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.
2026-08-24 02:14:15 +00:00
parent 48cf93ec7e
commit b235a5d48e
13 changed files with 815 additions and 63 deletions
--- a/README.md
+++ b/README.md
@@ -71,8 +71,18 @@ make clean      # Remove bin/
 ### Configuration

 All configuration is via environment variables. For local development,
-you can place variables in a `.env` file in the project root (loaded
-automatically via `godotenv/autoload`).
+you can place variables in a `.env` file in the process working
+directory, read once at startup before anything else looks at the
+environment.
+
+The file is optional and having none is the normal case for a
+deployment. A file that is there but cannot be parsed aborts startup
+with a message naming it, because a single malformed line makes none
+of the file apply: every variable in it silently reverts to its
+default, which is exactly the failure [Invalid values abort
+startup](#invalid-values-abort-startup) exists to prevent, for all of
+them at once. A variable already present in the real environment wins
+over the file's value for the same name.

 The environment is selected by setting `WEBHOOKER_ENVIRONMENT` to `dev`
 or `prod` (default: `dev`). The setting controls exactly one behavior:
@@ -125,7 +135,7 @@ TTY detection, and security headers are always applied.
 | `MAINTENANCE_MODE`      | Report `maintenanceMode: true` in the healthcheck JSON. It does not change how any request is served — no maintenance page exists | `false`  |
 | `METRICS_USERNAME`      | Basic auth username for `/metrics`. Must be set together with `METRICS_PASSWORD`; one without the other fails startup | `""`     |
 | `METRICS_PASSWORD`      | Basic auth password for `/metrics`. Must be set together with `METRICS_USERNAME`; one without the other fails startup | `""`     |
-| `SENTRY_DSN`            | Sentry error reporting DSN          | `""`     |
+| `SENTRY_DSN`            | Sentry error reporting DSN. Unset leaves error reporting off; a value the Sentry SDK cannot parse fails startup rather than serving with reporting silently off | `""`     |
 | `RETENTION_SWEEP_INTERVAL` | How often the retention reaper and archive sweeper run (Go duration, must be positive) | `1h` |
 | `SESSION_IDLE_TIMEOUT`  | Idle session timeout (Go duration)  | `24h`    |
 | `RECEIVER_RATE_LIMIT`   | Receiver requests/minute per IP per entrypoint (10x that per IP across the route) | `120` |
@@ -464,10 +474,20 @@ startup), every entry in `TRUSTED_PROXIES` and
 `ALLOWED_EGRESS_CIDRS` must be a CIDR block or a bare IP address, and
 `BIND_ADDRESS` must be an IP address literal — `localhost`,
 `127.0.0.1:8080` and `10.0.0.0/8` are each rejected rather than
-resolved, split, or narrowed to something they do not say.
+resolved, split, or narrowed to something they do not say — and
+`SENTRY_DSN` must parse as a Sentry DSN.
 `SESSION_IDLE_TIMEOUT` is the exception: a
 non-positive value there means idle expiry is disabled, not invalid.

+`SENTRY_DSN` is checked with the Sentry SDK's own parser, the same call
+the SDK makes on the DSN it is later handed, so what configuration
+accepts is exactly what will initialise. A typo in it is the one
+configuration mistake nothing downstream can ever notice — the variable
+is still set, so every later signal reports error reporting as on while
+no report is being sent — which is why it aborts rather than starting
+with reporting off. Leaving it unset is not a mistake and not affected:
+error reporting is simply off and startup is normal.
+
 Boolean variables (`DEBUG`, `MAINTENANCE_MODE`) accept exactly the
 spellings Go's `strconv.ParseBool` accepts — `1`, `t`, `T`, `TRUE`,
 `true`, `True`, `0`, `f`, `F`, `FALSE`, `false`, `False` — and nothing
--- a/cmd/webhooker/dotenv_test.go
+++ b/cmd/webhooker/dotenv_test.go
@@ -0,0 +1,107 @@
+package main
+
+import (
+	"bytes"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/webhooker/internal/config"
+)
+
+// dotEnvKey is a throwaway variable name these tests write and read,
+// so they cannot disturb real configuration.
