feat(alerting): Implement alert-level provider overrides (#929)

* feat(alerting): Implement alert-level provider overrides

Fixes #96

* Fix tests

* Add missing test cases for alerting providers

* feat(alerting): Implement alert-level overrides on all providers

* chore: Add config.yaml to .gitignore

* fix typo in discord provider

* test: Start fixing tests for alerting providers

* test: Fix GitLab tests

* Fix all tests

* test: Improve coverage

* test: Improve coverage

* Rename override to provider-override

* docs: Mention new provider-override config

* test: Improve coverage

* test: Improve coverage

* chore: Rename Alert.OverrideAsBytes to Alert.ProviderOverrideAsBytes

alerting/provider/slack/slack.go

@@ -3,6 +3,7 @@ package slack
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -10,41 +11,70 @@ import (
 	"github.com/TwiN/gatus/v5/alerting/alert"
 	"github.com/TwiN/gatus/v5/client"
 	"github.com/TwiN/gatus/v5/config/endpoint"
+	"gopkg.in/yaml.v3"
 )
 
+var (
+	ErrWebhookURLNotSet       = errors.New("webhook-url not set")
+	ErrDuplicateGroupOverride = errors.New("duplicate group override")
+)
+
+type Config struct {
+	WebhookURL string `yaml:"webhook-url"` // Slack webhook URL
+}
+
+func (cfg *Config) Validate() error {
+	if len(cfg.WebhookURL) == 0 {
+		return ErrWebhookURLNotSet
+	}
+	return nil
+}
+
+func (cfg *Config) Merge(override *Config) {
+	if len(override.WebhookURL) > 0 {
+		cfg.WebhookURL = override.WebhookURL
+	}
+}
+
 // AlertProvider is the configuration necessary for sending an alert using Slack
 type AlertProvider struct {
-	WebhookURL string `yaml:"webhook-url"` // Slack webhook URL
+	DefaultConfig Config `yaml:",inline"`
+
 	// DefaultAlert is the default alert configuration to use for endpoints with an alert of the appropriate type
 	DefaultAlert *alert.Alert `yaml:"default-alert,omitempty"`
+
 	// Overrides is a list of Override that may be prioritized over the default configuration
 	Overrides []Override `yaml:"overrides,omitempty"`
 }
 
 // Override is a case under which the default integration is overridden
 type Override struct {
-	Group      string `yaml:"group"`
-	WebhookURL string `yaml:"webhook-url"`
+	Group  string `yaml:"group"`
+	Config `yaml:",inline"`
 }
 
-// IsValid returns whether the provider's configuration is valid
-func (provider *AlertProvider) IsValid() bool {
+// Validate the provider's configuration
+func (provider *AlertProvider) Validate() error {
 	registeredGroups := make(map[string]bool)
 	if provider.Overrides != nil {
 		for _, override := range provider.Overrides {
-			if isAlreadyRegistered := registeredGroups[override.Group]; isAlreadyRegistered || override.Group == "" || len(override.WebhookURL) == 0 {
-				return false
+			if isAlreadyRegistered := registeredGroups[override.Group]; isAlreadyRegistered || override.Group == "" {
+				return ErrDuplicateGroupOverride
 			}
 			registeredGroups[override.Group] = true
 		}
 	}
-	return len(provider.WebhookURL) > 0
+	return provider.DefaultConfig.Validate()
 }
 
 // Send an alert using the provider
 func (provider *AlertProvider) Send(ep *endpoint.Endpoint, alert *alert.Alert, result *endpoint.Result, resolved bool) error {
+	cfg, err := provider.GetConfig(ep.Group, alert)
+	if err != nil {
+		return err
+	}
 	buffer := bytes.NewBuffer(provider.buildRequestBody(ep, alert, result, resolved))
-	request, err := http.NewRequest(http.MethodPost, provider.getWebhookURLForGroup(ep.Group), buffer)
+	request, err := http.NewRequest(http.MethodPost, cfg.WebhookURL, buffer)
 	if err != nil {
 		return err
 	}
@@ -126,19 +156,38 @@ func (provider *AlertProvider) buildRequestBody(ep *endpoint.Endpoint, alert *al
 	return bodyAsJSON
 }
 
-// getWebhookURLForGroup returns the appropriate Webhook URL integration to for a given group
-func (provider *AlertProvider) getWebhookURLForGroup(group string) string {
-	if provider.Overrides != nil {
-		for _, override := range provider.Overrides {
-			if group == override.Group {
-				return override.WebhookURL
-			}
-		}
-	}
-	return provider.WebhookURL
-}
-
 // GetDefaultAlert returns the provider's default alert configuration
 func (provider *AlertProvider) GetDefaultAlert() *alert.Alert {
 	return provider.DefaultAlert
 }
+
+// GetConfig returns the configuration for the provider with the overrides applied
+func (provider *AlertProvider) GetConfig(group string, alert *alert.Alert) (*Config, error) {
+	cfg := provider.DefaultConfig
+	// Handle group overrides
+	if provider.Overrides != nil {
+		for _, override := range provider.Overrides {
+			if group == override.Group {
+				cfg.Merge(&override.Config)
+				break
+			}
+		}
+	}
+	// Handle alert overrides
+	if len(alert.ProviderOverride) != 0 {
+		overrideConfig := Config{}
+		if err := yaml.Unmarshal(alert.ProviderOverrideAsBytes(), &overrideConfig); err != nil {
+			return nil, err
+		}
+		cfg.Merge(&overrideConfig)
+	}
+	// Validate the configuration
+	err := cfg.Validate()
+	return &cfg, err
+}
+
+// ValidateOverrides validates the alert's provider override and, if present, the group override
+func (provider *AlertProvider) ValidateOverrides(group string, alert *alert.Alert) error {
+	_, err := provider.GetConfig(group, alert)
+	return err
+}