matched field name could resolve to `_time`.</comment>

<file context>
@@ -58,9 +58,7 @@ func (pu *pipeUnpackLogfmt) canLiveTail() bool {
-	// TODO: verify that the unpacked fields do not overwrite _time with non-timestamp values.
-
-	return true
+	return pu.resultPrefix != "" || !prefixfilter.MatchFilters(pu.fieldFilters, "_time")
 }
 
</file context>

}

func (pu *pipeUnpackLogfmt) isFixedOutputFieldsOrder() bool {
Expand Down