mg4/server/api.go

package main

import (
	"crypto/subtle"
	"encoding/json"
	"errors"
	"io"
	"log/slog"
	"net/http"
	"strings"
	"time"
)

// API serves the car. Every endpoint but /v1/health needs the ingest token.
//
//	POST /v1/telemetry                           live samples, stored
//	PUT  /v1/consumption/daily                   one delta chunk of a day's consumption
//	PUT  /v1/charging-sessions/{sessionId}       one finished DC charging session
//	GET  /v1/consumption/history/{installationId} the car reads its own totals back
//	GET  /v1/health                              unauthenticated liveness probe
//
// A car authenticates with the shared INGEST_TOKEN or with its own token from pairing
// (pairing_http.go).
type API struct {
	Store       Store
	IngestToken string
	Pairing     *Pairing
	// PublicURL is the server's address as a phone reaches it, for the pairing QR code;
	// derived from the request when empty.
	PublicURL string
}

const maxBodyBytes = 512 * 1024

func (a *API) Routes() http.Handler {
	mux := http.NewServeMux()
	mux.HandleFunc("GET /v1/health", func(w http.ResponseWriter, r *http.Request) {
		writeJSON(w, http.StatusOK, map[string]string{"status": "ok"})
	})
	mux.HandleFunc("POST /v1/telemetry", a.ingest(a.telemetry))
	mux.HandleFunc("PUT /v1/consumption/daily", a.ingest(a.consumption))
	mux.HandleFunc("PUT /v1/charging-sessions/{sessionId}", a.ingest(a.chargingSession))
	mux.HandleFunc("GET /v1/consumption/history/{installationId}", a.ingest(a.history))
	a.pairingRoutes(mux)
	return mux
}

// caller is who sent an ingest request. A paired car's token is bound to the installation
// id it was paired with; the shared INGEST_TOKEN may write and read for any.
type caller struct {
	installationID string // empty for the shared token
}

// mayUse reports whether the caller may write or read data stored under id.
func (c caller) mayUse(id string) bool {
	return c.installationID == "" || strings.EqualFold(c.installationID, id)
}

// errOtherInstallation is a 403: a paired car's token used for another car's data.
var errOtherInstallation = errors.New("this token belongs to another installation; pair again after changing the installation id")

// authenticate accepts the shared token (compared in constant time; main refuses to start
// with one under 24 characters, so an empty or short one never matches) or a paired car's
// own, unrevoked token.
func (a *API) authenticate(r *http.Request) (caller, bool) {
	header := r.Header.Get("Authorization")
	supplied, ok := strings.CutPrefix(header, "Bearer ")
	if !ok || supplied == "" {
		return caller{}, false
	}
	if len(a.IngestToken) >= minIngestTokenLength &&
		subtle.ConstantTimeCompare([]byte(supplied), []byte(a.IngestToken)) == 1 {
		return caller{}, true
	}
	d, ok := a.Pairing.Authorized(supplied)
	if !ok {
		return caller{}, false
	}
	return caller{installationID: d.InstallationID}, true
}

type handler func(w http.ResponseWriter, r *http.Request, c caller, body []byte) error

// ingest wraps a handler with the token check, the body limit and error mapping: a
// validation failure is a 400 naming the field, another car's data a 403; anything else
// is a 500 whose detail stays in the server log.
func (a *API) ingest(h handler) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		c, ok := a.authenticate(r)
		if !ok {
			writeJSON(w, http.StatusUnauthorized, map[string]string{"error": "unauthorized"})
			return
		}
		body, err := io.ReadAll(io.LimitReader(r.Body, maxBodyBytes+1))
		if err != nil {
			writeJSON(w, http.StatusBadRequest, map[string]string{"error": "body could not be read"})
			return
		}
		if len(body) > maxBodyBytes {
			writeJSON(w, http.StatusBadRequest, map[string]string{"error": "body is too large"})
			return
		}
		if err := h(w, r, c, body); err != nil {
			if errors.Is(err, errOtherInstallation) {
				writeJSON(w, http.StatusForbidden, map[string]string{"error": err.Error()})
				return
			}
			var verr *ValidationError
			if errors.As(err, &verr) {
				writeJSON(w, http.StatusBadRequest, map[string]string{"error": verr.Error()})
				return
			}
			slog.Error("request failed", "method", r.Method, "path", r.URL.Path, "error", err)
			writeJSON(w, http.StatusInternalServerError, map[string]string{"error": "internal server error"})
		}
	}
}

