AMLII-2249 - fortify log tailer pipeline for journald and windows events

Pass the specified tailers through a no-op line handler, as the messages these tailers
produce are not viable candidates for truncation.

Develop and use a new NoopDecoder constructor to make it clear that these entities
do not yet have meaningful work being performed on them in the decoder flow.

pkg/logs/internal/decoder/decoder.go

@@ -14,6 +14,7 @@ import (
 	pkgconfigsetup "github.com/DataDog/datadog-agent/pkg/config/setup"
 	"github.com/DataDog/datadog-agent/pkg/logs/internal/framer"
 	"github.com/DataDog/datadog-agent/pkg/logs/internal/parsers"
+	"github.com/DataDog/datadog-agent/pkg/logs/internal/parsers/noop"
 	"github.com/DataDog/datadog-agent/pkg/logs/message"
 	"github.com/DataDog/datadog-agent/pkg/logs/sources"
 	status "github.com/DataDog/datadog-agent/pkg/logs/status/utils"
@@ -79,14 +80,44 @@ func syncSourceInfo(source *sources.ReplaceableSource, lh *MultiLineHandler) {
 	}
 }
 
+// NewNoopDecoder initializes a decoder with all dependent components in passthrough mode.
+func NewNoopDecoder() *Decoder {
+	inputChan := make(chan *message.Message)
+	outputChan := make(chan *message.Message)
+	detectedPattern := &DetectedPattern{}
+
+	lineHandler := NewNoopLineHandler(outputChan)
+	framer, lineParser := buildFramerAndParser(lineHandler, noop.New(), framer.NoFraming)
+
+	return New(inputChan, outputChan, framer, lineParser, lineHandler, detectedPattern)
+}
+
 // NewDecoderWithFraming initialize a decoder with given endline strategy.
 func NewDecoderWithFraming(source *sources.ReplaceableSource, parser parsers.Parser, framing framer.Framing, multiLinePattern *regexp.Regexp, tailerInfo *status.InfoRegistry) *Decoder {
 	inputChan := make(chan *message.Message)
 	outputChan := make(chan *message.Message)
-	maxContentSize := config.MaxMessageSizeBytes(pkgconfigsetup.Datadog())
 	detectedPattern := &DetectedPattern{}
 
+	lineHandler := buildLineHandler(source, multiLinePattern, tailerInfo, outputChan, detectedPattern)
+	framer, lineParser := buildFramerAndParser(lineHandler, parser, framing)
+
+	return New(inputChan, outputChan, framer, lineParser, lineHandler, detectedPattern)
+}
+
+func buildFramerAndParser(lineHandler LineHandler, parser parsers.Parser, framing framer.Framing) (*framer.Framer, LineParser) {
+	maxMessageSize := config.MaxMessageSizeBytes(pkgconfigsetup.Datadog())
+	var lineParser LineParser
+	if parser.SupportsPartialLine() {
+		lineParser = NewMultiLineParser(lineHandler, config.AggregationTimeout(pkgconfigsetup.Datadog()), parser, maxMessageSize)
+	} else {
+		lineParser = NewSingleLineParser(lineHandler, parser)
+	}
+	return framer.NewFramer(lineParser.process, framing, maxMessageSize), lineParser
+}
+
+func buildLineHandler(source *sources.ReplaceableSource, multiLinePattern *regexp.Regexp, tailerInfo *status.InfoRegistry, outputChan chan *message.Message, detectedPattern *DetectedPattern) LineHandler {
 	outputFn := func(m *message.Message) { outputChan <- m }
+	maxContentSize := config.MaxMessageSizeBytes(pkgconfigsetup.Datadog())
 
 	// construct the lineHandler
 	var lineHandler LineHandler
@@ -122,18 +153,7 @@ func NewDecoderWithFraming(source *sources.ReplaceableSource, parser parsers.Par
 		}
 	}
 
-	// construct the lineParser, wrapping the parser
-	var lineParser LineParser
-	if parser.SupportsPartialLine() {
-		lineParser = NewMultiLineParser(lineHandler, config.AggregationTimeout(pkgconfigsetup.Datadog()), parser, maxContentSize)
-	} else {
-		lineParser = NewSingleLineParser(lineHandler, parser)
-	}
-
-	// construct the framer
-	framer := framer.NewFramer(lineParser.process, framing, maxContentSize)
-
-	return New(inputChan, outputChan, framer, lineParser, lineHandler, detectedPattern)
+	return lineHandler
 }
 
