webhooker/internal/handlers/event_body.go

package handlers

import (
	"database/sql"
	"errors"
	"net/http"
	"strconv"

	"github.com/go-chi/chi"
	"github.com/google/uuid"
	"gorm.io/gorm"
	"sneak.berlin/go/webhooker/internal/database"
)

// eventBodyQuery reads one event's stored body as bytes. The cast
// to blob is what makes the driver hand back the stored bytes
// rather than a string conversion, so Content-Length taken from
// the result matches what goes on the wire. The soft-delete
// predicate is spelled out because Raw bypasses GORM's default
// scope, and it is what stops a reaped event still being
// downloadable.
const eventBodyQuery = "SELECT cast(body as blob) " +
	"FROM events WHERE id = ? AND webhook_id = ? AND deleted_at IS NULL"

// HandleEventBodyDownload serves one event's stored body in
// full, which the event log page cannot: it caps each rendered
// body at maxRenderedBodyBytes.
//
// The bytes are attacker-supplied — anyone who can reach the
// public receiver chooses them — and this route hands them back
// inside the operator's own authenticated origin, so the
// response is deliberately not renderable. Content-Disposition
// makes the browser download rather than display it, and the
// octet-stream type plus nosniff stop it being interpreted as
// HTML or script. Without those a stored payload would execute
// as the logged-in operator. The application's CSP does not
// help here: script-src allows 'unsafe-inline' from 'self', so
// a document served from this origin could run its own inline
// script.
func (h *Handlers) HandleEventBodyDownload() http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		webhook, ok := h.ownedWebhook(w, r)
		if !ok {
			return
		}

		// Parsing the id before use serves two purposes: a
		// malformed id can never reach the SQL or the response
		// header, and the canonical form below is drawn from
		// uuid's own fixed alphabet rather than from the
		// request, so the Content-Disposition value cannot be
		// steered by a client.
		eventID, err := uuid.Parse(chi.URLParam(r, "eventID"))
		if err != nil {
			http.NotFound(w, r)

			return
		}

		h.serveEventBody(w, r, webhook, eventID.String())
	}
}

// serveEventBody writes the named event's stored body to w.
//
// The event must belong to webhook, which is what keeps this
// route from reading any event in the system by id alone. Two
// things enforce that and they are not equally strong. The
// operative one is that events live in a per-webhook SQLite
// file, so a sibling webhook's event is not in the database
// being queried at all. The webhook_id predicate on the query
// below is the second guard, and it is currently redundant
// against that isolation; it is there so the scoping survives
// any future change that puts more than one webhook's events in
// one file.
//
// The body is read in one query and held whole in memory while
// it is written. That costs roughly two body-sized allocations
// per concurrent download, not one: the driver's column buffer
// and the copy database/sql makes in convertAssign when a
// []byte column is scanned into a *[]byte are live at the same
// time. Measured allocation is ~2x the body plus ~45 KB, so at
// the 1 MB ingest cap a download costs ~2 MB of Go heap. On
// top of that, SQLite's own materialisation of the column
// value sits in the driver's allocator outside the Go heap, so
// process peak is higher again: 2x is a floor, not a ceiling.
// There is no cheaper bound available — database/sql exposes
// no incremental handle on a SQLite BLOB, and reading byte
// ranges with substr does not avoid the cost either, because
// SQLite materialises the whole column value to evaluate each
// substr call. Range reads only pay for that materialisation
// once per range.
//
// One consequence is worth keeping in view: the read finishes
// before the client is written to, so no read lock is held for
// the length of a slow download. These per-webhook databases
// run in SQLite's default journal mode rather than WAL, so a
// lock held that long would block the receiver from recording
// new events.
func (h *Handlers) serveEventBody(
	w http.ResponseWriter,
	r *http.Request,
	webhook database.Webhook,
	eventID string,
) {
	if !h.dbMgr.DBExists(webhook.ID) {
		http.NotFound(w, r)

		return
	}

	webhookDB, err := h.dbMgr.GetDB(webhook.ID)
	if err != nil {
		h.serverError(w, "failed to get webhook database", err)

		return
	}

	body, found, err := eventBody(webhookDB, webhook.ID, eventID)
	if err != nil {
		h.serverError(w, "failed to read event body", err)

		return
	}

	// A miss is a 404 whether the event belongs to another
	// webhook or does not exist at all, so the response does
	// not report which. Reading the body before any header is
	// written is also what keeps an event reaped mid-request
	// from producing a torn response: either the read finds the
	// row and the whole body is served, or it does not and the
	// response is a clean 404.
	if !found {
		http.NotFound(w, r)

		return
	}

	setEventBodyHeaders(w, eventID, int64(len(body)))

	_, err = w.Write(body)
	if err != nil {
		// The status and Content-Length are already committed,
		// so the client sees a short download. There is no way
		// to report a 500 from here; the log is the record.
		h.log.Error(
			"failed to write event body",
			"webhook_id", webhook.ID,
			"event_id", eventID,
			"error", err,
		)
	}
}

// eventBody returns an event's stored body and whether the event
// exists within the webhook.
func eventBody(
	webhookDB *gorm.DB,
	webhookID, eventID string,
) ([]byte, bool, error) {
	var body []byte

	err := webhookDB.Raw(
		eventBodyQuery, eventID, webhookID,
	).Row().Scan(&body)

	if errors.Is(err, sql.ErrNoRows) {
		return nil, false, nil
	}

	if err != nil {
		return nil, false, err
	}

	return body, true, nil
}

// setEventBodyHeaders applies the response headers that make
// this route safe to hand attacker-supplied bytes through. See
// HandleEventBodyDownload for why they are a security control
// and not a formatting choice.
//
// nosniff is also set by the global SecurityHeaders middleware.
// It is repeated here so the guarantee belongs to the route
// that needs it rather than to a middleware someone could
// reorder or scope away.
func setEventBodyHeaders(
	w http.ResponseWriter,
	eventID string,
	size int64,
) {
	w.Header().Set("Content-Type", "application/octet-stream")
	w.Header().Set("X-Content-Type-Options", "nosniff")
	w.Header().Set(
		"Content-Disposition",
		`attachment; filename="webhooker-event-`+eventID+`.bin"`,
	)
	w.Header().Set("Content-Length", strconv.FormatInt(size, 10))
}