// maxDroppedListed caps the dropped values named in a telemetry response and log line.
const maxDroppedListed = 20

// telemetry stores the batch. The answer says how many samples were stored and what was
// dropped; a batch with nothing usable is still a 200, since sending it again would not help.
func (a *API) telemetry(w http.ResponseWriter, r *http.Request, c caller, body []byte) error {
	batch, err := validateTelemetryBatch(body, time.Now())
	if err != nil {
		return err
	}
	if !c.mayUse(batch.InstallationID) {
		return errOtherInstallation
	}
	if len(batch.Samples) > 0 {
		if err := a.Store.WriteTelemetry(r.Context(), batch.InstallationID, batch.Samples); err != nil {
			return err
		}
	}
	dropped := batch.Dropped
	if len(dropped) > maxDroppedListed {
		dropped = dropped[:maxDroppedListed]
	}
	if len(batch.Dropped) > 0 {
		slog.Warn("telemetry values dropped", "installation", batch.InstallationID,
			"count", len(batch.Dropped), "first", dropped)
	}
	writeJSON(w, http.StatusOK, map[string]any{
		"stored":       len(batch.Samples),
		"droppedCount": len(batch.Dropped),
		"dropped":      append([]string{}, dropped...),
	})
	return nil
}

// consumption stores one delta chunk at its recordedAt. Reads add a day's chunks up, so a
// chunk the car re-sends from its offline cache replaces itself instead of counting twice.
func (a *API) consumption(w http.ResponseWriter, r *http.Request, c caller, body []byte) error {
	chunk, err := validateConsumption(body)
	if err != nil {
		return err
	}
	if !c.mayUse(chunk.InstallationID) {
		return errOtherInstallation
	}
	if err := a.Store.WriteConsumptionChunk(r.Context(), chunk); err != nil {
		return err
	}
	w.WriteHeader(http.StatusNoContent)
	return nil
}

func (a *API) chargingSession(w http.ResponseWriter, r *http.Request, c caller, body []byte) error {
	session, err := validateChargingSession(body, r.PathValue("sessionId"))
	if err != nil {
		return err
	}
	if !c.mayUse(session.InstallationID) {
		return errOtherInstallation
	}
	if err := a.Store.WriteChargingSession(r.Context(), session); err != nil {
		return err
	}
	w.WriteHeader(http.StatusNoContent)
	return nil
}

// history is the car's read-back of its own uploaded totals, so an app reinstall does not
// blank its Consumption tab. Up to 400 days, newest first, plus the lifetime sum of all.
func (a *API) history(w http.ResponseWriter, r *http.Request, c caller, _ []byte) error {
	id, err := validUUID(r.PathValue("installationId"), "installationId")
	if err != nil {
		return err
	}
	if !c.mayUse(id) {
		return errOtherInstallation
	}
	chunks, err := a.Store.ReadConsumptionChunks(r.Context(), id)
	if err != nil {
		return err
	}
	days := aggregateDays(chunks)
	var lifetime Totals
	if len(days) > 0 {
		lifetime = days[0].Lifetime
	}
	if len(days) > 400 {
		days = days[:400]
	}
	writeJSON(w, http.StatusOK, struct {
		Records  []DailyConsumption `json:"records"`
		Lifetime Totals             `json:"lifetime"`
	}{Records: days, Lifetime: lifetime})
	return nil
}

func writeJSON(w http.ResponseWriter, status int, value any) {
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(status)
	if err := json.NewEncoder(w).Encode(value); err != nil {
		slog.Warn("response not written", "error", err)
	}
}