+const dotEnvKey = "WEBHOOKER_TEST_DISPATCH_VALUE"
+
+// writeDotEnvInWorkingDir puts contents in a .env file in a fresh
+// temporary directory and moves the process there.
+//
+// The callers are deliberately not parallel and must stay that way:
+// t.Chdir moves the whole process. Go releases parallel tests only
+// after every sequential test in the package has finished, so nothing
+// else runs while these do.
+func writeDotEnvInWorkingDir(t *testing.T, contents string) {
+	t.Helper()
+
+	dir := t.TempDir()
+	require.NoError(t, os.WriteFile(
+		filepath.Join(dir, config.DotEnvPath),
+		[]byte(contents), 0o600,
+	))
+	t.Chdir(dir)
+}
+
+// TestDispatch_MalformedDotEnvRefuses pins the second half of the
+// defect. godotenv applies nothing at all when a file will not parse,
+// so one mistyped line used to revert every variable in it to its
+// default and start the server anyway, with no log line naming the
+// file. The refusal has to arrive before any subcommand runs, which
+// is why `help` — the one subcommand that touches nothing — is still
+// refused here.
+//
+//nolint:paralleltest // t.Chdir moves the whole process.
+func TestDispatch_MalformedDotEnvRefuses(t *testing.T) {
+	writeDotEnvInWorkingDir(t, "PORT 19615\n")
+
+	var stdout, stderr bytes.Buffer
+
+	code := dispatch(
+		[]string{helpCommand}, strings.NewReader(""), &stdout, &stderr,
+	)
+
+	require.Equal(t, 1, code, "a broken .env must exit non-zero")
+	assert.Contains(
+		t, stderr.String(), config.DotEnvPath,
+		"the refusal must name the file",
+	)
+	assert.Empty(
+		t, stdout.String(),
+		"the subcommand must not have run",
+	)
+}
+
+// TestDispatch_LoadsDotEnvBeforeSubcommands pins the ordering the
+// godotenv/autoload import used to provide for free. It ran in an
+// init(), so .env was in the environment before anything read it —
+// including 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.
+func TestDispatch_LoadsDotEnvBeforeSubcommands(t *testing.T) {
+	t.Setenv(dotEnvKey, "placeholder")
+	require.NoError(t, os.Unsetenv(dotEnvKey))
+
+	writeDotEnvInWorkingDir(t, dotEnvKey+"=from-dot-env\n")
+
+	var stdout, stderr bytes.Buffer
+
+	code := dispatch(
+		[]string{helpCommand}, strings.NewReader(""), &stdout, &stderr,
+	)
+
+	require.Equal(t, 0, code)
+	assert.Equal(
+		t, "from-dot-env", os.Getenv(dotEnvKey),
+		"the file must be applied before the subcommand runs",
+	)
+}
+
+// TestDispatch_MissingDotEnvIsFine pins the case most deployments are
+// in: no .env at all, which must stay a normal start.
+//
+//nolint:paralleltest // t.Chdir moves the whole process.
+func TestDispatch_MissingDotEnvIsFine(t *testing.T) {
+	t.Chdir(t.TempDir())
+
+	var stdout, stderr bytes.Buffer
+
+	code := dispatch(
+		[]string{helpCommand}, strings.NewReader(""), &stdout, &stderr,
+	)
+
+	require.Equal(t, 0, code)
+	assert.Empty(t, stderr.String())
+}
--- a/cmd/webhooker/main.go
+++ b/cmd/webhooker/main.go
@@ -54,6 +54,11 @@ const stopTimeout = 5 * time.Second
 // caller can tell "called wrong" from "declined".
 const exitUsage = 2

+// helpCommand is the subcommand that prints usage. The flag spellings
+// beside it in the switch are aliases; this is the name the usage text
+// documents and the one tests invoke.
