Refactor delivery targets to a Target interface (closes #77) (#81)

Refactors the delivery engine so each target TYPE is an implementation of a `Target` interface, dispatched from a registry, with each target owning its full delivery including durable retries. Implements the authoritative design from issue #77 (the corrected "hand the DB + Scheduler to the target" design).

## The new interface

```go
type Scheduler interface {
    ScheduleRetry(task Task, delay time.Duration)
}

type Target interface {
    Deliver(ctx context.Context, webhookDB *gorm.DB,
        d *database.Delivery, task *Task, sched Scheduler)
}
```

`Deliver` receives everything a target needs to be autonomous and durable: the request context, the per-webhook `*gorm.DB`, the `*database.Delivery`, the attempt `*Task`, and a `Scheduler` (the engine) for durable re-enqueue. The target makes one attempt, writes the `DeliveryResult`, updates `DeliveryStatus`, and — for retry targets — decides whether to retry, computes its own backoff, gates with its own circuit breaker, and reschedules via the injected `Scheduler`.

`processDelivery` collapses to a registry lookup (`map[database.TargetType]Target`) and a `Deliver` call; an unknown target type still fails the delivery as before.

## Per-target ownership

- `httpTarget` and `slackTarget` share a retry core (`httpCore`) that owns retry, exponential backoff, and the per-target circuit breaker. The core is fire-and-forget when `MaxRetries == 0` and adds breaker-gated backed-off retries when `MaxRetries > 0`. The per-attempt request differs (HTTP forwards the body + filtered headers; Slack posts a formatted message) and is supplied as a closure, so each keeps its exact recording semantics (e.g. HTTP records no error string for a non-2xx, Slack records `HTTP <code>`).
- `databaseTarget` and `logTarget` are fire-and-forget: they record a single successful attempt.

Moved wholesale into the http/slack targets: `deliverHTTP*`, `handleHTTPRetry`, `circuitBreakerBlock`, `calcBackoff` / `calcRemainingBackoff` / `backoffElapsed`, the circuit-breaker `sync.Map` + `getCircuitBreaker`, `clientForConfig`, `doHTTPRequest`, `applyRequestHeaders`, and the config parsers. The engine keeps `recordResult`, `updateDeliveryStatus`, and `ScheduleRetry`.

## Slack MaxRetries gating

Slack is now on the same shared core as HTTP, with retry + breaker gated on `MaxRetries`. A `MaxRetries` of 0 stays single-attempt fire-and-forget, so **every existing Slack target is unchanged**; a Slack target configured with retries gets backoff + circuit breaker.

## Log-target full content

`logTarget` now logs the ENTIRE inbound webhook — full request body and full request headers, plus method, content type, and the webhook id and entrypoint id — rather than a summary line. This supersedes the smaller log-summary work (#70).

## `Task.EntrypointID`

To carry the entrypoint id to the log target, `Task` gains an `EntrypointID` field, populated in the webhook handler's `buildDeliveryTasks`, the engine's recovery-task builder, and `buildEventFromTask`.

## Durability / recovery

The crash-durable async retry model is preserved unchanged: one attempt per worker turn; on failure the status is set `retrying`, backoff is computed, and the task is re-enqueued via `ScheduleRetry` (a `time.AfterFunc` onto the retry channel). On restart, `recoverRetryingDeliveries` and the 60s sweep hand each orphaned `retrying` delivery back to its target to recompute the remaining backoff and reschedule (targets that own retries implement an internal `rescheduler`; fire-and-forget targets, which never produce `retrying` deliveries, are skipped).

## How behaviour is preserved

No external behaviour changes except the two called out above (log target full content; Slack gaining `MaxRetries`-gated retries). All existing delivery tests pass with only their `export_test.go` wrappers re-pointed at the new structure — `ExportDeliverHTTP/Slack/Database/Log` now call the targets, `ExportGetCircuitBreaker` / `ExportClient` / `ExportClientForConfig` / `ExportDoHTTPRequest` resolve against the HTTP target's shared client and breaker map, and `ExportParseHTTPConfig` / `ExportParseSlackConfig` call the relocated free functions. Added: a `logTarget` test asserting the log line contains the full body, headers, and ids, and a Slack `MaxRetries`-gated retry test.

`docker build .` is green (fmt-check, lint, test, static build all pass).

