Fail loudly on an unparseable SENTRY_DSN and a malformed .env (closes #283)

Two configuration paths still failed silently, against the rule every
other variable follows: a value that is set but cannot be parsed must
abort startup rather than substitute a default.

SENTRY_DSN is now parsed in loadFromEnv, with sentry.NewDsn — the same
call sentry.Init makes on the DSN it is handed, so configuration and
initialisation cannot disagree about what a valid DSN is. That costs
internal/config an import of the Sentry SDK, which is already a module
dependency already linked into the binary, and buys a single definition
of validity rather than a hand-rolled second one free to drift. A typo
in a DSN used to log one error line and leave the process serving with
error reporting off forever, which nothing downstream can notice: the
variable is still set, so every later signal reports it as on.

hasSentryDSN is replaced by Config.SentryEnabled(), following
MetricsAuthEnabled(): one method read by the startup log field, by the
SDK initialisation and by the sentryhttp middleware, so the log cannot
report reporting as on while nothing is sending. enableSentry's error
branch is now fatal, and Run gives up before it listens rather than
binding a port it is about to release. Fatal there means what a listen
failure already meant — Shutdowner.Shutdown(fx.ExitCode(1)), through
fx's normal stop sequence — so shutdownOnListenFailure is now
shutdownWithFailure and ListenFailureExitCode is
StartupFailureExitCode.

The godotenv/autoload blank import is replaced by config.LoadDotEnv,
called at the top of dispatch. autoload discarded Load's error, and
godotenv applies nothing at all when a file will not parse, so one
mistyped line reverted every variable in the file to its default and
started the server with no log line naming the file. A missing file
stays fine — it is optional and most deployments have none. The call
sits in dispatch rather than in loadFromEnv because autoload ran in an
init(), ahead of config.DataDir(), which both the DATA_DIR lock and
resetpw call outside the fx graph; loading any later would let a .env
that sets DATA_DIR lock one directory while the config opened
databases in another.

Both defects were reproduced against the previous build first: an
unparseable DSN served traffic while logging "hasSentryDSN":true, and
a malformed .env started on the default port with the file unmentioned.

internal/config/config.go

@@ -4,6 +4,7 @@ package config
 import (
 	"errors"
 	"fmt"
+	"io/fs"
 	"log/slog"
 	"net/netip"
 	"os"
@@ -11,13 +12,11 @@ import (
 	"strings"
 	"time"
 
+	"github.com/getsentry/sentry-go"
+	"github.com/joho/godotenv"
 	"go.uber.org/fx"
 	"sneak.berlin/go/webhooker/internal/globals"
 	"sneak.berlin/go/webhooker/internal/logger"
-
-	// Populates the environment from a ./.env file automatically for
-	// development configuration. Kept in one place only (here).
-	_ "github.com/joho/godotenv/autoload"
 )
 
 const (
@@ -84,6 +83,12 @@ const (
 	// IPv6 prefix spends on the ::ffff:0:0/96 wrapper, so a /104
 	// covers the same addresses as an IPv4 /8.
 	mappedV4Offset = 96
+
+	// DotEnvPath is the optional file of KEY=value lines read into the
+	// environment at startup, relative to the process working
+	// directory. Exported so that documentation and tests name the
+	// same path the loader opens.
+	DotEnvPath = ".env"
 )
 
 // ErrInvalidEnvironment is returned when WEBHOOKER_ENVIRONMENT
@@ -107,6 +112,15 @@ var ErrInvalidCIDR = errors.New("invalid CIDR")
 // something that is not an IP address literal.
 var ErrInvalidBindAddress = errors.New("invalid bind address")
 
+// ErrInvalidSentryDSN is returned when SENTRY_DSN is set to something
+// the Sentry SDK cannot parse as a DSN.
+var ErrInvalidSentryDSN = errors.New("invalid Sentry DSN")
+
+// ErrDotEnvUnreadable is returned when the optional .env file exists
+// but cannot be read or parsed. A file that is not there is not an
+// error; a file that is there and broken is.
+var ErrDotEnvUnreadable = errors.New("unreadable .env file")
+
 // ErrIncompleteMetricsAuth is returned when exactly one of
 // METRICS_USERNAME and METRICS_PASSWORD carries a value. Neither
 // fallback is acceptable: serving /metrics on the username alone
@@ -212,12 +226,62 @@ func (c *Config) MetricsAuthEnabled() bool {
 	return c.MetricsUsername != "" && c.MetricsPassword != ""
 }
 
+// SentryEnabled reports whether error reporting is shipped to Sentry.
+// It is the only answer to that question in the codebase: the SDK
+// initialisation, the sentryhttp middleware registration and the
+// startup log's sentryEnabled field all read this one method, so the
+// log cannot report reporting as on while nothing is sending.
+//
+// A non-empty DSN is enough because loadFromEnv already parsed it with
+// the SDK's own parser and refused to build a Config around one the
+// SDK would reject, and because initialising the SDK with a DSN that
+// parsed and failed anyway aborts the process rather than leaving this
+// true and the client absent.
+func (c *Config) SentryEnabled() bool {
+	return c.SentryDSN != ""
+}
+
 // envString returns the value of the named environment variable,
 // or an empty string if not set.
 func envString(key string) string {
 	return os.Getenv(key)
 }
 