+const helpCommand = "help"
+
 // Build-time variables set via -ldflags.
 //
 //nolint:gochecknoglobals // Build-time variables injected by the linker.
@@ -75,11 +80,27 @@ func main() {
 // every existing deployment invoke; that path is unchanged, including
 // where the DATA_DIR lock is taken relative to building the fx graph
 // and how fx propagates a non-zero exit itself.
+//
+// The optional .env file is read here, before any subcommand and so
+// before anything reads the environment — config.DataDir, which both
+// the DATA_DIR lock and resetpw call outside the fx graph, above all.
+// It used to be read from an init() in internal/config, which put it
+// earlier still but threw the error away: a single malformed line
+// applied none of the file and said nothing about it. A file that is
+// not there stays fine, since .env is optional and most deployments
+// do not have one.
 func dispatch(
 	args []string,
 	stdin io.Reader,
 	stdout, stderr io.Writer,
 ) int {
+	err := config.LoadDotEnv()
+	if err != nil {
+		_, _ = fmt.Fprintf(stderr, "%s: %v\n", appname, err)
+
+		return 1
+	}
+
 	if len(args) == 0 {
 		return run(stderr)
 	}
@@ -87,7 +108,7 @@ func dispatch(
 	switch args[0] {
 	case resetpw.Name:
 		return resetpw.Run(args[1:], stdin, stdout, stderr)
-	case "help", "-h", "-help", "--help":
+	case helpCommand, "-h", "-help", "--help":
 		usage(stdout)

 		return 0
--- a/cmd/webhooker/main_test.go
+++ b/cmd/webhooker/main_test.go
@@ -121,7 +121,7 @@ func TestDispatch_Help(t *testing.T) {
 	var stdout, stderr bytes.Buffer

 	code := dispatch(
-		[]string{"help"}, strings.NewReader(""), &stdout, &stderr,
+		[]string{helpCommand}, strings.NewReader(""), &stdout, &stderr,
 	)

 	require.Equal(t, 0, code)
--- a/internal/config/config.go
+++ b/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(),
 	)

--- a/internal/config/dotenv_test.go
+++ b/internal/config/dotenv_test.go
@@ -0,0 +1,158 @@
+package config_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/webhooker/internal/config"
+)
+
+// dotEnvKey is a throwaway variable name the .env tests write and
+// read, so they cannot disturb real configuration.
+const dotEnvKey = "WEBHOOKER_TEST_DOTENV_VALUE"
+
+// malformedDotEnv is a file godotenv cannot parse. The first line is
+// the realistic typo — a space where the `=` belongs — and the rest
+// make sure nothing downstream treats the file as salvageable line by
+// line.
+const malformedDotEnv = "PORT 19615\n" +
+	"this is not = valid ! syntax\n" +
+	"\"unclosed\n"
+
+// unsetDotEnvKey makes dotEnvKey genuinely absent for the duration of
+// the test and restores it afterwards. t.Setenv registers the restore;
+// the Unsetenv that follows is what the test actually needs, because a
+// variable set to the empty string is still present in os.Environ and
+// godotenv would refuse to overwrite it.
+func unsetDotEnvKey(t *testing.T) {
+	t.Helper()
+	t.Setenv(dotEnvKey, "placeholder")
+	require.NoError(t, os.Unsetenv(dotEnvKey))
+}
+
+// writeDotEnv writes contents to a .env file in a fresh temporary
+// directory and returns its path.
+func writeDotEnv(t *testing.T, contents string) string {
+	t.Helper()
+
+	path := filepath.Join(t.TempDir(), config.DotEnvPath)
+	require.NoError(t, os.WriteFile(path, []byte(contents), 0o600))
+
+	return path
+}
+
+// TestLoadDotEnv_MissingFileIsFine pins the case most deployments are
+// in. The file is optional: it is a development convenience, and a
+// deployment that configures the environment directly must start
+// normally rather than be refused for a file it was never meant to
+// have.