Closes #77

Co-authored-by: sneak <sneak@sneak.berlin>
Reviewed-on: #81
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/delivery/target_http.go

@@ -0,0 +1,499 @@
+package delivery
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"sync"
+	"time"
+
+	"gorm.io/gorm"
+	"sneak.berlin/go/webhooker/internal/database"
+)
+
+// Sentinel errors returned by the config parsers.
+var (
+	errEmptyTargetConfig = errors.New(
+		"empty target config",
+	)
+	errMissingTargetURL = errors.New(
+		"target URL is required",
+	)
+)
+
+// HTTPTargetConfig holds configuration for http target
+// types.
+type HTTPTargetConfig struct {
+	URL     string            `json:"url"`
+	Headers map[string]string `json:"headers,omitempty"`
+	Timeout int               `json:"timeout,omitempty"`
+}
+
+// httpCore holds the retry, backoff, and circuit-breaker
+// machinery shared by the HTTP and Slack targets. Each of
+// those targets owns its own httpCore instance (and thus its
+// own circuit breakers); the per-attempt request differs
+// between them and is supplied as a closure.
+type httpCore struct {
+	eng *Engine
+
+	// circuitBreakers stores a *CircuitBreaker per target ID.
+	circuitBreakers sync.Map
+}
+
+// deliver runs one delivery attempt through the retry core.
+// A maxRetries of 0 is fire-and-forget: a single attempt is
+// recorded and no circuit breaker is consulted. A positive
+// maxRetries gates the attempt on the circuit breaker and
+// schedules a backed-off retry on failure.
+func (c *httpCore) deliver(
+	webhookDB *gorm.DB,
+	d *database.Delivery,
+	task *Task,
+	sched Scheduler,
+	maxRetries int,
+	attempt func() attemptResult,
+) {
+	if maxRetries == 0 {
+		c.fireAndForget(webhookDB, d, attempt())
+
+		return
+	}
+
+	c.withRetry(
+		webhookDB, d, task, sched, maxRetries, attempt,
+	)
+}
+
+func (c *httpCore) fireAndForget(
+	webhookDB *gorm.DB,
+	d *database.Delivery,
+	res attemptResult,
+) {
+	c.eng.recordResult(
+		webhookDB, d, 1, res.success,
+		res.statusCode, res.respBody, res.errMsg,
+		res.duration,
+	)
+
+	if res.success {
+		c.eng.updateDeliveryStatus(
+			webhookDB, d,
+			database.DeliveryStatusDelivered,
+		)
+
+		return
+	}
+
+	c.eng.updateDeliveryStatus(
+		webhookDB, d, database.DeliveryStatusFailed,
+	)
+}
+
+func (c *httpCore) withRetry(
+	webhookDB *gorm.DB,
+	d *database.Delivery,
+	task *Task,
+	sched Scheduler,
+	maxRetries int,
+	attempt func() attemptResult,
+) {
+	cb := c.getCircuitBreaker(task.TargetID)
+	if c.circuitBreakerBlock(webhookDB, d, task, sched, cb) {
+		return
+	}
+
+	attemptNum := task.AttemptNum
+
+	res := attempt()
+
+	c.eng.recordResult(
+		webhookDB, d, attemptNum, res.success,
+		res.statusCode, res.respBody, res.errMsg,
+		res.duration,
+	)
+
+	if res.success {
+		cb.RecordSuccess()
+
+		c.eng.updateDeliveryStatus(
+			webhookDB, d,
+			database.DeliveryStatusDelivered,
+		)
+
+		return
+	}
+
+	cb.RecordFailure()
+
+	c.handleRetry(
+		webhookDB, d, task, sched, maxRetries, attemptNum,
+	)
+}
+
+func (c *httpCore) circuitBreakerBlock(
+	webhookDB *gorm.DB,
+	d *database.Delivery,
+	task *Task,
+	sched Scheduler,
+	cb *CircuitBreaker,
+) bool {
+	if cb.Allow() {
+		return false
+	}
+
+	remaining := cb.CooldownRemaining()
+
+	c.eng.log.Info(
+		"circuit breaker open, skipping delivery",
+		"target_id", task.TargetID,
+		"target_name", task.TargetName,
+		"delivery_id", d.ID,
+		"cooldown_remaining", remaining,
+	)
+
+	c.eng.updateDeliveryStatus(
+		webhookDB, d,
+		database.DeliveryStatusRetrying,
+	)
+
+	retryTask := *task
+	sched.ScheduleRetry(retryTask, remaining)
+
+	return true
+}
+
+func (c *httpCore) handleRetry(
