[8.18](backport #45810) x-pack/filebeat/input/httpjson: allow template termination without logging error (#45993)

* x-pack/filebeat/input/httpjson: allow template termination without logging error (#45810)

This adds a terminate helper that can be used to signal that a template
should terminate without an error. The termination can (should) be used
to report the reason for the termination which will be logged at DEBUG
level, but otherwise ignored.

(cherry picked from commit f324f29)

# Conflicts:
#	docs/reference/filebeat/filebeat-input-httpjson.md
#	x-pack/filebeat/input/httpjson/value_tpl.go

* remove irrelevant changelog entries

* remove markdown
* add asciidoc documentation
* resolve conflicts

---------

Co-authored-by: Dan Kortschak <dan.kortschak@elastic.co>

Author: mergify[bot]

CHANGELOG.next.asciidoc

@@ -241,6 +241,7 @@ https://github.com/elastic/beats/compare/v8.8.1\...main[Check the HEAD diff]
 - Jounrald input now supports filtering by facilities {pull}41061[41061]
 - Add ability to remove request trace logs from http_endpoint input. {pull}40005[40005]
 - Add ability to remove request trace logs from entityanalytics input. {pull}40004[40004]
+- Add mechanism to allow HTTP JSON templates to terminate without logging an error. {issue}45664[45664] {pull}45810[45810]
 
 *Auditbeat*
 

x-pack/filebeat/docs/inputs/input-httpjson.asciidoc

@@ -230,6 +230,7 @@ Some built-in helper functions are provided to work with the input state inside
 - `parseTimestamp`: parses a timestamp in seconds and returns a `time.Time` in UTC. Example: `[[parseTimestamp 1604582732]]` returns `2020-11-05 13:25:32 +0000 UTC`.
 - `replaceAll(old, new, s)`: replaces all non-overlapping instances of `old` with `new` in `s`. Example: `[[ replaceAll "some" "my" "some value" ]]` returns `my value`.
 - `sprintf`: formats according to a format specifier and returns the resulting string. Refer to https://pkg.go.dev/fmt#Sprintf[the Go docs] for usage. Example: `[[sprintf "%d:%q" 34 "quote this"]]`
+- `terminate`: exits the template without falling back to the default value and without causing an error. It takes a single string argument that is logged in debug logging.
 - `toInt`: converts a value of any type to an integer when possible. Returns 0 if the conversion fails.
 - `toJSON`: converts a value to a JSON string. This can be used with `value_type: json` to create an object from a template. Example: `[[ toJSON .last_response.body.pagingIdentifiers ]]`.
 - `urlEncode`: URL encodes the supplied string. Example `[[urlEncode "string1"]]`. Example `[[urlEncode "<string1>"]]` will return `%3Cstring1%3E`.

x-pack/filebeat/input/httpjson/request.go

@@ -386,6 +386,9 @@ func evaluateResponse(expression *valueTpl, data []byte, log *logp.Logger) (bool
 	if err != nil {
 		return false, fmt.Errorf("error while evaluating expression: %w", err)
 	}
+	if val == "" {
+		return false, nil
+	}
 	result, err := strconv.ParseBool(val)
 	if err != nil {
 		return false, fmt.Errorf("error while parsing boolean value of string: %w", err)

x-pack/filebeat/input/httpjson/value_tpl.go

@@ -82,6 +82,7 @@ func (t *valueTpl) Unpack(in string) error {
 			"urlEncode":           urlEncode,
 			"userAgent":           userAgentString,
 			"uuid":                uuidString,
+			"terminate":           func(s string) (any, error) { return nil, &errTerminate{s} },
 		}).
 		Delims(leftDelim, rightDelim).
 		Parse(in)
@@ -94,6 +95,17 @@ func (t *valueTpl) Unpack(in string) error {
 	return nil
 }
 
+type errTerminate struct {
+	Reason string
+}
+
+func (e *errTerminate) Error() string {
+	if e.Reason != "" {
+		return "terminated template: " + e.Reason
+	}
+	return "terminated template"
+}
+
 func (t *valueTpl) Execute(trCtx *transformContext, tr transformable, targetName string, defaultVal *valueTpl, log *logp.Logger) (val string, err error) {
 	fallback := func(err error) (string, error) {
 		if defaultVal != nil {
@@ -128,6 +140,11 @@ func (t *valueTpl) Execute(trCtx *transformContext, tr transformable, targetName
 	}
 
 	if err := t.Template.Execute(buf, data); err != nil {
+		var termErr *errTerminate
+		if errors.As(err, &termErr) {
+			log.Debugw("template execution terminated", "target", targetName, "reason", termErr.Reason)
+			return "", nil
+		}
 		return fallback(err)
 	}
 

x-pack/filebeat/input/httpjson/value_tpl_test.go

@@ -74,6 +74,14 @@ func TestValueTpl(t *testing.T) {
 			paramDefVal: "25",
 			expectedVal: "25",
 		},
+		{
+			name:        "terminate",
+			value:       `[[if false]]ok[[else]][[terminate "because reasons"]][[end]]`,
+			paramCtx:    emptyTransformContext(),
+			paramTr:     transformable{},
+			paramDefVal: "this should not be seen",
+			expectedVal: "",
+		},
 		{
 			name:          "returns error if result is empty and no default is set",
 			value:         "",