+//
+//nolint:paralleltest // unsetDotEnvKey uses t.Setenv.
+func TestLoadDotEnv_MissingFileIsFine(t *testing.T) {
+	unsetDotEnvKey(t)
+
+	absent := filepath.Join(t.TempDir(), config.DotEnvPath)
+	require.NoError(t, config.LoadDotEnvFileForTest(absent))
+
+	_, present := os.LookupEnv(dotEnvKey)
+	assert.False(t, present, "nothing may be set from an absent file")
+}
+
+// TestLoadDotEnv_AppliesValues pins that a well-formed file still
+// reaches the environment, which is the whole reason the file is read
+// at all.
+//
+//nolint:paralleltest // unsetDotEnvKey uses t.Setenv.
+func TestLoadDotEnv_AppliesValues(t *testing.T) {
+	unsetDotEnvKey(t)
+
+	path := writeDotEnv(t, "# a comment\n"+dotEnvKey+"=from-dot-env\n")
+
+	require.NoError(t, config.LoadDotEnvFileForTest(path))
+	assert.Equal(t, "from-dot-env", os.Getenv(dotEnvKey))
+}
+
+// TestLoadDotEnv_RealEnvironmentWins pins that the file cannot
+// override a variable the process was actually started with. A
+// deployment that sets DATA_DIR in its unit file must not have it
+// silently replaced by a stale .env left in the working directory.
+func TestLoadDotEnv_RealEnvironmentWins(t *testing.T) {
+	t.Setenv(dotEnvKey, "from-environment")
+
+	path := writeDotEnv(t, dotEnvKey+"=from-dot-env\n")
+
+	require.NoError(t, config.LoadDotEnvFileForTest(path))
+	assert.Equal(t, "from-environment", os.Getenv(dotEnvKey))
+}
+
+// TestLoadDotEnv_MalformedFileAborts is the defect this fixes. One bad
+// line makes godotenv apply none of the file, so every variable in it
+// reverts to its default; the process used to start that way with no
+// log line naming the file at all.
+//
+//nolint:paralleltest // unsetDotEnvKey uses t.Setenv.
+func TestLoadDotEnv_MalformedFileAborts(t *testing.T) {
+	unsetDotEnvKey(t)
+
+	path := writeDotEnv(
+		t, malformedDotEnv+dotEnvKey+"=from-dot-env\n",
+	)
+
+	err := config.LoadDotEnvFileForTest(path)
+
+	require.Error(t, err)
+	require.ErrorIs(t, err, config.ErrDotEnvUnreadable)
+	assert.Contains(
+		t, err.Error(), config.DotEnvPath,
+		"the failure must name the file it could not read",
+	)
+
+	_, present := os.LookupEnv(dotEnvKey)
+	assert.False(
+		t, present,
+		"a rejected file must apply nothing, not part of itself",
+	)
+}
+
+// TestLoadDotEnv_UnreadableFileAborts pins that only absence is
+// tolerated. A .env that exists but cannot be read is a file the
+// operator meant to be applied, so it fails like a malformed one
+// rather than being treated as though it were not there.
+func TestLoadDotEnv_UnreadableFileAborts(t *testing.T) {
+	t.Parallel()
+
+	// A directory in the file's place: open succeeds and the read
+	// fails, which no umask or root-ness can turn back into success
+	// the way a chmod could.
+	path := filepath.Join(t.TempDir(), config.DotEnvPath)
+	require.NoError(t, os.Mkdir(path, 0o750))
+
+	err := config.LoadDotEnvFileForTest(path)
+
+	require.Error(t, err)
+	require.ErrorIs(t, err, config.ErrDotEnvUnreadable)
+}
+
+// TestLoadDotEnv_ReadsTheWorkingDirectory pins the path LoadDotEnv
+// itself opens, which the tests above bypass. It is relative to the
+// process working directory, as it was under godotenv/autoload and as
+// the README documents.
