Fix: logs pipelines: ignore coalesce op when generating nil check for add value expressions (#4305)

* chore: panic if agent config recommendation can't be generated

* chore: add case with coalesce op in field nil check generation tests

* fix: ignore expr ast member nodes that contain coalesce op in them

pkg/query-service/app/logparsingpipeline/pipelineBuilder.go

@@ -352,6 +352,13 @@ type logFieldsInExprExtractor struct {
 func (v *logFieldsInExprExtractor) Visit(node *ast.Node) {
 	if n, ok := (*node).(*ast.MemberNode); ok {
 		memberRef := n.String()
+
+		// coalesce ops end up as MemberNode right now for some reason.
+		// ignore such member nodes.
+		if strings.Contains(memberRef, "??") {
+			return
+		}
+
 		if strings.HasPrefix(memberRef, "attributes") || strings.HasPrefix(memberRef, "resource") {
 			v.referencedFields = append(v.referencedFields, memberRef)
 		}

pkg/query-service/app/logparsingpipeline/pipelineBuilder_test.go

@@ -698,6 +698,13 @@ func TestMembershipOpInProcessorFieldExpressions(t *testing.T) {
 				Name:    "add3",
 				Field:   `attributes["attrs.test.value"]`,
 				Value:   `EXPR(attributes.test?.value)`,
+			}, {
+				ID:      "add4",
+				Type:    "add",
+				Enabled: true,
+				Name:    "add4",
+				Field:   `attributes["attrs.test.value"]`,
+				Value:   `EXPR((attributes.temp?.request_context?.scraper ?? [nil])[0])`,
 			},
 		},
 	}

pkg/query-service/app/opamp/model/agent.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"crypto/sha256"
+	"fmt"
 	"sync"
 	"time"
 
@@ -276,8 +277,8 @@ func (agent *Agent) processStatusUpdate(
 func (agent *Agent) updateRemoteConfig(configProvider AgentConfigProvider) bool {
 	recommendedConfig, confId, err := configProvider.RecommendAgentConfig([]byte(agent.EffectiveConfig))
 	if err != nil {
-		zap.S().Errorf("could not generate config recommendation for agent %d: %w", agent.ID, err)
-		return false
+		// The server must always recommend a config.
+		panic(fmt.Errorf("could not generate config recommendation for agent %s: %w", agent.ID, err))
 	}
 
 	cfg := protobufs.AgentRemoteConfig{