+	webhookDB *gorm.DB,
+	d *database.Delivery,
+	task *Task,
+	sched Scheduler,
+	maxRetries int,
+	attemptNum int,
+) {
+	if attemptNum >= maxRetries {
+		c.eng.updateDeliveryStatus(
+			webhookDB, d,
+			database.DeliveryStatusFailed,
+		)
+
+		return
+	}
+
+	c.eng.updateDeliveryStatus(
+		webhookDB, d, database.DeliveryStatusRetrying,
+	)
+
+	backoff := calcBackoff(attemptNum)
+
+	retryTask := *task
+	retryTask.AttemptNum = attemptNum + 1
+	sched.ScheduleRetry(retryTask, backoff)
+}
+
+func (c *httpCore) getCircuitBreaker(
+	targetID string,
+) *CircuitBreaker {
+	if val, ok := c.circuitBreakers.Load(targetID); ok {
+		cb, _ := val.(*CircuitBreaker)
+
+		return cb
+	}
+
+	fresh := NewCircuitBreaker()
+
+	actual, _ := c.circuitBreakers.LoadOrStore(
+		targetID, fresh,
+	)
+
+	cb, _ := actual.(*CircuitBreaker)
+
+	return cb
+}
+
+// remainingBackoff returns how long remains of the backoff
+// window for the last attempt of a recovered retrying
+// delivery. It implements rescheduler.
+func (c *httpCore) remainingBackoff(
+	webhookDB *gorm.DB,
+	deliveryID string,
+	attemptNum int,
+) time.Duration {
+	var lastResult database.DeliveryResult
+
+	err := webhookDB.
+		Where("delivery_id = ?", deliveryID).
+		Order("created_at DESC").
+		First(&lastResult).Error
+	if err != nil {
+		return 0
+	}
+
+	backoff := calcBackoff(attemptNum)
+	elapsed := time.Since(lastResult.CreatedAt)
+	remaining := backoff - elapsed
+
+	return max(remaining, 0)
+}
+
+// backoffElapsed reports whether the backoff window for the
+// last attempt of a retrying delivery has passed. It
+// implements rescheduler.
+func (c *httpCore) backoffElapsed(
+	webhookDB *gorm.DB,
+	deliveryID string,
+	attemptNum int,
+) bool {
+	var lastResult database.DeliveryResult
+
+	err := webhookDB.
+		Where("delivery_id = ?", deliveryID).
+		Order("created_at DESC").
+		First(&lastResult).Error
+	if err != nil {
+		return true
+	}
+
+	backoff := calcBackoff(attemptNum)
+
+	return time.Since(lastResult.CreatedAt) >= backoff
+}
+
+func calcBackoff(attemptNum int) time.Duration {
+	shift := max(attemptNum-1, 0)
+	shift = min(shift, maxBackoffShift)
+
+	return time.Duration(1<<uint(shift)) * time.Second
+}
+
+// httpTarget delivers events to http targets. It forwards the
+// event body and (filtered) request headers to the configured
+// URL and owns retry, backoff, and circuit breaking through
+// the shared httpCore.
+type httpTarget struct {
+	*httpCore
+
+	client *http.Client
+}
+
+// Deliver implements Target.
+func (t *httpTarget) Deliver(
+	ctx context.Context,
+	webhookDB *gorm.DB,
+	d *database.Delivery,
+	task *Task,
+	sched Scheduler,
+) {
+	cfg, err := parseHTTPConfig(d.Target.Config)
+	if err != nil {
+		t.eng.log.Error(
+			"invalid HTTP target config",
+			"target_id", d.TargetID,
+			"error", err,
+		)
+
+		t.eng.recordResult(
+			webhookDB, d, task.AttemptNum,
+			false, 0, "", err.Error(), 0,
+		)
+
+		t.eng.updateDeliveryStatus(
+			webhookDB, d, database.DeliveryStatusFailed,
+		)
+
+		return
+	}
+
+	attempt := func() attemptResult {
+		return t.attempt(ctx, cfg, &d.Event)
+	}
+
+	t.deliver(
+		webhookDB, d, task, sched,
+		d.Target.MaxRetries, attempt,
+	)
+}
+
+// attempt performs a single HTTP delivery attempt and derives
+// the success flag and error message the same way the engine
+// did: a non-2xx response is a failure but carries no error
+// string; only a transport-level error does.
+func (t *httpTarget) attempt(
+	ctx context.Context,
+	cfg *HTTPTargetConfig,
+	event *database.Event,
+) attemptResult {
+	statusCode, respBody, duration, reqErr :=