+//
+//nolint:paralleltest // t.Chdir moves the whole process.
+func TestLoadDotEnv_ReadsTheWorkingDirectory(t *testing.T) {
+	unsetDotEnvKey(t)
+
+	dir := t.TempDir()
+	require.NoError(t, os.WriteFile(
+		filepath.Join(dir, config.DotEnvPath),
+		[]byte(dotEnvKey+"=from-working-directory\n"),
+		0o600,
+	))
+	t.Chdir(dir)
+
+	require.NoError(t, config.LoadDotEnv())
+	assert.Equal(t, "from-working-directory", os.Getenv(dotEnvKey))
+}
--- a/internal/config/env_test.go
+++ b/internal/config/env_test.go
@@ -518,8 +518,20 @@ type badEnvValueCase struct {
 }

 // badEnvValueCases is the config.New table, kept out of the test body
-// so the test itself stays readable.
+// so the test itself stays readable. It is assembled from per-variable
+// groups because one literal covering every variable outgrew the
+// function-length budget.
 func badEnvValueCases() []badEnvValueCase {
+	cases := listenerEnvValueCases()
+	cases = append(cases, flagEnvValueCases()...)
+	cases = append(cases, sentryEnvValueCases()...)
+
+	return cases
+}
+
+// listenerEnvValueCases covers the two variables that describe the
+// HTTP listener.
+func listenerEnvValueCases() []badEnvValueCase {
 	return []badEnvValueCase{
 		{
 			name:  "valid PORT is used",
@@ -542,27 +554,6 @@ func badEnvValueCases() []badEnvValueCase {
 			value:       "70000",
 			expectError: true,
 		},
-		{
-			name:  "valid DEBUG is used",
-			key:   envKeyDebug,
-			value: "true",
-			check: func(t *testing.T, cfg *config.Config) {
-				t.Helper()
-				assert.True(t, cfg.Debug)
-			},
-		},
-		{
-			name:        "unparseable DEBUG aborts startup",
-			key:         envKeyDebug,
-			value:       "ture",
-			expectError: true,
-		},
-		{
-			name:        "unparseable MAINTENANCE_MODE aborts startup",
-			key:         envKeyMaintenanceMode,
-			value:       "sometimes",
-			expectError: true,
-		},
 		{
 			name:  "valid BIND_ADDRESS is used",
 			key:   envKeyBindAddress,
@@ -595,6 +586,69 @@ func badEnvValueCases() []badEnvValueCase {
 	}
 }

+// flagEnvValueCases covers the boolean variables.
+func flagEnvValueCases() []badEnvValueCase {
+	return []badEnvValueCase{
+		{
+			name:  "valid DEBUG is used",
+			key:   envKeyDebug,
+			value: "true",
+			check: func(t *testing.T, cfg *config.Config) {
+				t.Helper()
+				assert.True(t, cfg.Debug)
+			},
+		},
+		{
+			name:        "unparseable DEBUG aborts startup",
+			key:         envKeyDebug,
+			value:       "ture",
+			expectError: true,
+		},
+		{
+			name:        "unparseable MAINTENANCE_MODE aborts startup",
+			key:         envKeyMaintenanceMode,
+			value:       "sometimes",
+			expectError: true,
+		},
+	}
+}
+
+// sentryEnvValueCases covers SENTRY_DSN. The three rejected values are
+// the ones measured on the defect: each initialised the SDK with an
+// error and left the process serving with error reporting off.
