Rename the program to sww and its deciding daemon to discriminator (closes #218) (#221)

internal/sww/healthcheck.go

@@ -0,0 +1,145 @@
+package sww
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"net/http"
+	"net/url"
+	"time"
+
+	"sneak.berlin/go/smallwebwaf/internal/config"
+	"sneak.berlin/go/smallwebwaf/internal/discriminator"
+)
+
+// healthCheckTimeout bounds the whole health check.
+const healthCheckTimeout = 5 * time.Second
+
+var errHealthEndpoint = errors.New("the health endpoint answered")
+
+// HealthCheck is the container's health check. In discriminator's container, where
+// SWW_COMMAND is discriminator, it returns 0 while discriminator answers its health
+// endpoint on 127.0.0.1, at the port in SWW_LISTEN_ADDR. In rproxy's container,
+// where SWW_COMMAND is rproxy, unset or empty, it returns 0 while rproxy
+// answers its health endpoint on 127.0.0.1, at the port in SWRP_LISTEN_ADDR, and
+// the app accepts connections at the address in SWRP_UPSTREAM_URL. Otherwise,
+// any other value of SWW_COMMAND included, it writes why to stderr and
+// returns 1. It reads no other setting, nor a file that another names, so neither
+// can fail it.
+// args are the arguments after `healthcheck`; it takes none, and given
+// one it names it on stderr and returns 1 without checking anything.
+func HealthCheck(
+	ctx context.Context, args []string, lookupEnv func(string) (string, bool),
+	stderr io.Writer,
+) int {
+	if len(args) > 0 {
+		_, _ = fmt.Fprintf(stderr,
+			"sww healthcheck: unexpected argument %q\n", args[0])
+
+		return 1
+	}
+
+	ctx, cancel := context.WithTimeout(ctx, healthCheckTimeout)
+	defer cancel()
+
+	command, err := config.Command(lookupEnv)
+
+	switch {
+	case err != nil:
+		err = fmt.Errorf("invalid setting: %w", err)
+	case command == "discriminator":
+		err = checkDiscriminator(ctx, lookupEnv)
+	default:
+		err = checkRProxyAndApp(ctx, lookupEnv)
+	}
+
+	if err != nil {
+		_, _ = fmt.Fprintln(stderr, "unhealthy:", err)
+
+		return 1
+	}
+
+	return 0
+}
+
+// checkDiscriminator asks discriminator's health endpoint.
+func checkDiscriminator(
+	ctx context.Context, lookupEnv func(string) (string, bool),
+) error {
+	listenAddr, err := config.DiscriminatorListenAddr(lookupEnv)
+	if err != nil {
+		return fmt.Errorf("invalid setting: %w", err)
+	}
+
+	err = askHealthEndpoint(ctx, listenAddr)
+	if err != nil {
+		return fmt.Errorf("ask discriminator: %w", err)
+	}
+
+	return nil
+}
+
+// checkRProxyAndApp asks rproxy's health endpoint, and connects to the app.
+func checkRProxyAndApp(
+	ctx context.Context, lookupEnv func(string) (string, bool),
+) error {
+	listenAddr, upstreamURL, err := config.RProxyListenAddrAndUpstreamURL(lookupEnv)
+	if err != nil {
+		return fmt.Errorf("invalid setting: %w", err)
+	}
+
+	err = askHealthEndpoint(ctx, listenAddr)
+	if err != nil {
+		return fmt.Errorf("ask rproxy: %w", err)
+	}
+
+	conn, err := (&net.Dialer{}).DialContext(ctx, "tcp", appAddress(upstreamURL))
+	if err != nil {
+		return fmt.Errorf("connect to the app: %w", err)
+	}
+
+	_ = conn.Close()
+
+	return nil
+}
+
+// askHealthEndpoint asks the health endpoint on 127.0.0.1 at the port of
+// listenAddr, which the settings have checked has one.
+func askHealthEndpoint(ctx context.Context, listenAddr string) error {
+	_, port, _ := net.SplitHostPort(listenAddr)
+	health := "http://" + net.JoinHostPort("127.0.0.1", port) + discriminator.HealthPath
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, health, http.NoBody)
+	if err != nil {
+		return fmt.Errorf("make the request: %w", err)
+	}
+
+	res, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return fmt.Errorf("send the request: %w", err)
+	}
+
+	_ = res.Body.Close()
+
+	if res.StatusCode != http.StatusOK {
+		return fmt.Errorf("%w %s", errHealthEndpoint, res.Status)
+	}
+
+	return nil
+}
+
+// appAddress is the host and port of the app's URL, the port being the
+// scheme's own when the URL names none.
+func appAddress(app *url.URL) string {
+	port := app.Port()
+	if port == "" {
+		port = "80"
+		if app.Scheme == "https" {
+			port = "443"
+		}
+	}
+
+	return net.JoinHostPort(app.Hostname(), port)
+}