Add endpoints.onErrorAdd

Add endpoints[].onErrorAdd to add response body or parts of it as error, when the conditions fail for easier debugging.

Implements https://github.com/TwiN/gatus/issues/742

README.md

@@ -272,6 +272,7 @@ You can then configure alerts to be triggered when an endpoint is unhealthy once
 | `endpoints[].ui.hide-url`                       | Whether to ensure the URL is not displayed in the results. Useful if the URL contains a token.                                              | `false`                    |
 | `endpoints[].ui.dont-resolve-failed-conditions` | Whether to resolve failed conditions for the UI.                                                                                            | `false`                    |
 | `endpoints[].ui.badge.reponse-time`             | List of response time thresholds. Each time a threshold is reached, the badge has a different color.                                        | `[50, 200, 300, 500, 750]` |
+| `endpoints[].onErrorAdd`                        | Add whole response Body ("[BODY]") or part of JSON ("[BODY].error") when an error occurs.                                                   | `""`                       |
 
 
 ### External Endpoints

core/endpoint.go

@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"github.com/TwiN/gatus/v5/jsonpath"
 	"io"
 	"net"
 	"net/http"
@@ -120,6 +121,9 @@ type Endpoint struct {
 
 	// NumberOfSuccessesInARow is the number of successful evaluations in a row
 	NumberOfSuccessesInARow int `yaml:"-"`
+
+	// OnErrorAdd is the message or [BODY] or part of [BODY].json to add to the result's errors if the endpoint fails
+	OnErrorAdd string `yaml:"onErrorAdd,omitempty"`
 }
 
 // IsEnabled returns whether the endpoint is enabled or not
@@ -302,6 +306,25 @@ func (endpoint *Endpoint) EvaluateHealth() *Result {
 	if endpoint.UIConfig.HideConditions {
 		result.ConditionResults = nil
 	}
+
+	if endpoint.OnErrorAdd != "" && !result.Success {
+		if endpoint.OnErrorAdd == BodyPlaceholder && len(result.Body) > 0 {
+			result.Errors = append(result.Errors, string(result.Body))
+		} else if strings.Contains(endpoint.OnErrorAdd, BodyPlaceholder) && len(result.Body) > 0 {
+			resolvedElement, resolvedElementLength, err := jsonpath.Eval(strings.TrimPrefix(
+				strings.TrimPrefix(endpoint.OnErrorAdd, BodyPlaceholder), "."), result.Body)
+			if err != nil {
+				result.Errors = append(result.Errors, "Decoding of endpoint.OnErrorAdd failed: "+err.Error())
+			} else {
+				if resolvedElementLength > 0 {
+					result.Errors = append(result.Errors, resolvedElement)
+				}
+			}
+		} else {
+			result.Errors = append(result.Errors, endpoint.OnErrorAdd)
+		}
+	}
+
 	return result
 }
 
@@ -417,6 +440,9 @@ func (endpoint *Endpoint) buildHTTPRequest() *http.Request {
 
 // needsToReadBody checks if there's any condition that requires the response Body to be read
 func (endpoint *Endpoint) needsToReadBody() bool {
+	if strings.Contains(endpoint.OnErrorAdd, BodyPlaceholder) {
+		return true
+	}
 	for _, condition := range endpoint.Conditions {
 		if condition.hasBodyPlaceholder() {
 			return true

core/endpoint_test.go

@@ -204,6 +204,29 @@ func TestEndpoint(t *testing.T) {
 			},
 			MockRoundTripper: nil,
 		},
+		{
+			Name: "failed-status-with-error-from-body",
+			Endpoint: Endpoint{
+				Name:       "website-health",
+				URL:        "https://twin.sh/health",
+				Conditions: []Condition{"[STATUS] == 200"},
+				OnErrorAdd: "[BODY].error",
+			},
+			ExpectedResult: &Result{
+				Success:   false,
+				Connected: true,
+				Hostname:  "twin.sh",
+				ConditionResults: []*ConditionResult{
+					{Condition: "[STATUS] (502) == 200", Success: false},
+				},
+				DomainExpiration: 0, // Because there's no [DOMAIN_EXPIRATION] condition, this is not resolved, so it should be 0.
+				Errors:           []string{"something went wrong"},
+			},
+			MockRoundTripper: test.MockRoundTripper(func(r *http.Request) *http.Response {
+				return &http.Response{StatusCode: http.StatusBadGateway, Body: io.NopCloser(strings.NewReader(
+					"{\"error\":\"something went wrong\"}"))}
+			}),
+		},
 	}
 	for _, scenario := range scenarios {
 		t.Run(scenario.Name, func(t *testing.T) {