+func sentryEnvValueCases() []badEnvValueCase {
+	return []badEnvValueCase{
+		{
+			name:  "valid SENTRY_DSN is used",
+			key:   envKeySentryDSN,
+			value: validSentryDSN,
+			check: func(t *testing.T, cfg *config.Config) {
+				t.Helper()
+				assert.Equal(t, validSentryDSN, cfg.SentryDSN)
+				assert.True(t, cfg.SentryEnabled())
+			},
+		},
+		{
+			name:        "unparseable SENTRY_DSN aborts startup",
+			key:         envKeySentryDSN,
+			value:       "not-a-dsn",
+			expectError: true,
+		},
+		{
+			name:        "SENTRY_DSN that is not a URL aborts startup",
+			key:         envKeySentryDSN,
+			value:       "%%%",
+			expectError: true,
+		},
+		{
+			name:        "keyless SENTRY_DSN aborts startup",
+			key:         envKeySentryDSN,
+			value:       "https://example.invalid/1",
+			expectError: true,
+		},
+	}
+}
+
 // TestNewUsesDefaultsWhenUnset proves the fail-loud behaviour did not
 // break the legitimate unset case: absent variables still get their
 // documented defaults.
@@ -603,7 +657,7 @@ func TestNewUsesDefaultsWhenUnset(t *testing.T) {

 	for _, key := range []string{
 		envKeyPort, envKeyDebug, envKeyMaintenanceMode,
-		envKeyBindAddress,
+		envKeyBindAddress, envKeySentryDSN,
 	} {
 		require.NoError(t, os.Unsetenv(key))
 	}
@@ -626,4 +680,9 @@ func TestNewUsesDefaultsWhenUnset(t *testing.T) {
 		t, config.DefaultBindAddressForTest, cfg.BindAddress,
 	)
 	assert.Equal(t, bindAddressDefault, cfg.BindAddress)
+
+	// An absent SENTRY_DSN is the common case and must stay a normal
+	// start with error reporting off, not a refusal.
+	assert.Empty(t, cfg.SentryDSN)
+	assert.False(t, cfg.SentryEnabled())
 }
--- a/internal/config/export_test.go
+++ b/internal/config/export_test.go
@@ -51,6 +51,18 @@ func EnvPortForTest(key string, defaultValue int) (int, error) {
 	return envPort(key, defaultValue)
 }

+// EnvSentryDSNForTest exposes envSentryDSN.
+func EnvSentryDSNForTest(key string) (string, error) {
+	return envSentryDSN(key)
+}
+
+// LoadDotEnvFileForTest exposes the loader LoadDotEnv runs, over a
+// caller-named file rather than the process working directory, so
+// each .env state can be covered without moving the test process.
+func LoadDotEnvFileForTest(path string) error {
+	return loadDotEnvFile(path)
+}
+
 // EnvBindAddressForTest exposes envBindAddress.
 func EnvBindAddressForTest(key, defaultValue string) (string, error) {
 	return envBindAddress(key, defaultValue)
--- a/internal/config/sentry_test.go
+++ b/internal/config/sentry_test.go
@@ -0,0 +1,141 @@
+package config_test
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/webhooker/internal/config"
+)
+
+// envKeySentryDSN is the variable envSentryDSN reads in production.
+const envKeySentryDSN = "SENTRY_DSN"
+
+// validSentryDSN is a syntactically complete DSN. The host is under
+// .invalid (RFC 2606), so nothing a test builds around it can reach a
+// real Sentry installation.
+const validSentryDSN = "https://abc123@sentry.invalid/42"
+
+// envSentryDSNCase is one row of the envSentryDSN table.
+type envSentryDSNCase struct {
+	name        string
+	set         bool
+	value       string
+	expectError bool
+	expected    string
+}
+
+// envSentryDSNCases is the envSentryDSN table. The three invalid
+// values are the ones measured on the defect: each initialised the SDK
+// with an error and left the process serving with reporting off.