+		t.doHTTPRequest(ctx, cfg, event)
+
+	success := reqErr == nil &&
+		statusCode >= httpSuccessMin &&
+		statusCode < httpSuccessMax
+
+	errMsg := ""
+	if reqErr != nil {
+		errMsg = reqErr.Error()
+	}
+
+	return attemptResult{
+		statusCode: statusCode,
+		respBody:   respBody,
+		duration:   duration,
+		success:    success,
+		errMsg:     errMsg,
+	}
+}
+
+func (t *httpTarget) doHTTPRequest(
+	ctx context.Context,
+	cfg *HTTPTargetConfig,
+	event *database.Event,
+) (int, string, int64, error) {
+	start := time.Now()
+
+	req, reqErr := http.NewRequestWithContext(
+		ctx,
+		http.MethodPost,
+		cfg.URL,
+		bytes.NewReader([]byte(event.Body)),
+	)
+	if reqErr != nil {
+		return 0, "", 0, fmt.Errorf(
+			"creating request: %w", reqErr,
+		)
+	}
+
+	applyRequestHeaders(req, event, cfg)
+
+	client := t.clientForConfig(cfg)
+
+	resp, doErr := executeHTTPRequest(client, req)
+
+	dur := time.Since(start).Milliseconds()
+	if doErr != nil {
+		return 0, "", dur, fmt.Errorf(
+			"sending request: %w", doErr,
+		)
+	}
+
+	defer func() { _ = resp.Body.Close() }()
+
+	body, readErr := io.ReadAll(
+		io.LimitReader(resp.Body, maxBodyLog),
+	)
+	if readErr != nil {
+		return resp.StatusCode, "", dur,
+			fmt.Errorf(
+				"reading response body: %w", readErr,
+			)
+	}
+
+	return resp.StatusCode, string(body), dur, nil
+}
+
+func (t *httpTarget) clientForConfig(
+	cfg *HTTPTargetConfig,
+) *http.Client {
+	if cfg.Timeout > 0 {
+		// Reuse the shared client's SSRF-safe transport so
+		// a per-target timeout does not drop the
+		// request-time private-IP guard. Only the timeout
+		// is overridden.
+		return &http.Client{
+			Timeout: time.Duration(
+				cfg.Timeout,
+			) * time.Second,
+			Transport: t.client.Transport,
+		}
+	}
+
+	return t.client
+}
+
+func parseHTTPConfig(
+	configJSON string,
+) (*HTTPTargetConfig, error) {
+	if configJSON == "" {
+		return nil, errEmptyTargetConfig
+	}
+
+	var cfg HTTPTargetConfig
+
+	err := json.Unmarshal(
+		[]byte(configJSON), &cfg,
+	)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"parsing config JSON: %w", err,
+		)
+	}
+
+	if cfg.URL == "" {
+		return nil, errMissingTargetURL
+	}
+
+	return &cfg, nil
+}
+
+// isForwardableHeader returns true if the header should
+// be forwarded to targets.
+func isForwardableHeader(name string) bool {
+	switch http.CanonicalHeaderKey(name) {
+	case "Host", "Connection", "Keep-Alive",
+		"Transfer-Encoding", "Te", "Trailer",
+		"Upgrade", "Proxy-Authorization",
+		"Proxy-Connection", "Content-Length":
+		return false
+	default:
+		return true
+	}
+}
+
+func applyRequestHeaders(
+	req *http.Request,
+	event *database.Event,
+	cfg *HTTPTargetConfig,
+) {
+	if event.ContentType != "" {
+		req.Header.Set(
+			"Content-Type", event.ContentType,
+		)
+	}
+
+	var originalHeaders map[string][]string
+
+	if event.Headers != "" {
+		jsonErr := json.Unmarshal(
+			[]byte(event.Headers),
+			&originalHeaders,
+		)
+		if jsonErr == nil {
+			for k, vals := range originalHeaders {
+				if isForwardableHeader(k) {
+					for _, v := range vals {
+						req.Header.Add(k, v)
+					}
+				}
+			}
+		}
+	}
+
+	for k, v := range cfg.Headers {
+		req.Header.Set(k, v)
+	}
+
+	req.Header.Set("User-Agent", "webhooker/1.0")
+}
+
+// executeHTTPRequest sends an HTTP request using the provided
+// client. URLs are validated by the config parsers and the
+// SSRF-safe transport before reaching here.
+func executeHTTPRequest(
+	client *http.Client, req *http.Request,
+) (*http.Response, error) {
+	return client.Do(req) //#nosec G704 -- URL validated by parseHTTPConfig/parseSlackConfig and SSRF-safe transport
+}