webhooker/internal/handlers/target_download.go

package handlers

import (
	"context"
	"errors"
	"net/http"
	"time"

	"sneak.berlin/go/webhooker/internal/archive"
	"sneak.berlin/go/webhooker/internal/database"
)

// downloadWriteTimeout is how long one write of a download may wait
// for a client that has stopped reading.
const downloadWriteTimeout = 60 * time.Second

// HandleTargetDownload serves a database target's archive as one
// gzipped JSON file, named for the webhook, the target and the time;
// see archive.Export.WriteGzipJSON for what it holds. Other
// target types have no archive and are a 404.
//
// A download runs for as long as the client keeps reading: it reads
// under a context the request limit does not cancel, and gives each
// write its own deadline in place of the server's write timeout. It
// stops when a write fails.
func (h *Handlers) HandleTargetDownload() http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		ctx := context.WithoutCancel(r.Context())

		webhook, target, export, ok := h.listTargetArchive(w, r)
		if !ok {
			return
		}

		now := time.Now()

		w.Header().Set("Content-Type", "application/gzip")
		w.Header().Set(
			"Content-Disposition",
			`attachment; filename="`+archive.ExportFileName(
				webhook.Name, target.Name, now,
			)+`"`,
		)

		err := export.WriteGzipJSON(
			ctx,
			downloadWriter{w: w, rc: http.NewResponseController(w)},
			&webhook, target, now,
		)
		if err != nil {
			h.log.Error(
				"failed to export archive",
				"target_id", target.ID,
				"error", err,
			)

			// The 200 has gone out. Aborting the connection is what
			// tells the client the file is incomplete.
			panic(http.ErrAbortHandler)
		}
	}
}

// downloadWriter writes a download to the client, giving each write
// downloadWriteTimeout to finish.
type downloadWriter struct {
	w  http.ResponseWriter
	rc *http.ResponseController
}

func (d downloadWriter) Write(b []byte) (int, error) {
	// A writer that has no write deadline, such as a test's recorder,
	// answers http.ErrNotSupported and needs none extended.
	err := d.rc.SetWriteDeadline(time.Now().Add(downloadWriteTimeout))
	if err != nil && !errors.Is(err, http.ErrNotSupported) {
		return 0, err
	}

	return d.w.Write(b)
}

// listTargetArchive lists the archive files of the request's database
// target for export. It reports false once it has written the response.
//
// It holds the rename lock, which every archive rename runs under,
// while it reads the stored names and lists the files, so the files it
// lists are the ones those names give. It lets go before the export is
// streamed, which takes the lock again for each file only while it
// finds the file under the names stored then and opens it.
func (h *Handlers) listTargetArchive(
	w http.ResponseWriter,
	r *http.Request,
) (database.Webhook, *database.Target, *archive.Export, bool) {
	h.archives.RenameLock().Lock()
	defer h.archives.RenameLock().Unlock()

	webhook, target, ok := h.ownedTarget(w, r)
	if !ok {
		return database.Webhook{}, nil, nil, false
	}

	if target.Type != database.TargetTypeDatabase {
		h.renderError(w, r, http.StatusNotFound)

		return database.Webhook{}, nil, nil, false
	}

	export, err := archive.NewExport(
		archive.Path(h.dbMgr, &webhook, target),
		h.archives.RenameLock(),
		func() (string, error) {
			return h.storedArchivePath(webhook.ID, target.ID)
		},
		h.log,
	)
	if err != nil {
		h.serverError(w, r, "failed to list archive for export", err)

		return database.Webhook{}, nil, nil, false
	}

	return webhook, target, export, true
}

// storedArchivePath returns the path archive.Path gives a
// database target under the names stored for it and its webhook now.
// Its caller holds the rename lock. A webhook or target deleted since is still
// found, since deleting one leaves its archive files under their names.
func (h *Handlers) storedArchivePath(
	webhookID, targetID string,
) (string, error) {
	var webhook database.Webhook

	err := h.db.DB().Unscoped().First(&webhook, "id = ?", webhookID).Error
	if err != nil {
		return "", err
	}

	var target database.Target

	err = h.db.DB().Unscoped().First(&target, "id = ?", targetID).Error
	if err != nil {
		return "", err
	}

	return archive.Path(h.dbMgr, &webhook, &target), nil
}