Cache conditions when applying variables to config (#6229) (#6281)

* Cache conditions when applying variables to config

# Conflicts:
#	internal/pkg/agent/transpiler/ast_test.go

* Add test for AST insertion

(cherry picked from commit cab5754)

# Conflicts:
#	internal/pkg/agent/transpiler/ast_test.go

# Conflicts:
#	internal/pkg/agent/transpiler/ast_test.go

Co-authored-by: Mikołaj Świątek <mail@mikolajswiatek.com>

changelog/fragments/1733411331-cache-conditions-in-config.yaml

@@ -0,0 +1,32 @@
+# Kind can be one of:
+# - breaking-change: a change to previously-documented behavior
+# - deprecation: functionality that is being removed in a later release
+# - bug-fix: fixes a problem in a previous version
+# - enhancement: extends functionality but does not break or fix existing behavior
+# - feature: new functionality
+# - known-issue: problems that we are aware of in a given version
+# - security: impacts on the security of a product or a user’s deployment.
+# - upgrade: important information for someone upgrading from a prior version
+# - other: does not fit into any of the other categories
+kind: enhancement
+
+# Change summary; a 80ish characters long description of the change.
+summary: Cache conditions when applying variables to config
+
+# Long description; in case the summary is not enough to describe the change
+# this field accommodate a description without length limits.
+# NOTE: This field will be rendered only for breaking-change and known-issue kinds at the moment.
+#description:
+
+# Affected component; usually one of "elastic-agent", "fleet-server", "filebeat", "metricbeat", "auditbeat", "all", etc.
+component: elastic-agent
+
+# PR URL; optional; the PR number that added the changeset.
+# If not present is automatically filled by the tooling finding the PR where this changelog fragment has been added.
+# NOTE: the tooling supports backports, so it's able to fill the original PR number instead of the backport PR number.
+# Please provide it if you are adding a fragment for a different PR.
+#pr: https://github.com/owner/repo/1234
+
+# Issue URL; optional; the GitHub issue related to this changeset (either closes or is part of).
+# If not present is automatically filled by the tooling with the issue linked to the PR number.
+#issue: https://github.com/owner/repo/1234

internal/pkg/agent/transpiler/ast.go

@@ -211,13 +211,14 @@ func (d *Dict) sort() {
 
 // Key represents a Key / value pair in the dictionary.
 type Key struct {
-	name  string
-	value Node
+	name      string
+	value     Node
+	condition *eql.Expression
 }
 
 // NewKey creates a new key with provided name node pair.
 func NewKey(name string, val Node) *Key {
-	return &Key{name, val}
+	return &Key{name: name, value: val}
 }
 
 func (k *Key) String() string {
@@ -288,11 +289,18 @@ func (k *Key) Apply(vars *Vars) (Node, error) {
 		case *BoolVal:
 			return k, nil
 		case *StrVal:
-			cond, err := eql.Eval(v.value, vars, true)
+			var err error
+			if k.condition == nil {
+				k.condition, err = eql.New(v.value)
+				if err != nil {
+					return nil, fmt.Errorf(`invalid condition "%s": %w`, v.value, err)
+				}
+			}
+			cond, err := k.condition.Eval(vars, true)
 			if err != nil {
 				return nil, fmt.Errorf(`condition "%s" evaluation failed: %w`, v.value, err)
 			}
-			return &Key{k.name, NewBoolVal(cond)}, nil
+			return &Key{name: k.name, value: NewBoolVal(cond)}, nil
 		}
 		return nil, fmt.Errorf("condition key's value must be a string; received %T", k.value)
 	}
@@ -303,7 +311,7 @@ func (k *Key) Apply(vars *Vars) (Node, error) {
 	if v == nil {
 		return nil, nil
 	}
-	return &Key{k.name, v}, nil
+	return &Key{name: k.name, value: v}, nil
 }
 
 // Processors returns any attached processors, because of variable substitution.

internal/pkg/agent/transpiler/ast_test.go

@@ -11,6 +11,8 @@ import (
 
 	"github.com/elastic/elastic-agent-libs/mapstr"
 
+	"github.com/elastic/elastic-agent/internal/pkg/eql"
+
 	"github.com/google/go-cmp/cmp"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -957,6 +959,41 @@ func TestShallowClone(t *testing.T) {
 	}
 }
 
+func TestCondition(t *testing.T) {
+	vars := mustMakeVars(map[string]interface{}{
+		"other": map[string]interface{}{
+			"data": "info",
+		}})
+
+	input := NewKey("condition", NewStrVal("${other.data} == 'info'"))
+	expected := NewKey("condition", NewBoolVal(true))
+
+	// the condition string hasn't been parsed yet
+	assert.Nil(t, input.condition)
+
+	output, err := input.Apply(vars)
+	require.NoError(t, err)
+	assert.Equal(t, expected, output)
+
+	// check if the condition was parsed and cached
+	assert.NotNil(t, input.condition)
+	condition, err := eql.New(input.value.Value().(string))
+	require.NoError(t, err)
+	assert.Equal(t, condition, input.condition)
+
+	// create a dict with the key
+	dict := NewDict([]Node{input})
+	ast := &AST{root: NewKey("key", dict)}
+	// the cached condition remains
+	assert.Equal(t, condition, input.condition)
+
+	// replace the key with a new one, without a cached condition
+	input2 := NewKey("condition", NewStrVal("${other.data} == 'info'"))
+	err = Insert(ast, input2, "")
+	require.NoError(t, err)
+	assert.Nil(t, input2.condition)
+}
+
 func mustMakeVars(mapping map[string]interface{}) *Vars {
 	v, err := NewVars("", mapping, nil)
 	if err != nil {