+func envSentryDSNCases() []envSentryDSNCase {
+	return []envSentryDSNCase{
+		{
+			name:     "unset means reporting off",
+			expected: "",
+		},
+		{
+			name:     "empty means reporting off",
+			set:      true,
+			value:    "",
+			expected: "",
+		},
+		{
+			name:     "whitespace means reporting off",
+			set:      true,
+			value:    "   ",
+			expected: "",
+		},
+		{
+			name:     "a valid DSN is kept",
+			set:      true,
+			value:    validSentryDSN,
+			expected: validSentryDSN,
+		},
+		{
+			name:     "surrounding whitespace is trimmed",
+			set:      true,
+			value:    "  " + validSentryDSN + "\t",
+			expected: validSentryDSN,
+		},
+		{
+			name:        "a value that is not a URL is rejected",
+			set:         true,
+			value:       "not-a-dsn",
+			expectError: true,
+		},
+		{
+			name:        "an unparseable URL is rejected",
+			set:         true,
+			value:       "%%%",
+			expectError: true,
+		},
+		{
+			name:        "a DSN without a public key is rejected",
+			set:         true,
+			value:       "https://example.invalid/1",
+			expectError: true,
+		},
+		{
+			name:        "a DSN without a project id is rejected",
+			set:         true,
+			value:       "https://abc123@sentry.invalid/",
+			expectError: true,
+		},
+		{
+			name:        "a non-HTTP scheme is rejected",
+			set:         true,
+			value:       "ftp://abc123@sentry.invalid/42",
+			expectError: true,
+		},
+	}
+}
+
+// TestEnvSentryDSN covers the helper directly. What it pins beyond the
+// value is the failure shape: a set-but-unparseable DSN names the
+// variable and the value, exactly as the other fail-loud helpers do,
+// so an operator reads the fix off the message.
+func TestEnvSentryDSN(t *testing.T) {
+	for _, tt := range envSentryDSNCases() {
+		t.Run(tt.name, func(t *testing.T) {
+			// Cannot use t.Parallel() here because t.Setenv
+			// is incompatible with parallel subtests.
+			if tt.set {
+				t.Setenv(envKeySentryDSN, tt.value)
+			} else {
+				require.NoError(t, os.Unsetenv(envKeySentryDSN))
+			}
+
+			got, err := config.EnvSentryDSNForTest(envKeySentryDSN)
+
+			if tt.expectError {
+				require.Error(t, err)
+				require.ErrorIs(t, err, config.ErrInvalidSentryDSN)
+				assert.Contains(t, err.Error(), envKeySentryDSN)
+				assert.Contains(t, err.Error(), tt.value)
+				assert.Empty(t, got)
+
+				return
+			}
+
+			require.NoError(t, err)
+			assert.Equal(t, tt.expected, got)
+		})
+	}
+}
+
+// TestSentryEnabled_TracksTheDSN pins that the one method answering
+// "is anything being reported" agrees with the DSN in every state. The
+// startup log, the SDK initialisation and the sentryhttp middleware
+// all read it, so a log field cannot report reporting as on while
+// nothing is sending.
+func TestSentryEnabled_TracksTheDSN(t *testing.T) {
+	t.Parallel()
+
+	assert.False(t, (&config.Config{}).SentryEnabled())
+	assert.True(
+		t,
+		(&config.Config{SentryDSN: validSentryDSN}).SentryEnabled(),
+	)
+}
--- a/internal/server/http.go
+++ b/internal/server/http.go
@@ -70,7 +70,7 @@ func (s *Server) serveUntilShutdown() {
 	err := s.httpServer.ListenAndServe()
 	if err != nil && !errors.Is(err, http.ErrServerClosed) {
 		s.log.Error("listen error", "error", err)
-		s.shutdownOnListenFailure()
+		s.shutdownWithFailure()
 	}
 }

--- a/internal/server/listen_failure_test.go
+++ b/internal/server/listen_failure_test.go
@@ -93,7 +93,7 @@ func requireListenFailureExit(t *testing.T, env *testEnv) {
 	select {
 	case sig := <-app.Wait():
 		require.Equal(
-			t, server.ListenFailureExitCode, sig.ExitCode,
+			t, server.StartupFailureExitCode, sig.ExitCode,
 			"listen failure must exit non-zero",
 		)
 	case <-time.After(listenFailureDeadline):
--- a/internal/server/sentry_failure_test.go
+++ b/internal/server/sentry_failure_test.go
@@ -0,0 +1,99 @@
+package server_test
+
+import (
+	"context"
+	"net"
+	"strconv"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+	"go.uber.org/fx"
+	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/globals"
+	"sneak.berlin/go/webhooker/internal/server"
+)
+
+// TestSentryInitFailure_ShutsDownTheApp pins that error reporting
+// which is configured and cannot be started ends the application
+// instead of serving without it.