+// LoadDotEnv reads DotEnvPath into the environment when that file is
+// present, and reports a file that is present but broken.
+//
+// It has to run before anything reads the environment, so that every
+// reader agrees on what the environment holds — the DATA_DIR lock
+// taken before the fx graph exists as much as loadFromEnv itself. A
+// variable already set in the real environment wins: godotenv never
+// overwrites one.
+//
+// A missing file is not an error. It is a development convenience and
+// most deployments set the environment directly.
+//
+// Any other failure is. godotenv parses the whole file before setting
+// anything, so a single malformed line applies none of it: every
+// variable in the file silently reverts to its default, which defeats
+// the fail-loud guarantee for all of them at once.
+func LoadDotEnv() error {
+	return loadDotEnvFile(DotEnvPath)
+}
+
+// loadDotEnvFile is LoadDotEnv over a named file, so tests can point
+// at a temporary one instead of the process working directory.
+func loadDotEnvFile(path string) error {
+	err := godotenv.Load(path)
+	if err == nil || errors.Is(err, fs.ErrNotExist) {
+		return nil
+	}
+
+	return fmt.Errorf(
+		"%w: %s: %w; nothing in it was applied, so fix the file or "+
+			"remove it",
+		ErrDotEnvUnreadable, path, err,
+	)
+}
+
 // DataDir resolves DATA_DIR, applying DefaultDataDir when it is unset
 // or empty. It is exported so that entry points which must act on the
 // data directory before the fx graph exists — taking the exclusive
@@ -462,6 +526,41 @@ func envBindAddress(key, defaultValue string) (string, error) {
 	return addr.String(), nil
 }
 
+// envSentryDSN returns the value of the named environment variable
+// checked as a Sentry DSN. An unset (or empty, or whitespace-only)
+// value yields "", which means error reporting stays off — the common
+// case, and a normal start.
+//
+// A set value is parsed with sentry.NewDsn, which is the call
+// sentry.Init makes on the DSN it is handed, so what passes here is
+// exactly what the SDK will accept later and the two cannot disagree.
+// Reproducing the check by hand instead would cost this package its
+// dependency on the SDK — already a module dependency, already linked
+// into the binary — in exchange for a second definition of "valid DSN"
+// free to drift from the one that decides.
+//
+// A set value that does not parse is a hard error naming the key, so
+// startup fails loudly. Losing error reporting is the failure this
+// variable exists to prevent, and a typo in a DSN is silent forever:
+// nothing later in the process can notice that reports are going
+// nowhere. The bad value is quoted because it is a URL to a public
+// endpoint carrying a public key, not a secret.
+func envSentryDSN(key string) (string, error) {
+	v := strings.TrimSpace(os.Getenv(key))
+	if v == "" {
+		return "", nil
+	}
+
+	_, err := sentry.NewDsn(v)
+	if err != nil {
+		return "", fmt.Errorf(
+			"%w: %s: %q: %w", ErrInvalidSentryDSN, key, v, err,
+		)
+	}
+
+	return v, nil
+}
+
 // resolveMetricsAuth reads the /metrics basic-auth credentials and
 // rejects a half-set pair, naming both variables either way. The
 // error carries neither value: the password is a secret.
@@ -594,6 +693,11 @@ func loadFromEnv() (*Config, error) {
 		return nil, err
 	}
 
+	sentryDSN, err := envSentryDSN("SENTRY_DSN")
+	if err != nil {
+		return nil, err
+	}
+
 	return &Config{
 		DataDir:                DataDir(),
 		Debug:                  debug,
@@ -603,7 +707,7 @@ func loadFromEnv() (*Config, error) {
 		MetricsPassword:        metricsPassword,
 		Port:                   port,
 		BindAddress:            bindAddress,
-		SentryDSN:              envString("SENTRY_DSN"),
+		SentryDSN:              sentryDSN,
 		RetentionSweepInterval: retentionSweepInterval,
 		SessionIdleTimeout:     sessionIdleTimeout,
 		ReceiverRateLimit:      receiverRateLimit,
@@ -738,7 +842,7 @@ func New(lc fx.Lifecycle, params ConfigParams) (*Config, error) {
 		"receiverRateLimit", s.ReceiverRateLimit,
 		"trustedProxies", len(s.TrustedProxies),
 		"allowedEgressCIDRs", len(s.AllowedEgressCIDRs),
-		"hasSentryDSN", s.SentryDSN != "",
+		"sentryEnabled", s.SentryEnabled(),
 		"hasMetricsAuth", s.MetricsAuthEnabled(),
 	)