 func buildLegacyAutoMultilineHandlerFromConfig(outputFn func(*message.Message), maxContentSize int, source *sources.ReplaceableSource, detectedPattern *DetectedPattern, tailerInfo *status.InfoRegistry) *LegacyAutoMultilineHandler {

pkg/logs/internal/decoder/noop_line_handler.go

@@ -0,0 +1,38 @@
+// Unless explicitly stated otherwise all files in this repository are licensed
+// under the Apache License Version 2.0.
+// This product includes software developed at Datadog (https://www.datadoghq.com/).
+// Copyright 2024-present Datadog, Inc.
+
+package decoder
+
+import (
+	"time"
+
+	"github.com/DataDog/datadog-agent/pkg/logs/message"
+)
+
+// NoopLineHandler provides a passthrough functionality for flows that don't need a functional line handler
+type NoopLineHandler struct {
+	outputChan chan *message.Message
+}
+
+// NewNoopLineHandler returns a new NoopLineHandler
+func NewNoopLineHandler(outputChan chan *message.Message) *NoopLineHandler {
+	return &NoopLineHandler{outputChan: outputChan}
+}
+
+// process handles a new line (message)
+func (noop *NoopLineHandler) process(msg *message.Message) {
+	noop.outputChan <- msg
+}
+
+// flushChan returns a channel which will deliver a message when `flush` should be called.
+func (noop *NoopLineHandler) flushChan() <-chan time.Time {
+	return nil
+}
+
+// flush flushes partially-processed data.  It should be called either when flushChan has
+// a message, or when the decoder is stopped.
+func (noop *NoopLineHandler) flush() {
+
+}

pkg/logs/tailers/journald/tailer.go

@@ -19,13 +19,10 @@ import (
 	tagger "github.com/DataDog/datadog-agent/comp/core/tagger/def"
 	"github.com/DataDog/datadog-agent/comp/logs/agent/config"
 	"github.com/DataDog/datadog-agent/pkg/logs/internal/decoder"
-	"github.com/DataDog/datadog-agent/pkg/logs/internal/framer"
-	"github.com/DataDog/datadog-agent/pkg/logs/internal/parsers/noop"
 	"github.com/DataDog/datadog-agent/pkg/logs/internal/tag"
 	"github.com/DataDog/datadog-agent/pkg/logs/message"
 	"github.com/DataDog/datadog-agent/pkg/logs/processor"
 	"github.com/DataDog/datadog-agent/pkg/logs/sources"
-	status "github.com/DataDog/datadog-agent/pkg/logs/status/utils"
 	"github.com/DataDog/datadog-agent/pkg/telemetry"
 	"github.com/DataDog/datadog-agent/pkg/util/log"
 )
@@ -69,7 +66,7 @@ func NewTailer(source *sources.LogSource, outputChan chan *message.Message, jour
 	}
 
 	return &Tailer{
-		decoder:           decoder.NewDecoderWithFraming(sources.NewReplaceableSource(source), noop.New(), framer.NoFraming, nil, status.NewInfoRegistry()),
+		decoder:           decoder.NewNoopDecoder(),
 		source:            source,
 		outputChan:        outputChan,
 		journal:           journal,

pkg/logs/tailers/windowsevent/tailer.go

@@ -18,12 +18,9 @@ import (
 	"github.com/cenkalti/backoff"
 
 	"github.com/DataDog/datadog-agent/pkg/logs/internal/decoder"
-	"github.com/DataDog/datadog-agent/pkg/logs/internal/framer"
-	"github.com/DataDog/datadog-agent/pkg/logs/internal/parsers/noop"
 	"github.com/DataDog/datadog-agent/pkg/logs/message"
 	"github.com/DataDog/datadog-agent/pkg/logs/processor"
 	"github.com/DataDog/datadog-agent/pkg/logs/sources"
-	status "github.com/DataDog/datadog-agent/pkg/logs/status/utils"
 	"github.com/DataDog/datadog-agent/pkg/logs/util/windowsevent"
 	"github.com/DataDog/datadog-agent/pkg/telemetry"
 	"github.com/DataDog/datadog-agent/pkg/util/log"
@@ -72,7 +69,7 @@ func NewTailer(evtapi evtapi.API, source *sources.LogSource, config *Config, out
 		evtapi:     evtapi,
 		source:     source,
 		config:     config,
-		decoder:    decoder.NewDecoderWithFraming(sources.NewReplaceableSource(source), noop.New(), framer.NoFraming, nil, status.NewInfoRegistry()),
+		decoder:    decoder.NewNoopDecoder(),
 		outputChan: outputChan,
 	}
 }