+//
+// The measured defect logged `sentry init failure` and kept running,
+// so the deployment served traffic with reporting off while every
+// other signal — SENTRY_DSN still set, the startup summary's own
+// field — said it was on. Nothing later in the process can notice
+// that reports are going nowhere, which is why this exits rather than
+// degrades.
+//
+// The DSN is placed on a hand-built Config, which is the only way to
+// reach this branch at all: loadFromEnv now parses SENTRY_DSN with
+// sentry.NewDsn, the same call sentry.Init makes, so a DSN that
+// survives configuration cannot fail initialisation in the SDK
+// version this pins. The branch stays because that is a property of
+// the SDK's current implementation rather than of its contract.
+func TestSentryInitFailure_ShutsDownTheApp(t *testing.T) {
+	t.Parallel()
+
+	port := freePort(t)
+
+	env := newTestEnvWithConfig(t, &config.Config{
+		DataDir:     t.TempDir(),
+		Environment: config.EnvironmentDev,
+		BindAddress: loopbackV4,
+		Port:        port,
+		SentryDSN:   "not-a-dsn",
+	})
+
+	app := fx.New(
+		fx.NopLogger,
+		fx.Supply(env.log, env.cfg, env.mw, env.hnd),
+		fx.Provide(globals.New, server.New),
+		fx.Invoke(func(*server.Server) {}),
+	)
+
+	startCtx, cancelStart := context.WithTimeout(
+		context.Background(), lifecycleTimeout,
+	)
+	defer cancelStart()
+
+	require.NoError(t, app.Start(startCtx))
+
+	select {
+	case sig := <-app.Wait():
+		require.Equal(
+			t, server.StartupFailureExitCode, sig.ExitCode,
+			"a sentry failure must exit non-zero",
+		)
+	case <-time.After(listenFailureDeadline):
+		t.Fatal("a sentry failure left the app running")
+	}
+
+	// The stop sequence still has to complete: the failure must reach
+	// shutdown through fx rather than around it.
+	stopCtx, cancelStop := context.WithTimeout(
+		context.Background(), lifecycleTimeout,
+	)
+	defer cancelStop()
+
+	require.NoError(t, app.Stop(stopCtx))
+
+	// And it must give up before it listens. A process that bound the
+	// port and then exited would have accepted requests it could not
+	// report on, which is the state under test in miniature.
+	requireBindable(t, port)
+}
+
+// requireBindable asserts that the port is free, which it is only if
+// the server under test never claimed it.
+func requireBindable(t *testing.T, port int) {
+	t.Helper()
+
+	var listenCfg net.ListenConfig
+
+	listener, err := listenCfg.Listen(
+		t.Context(), "tcp",
+		net.JoinHostPort(loopbackV4, strconv.Itoa(port)),
+	)
+	require.NoError(t, err, "the server bound a port it then gave up")
+	require.NoError(t, listener.Close())
+}
--- a/internal/server/server.go
+++ b/internal/server/server.go
@@ -51,12 +51,13 @@ const (
 	minSentryFlush = 250 * time.Millisecond
 )

-// ListenFailureExitCode is the status the process exits with when the
-// HTTP listener cannot be established, or dies for a reason other
-// than a requested shutdown. It must stay non-zero: systemd
-// `Restart=on-failure` and Docker's restart policies key off it, and a
-// zero exit would read as a deliberate stop.
-const ListenFailureExitCode = 1
+// StartupFailureExitCode is the status the process exits with when
+// the serving goroutine gives up: the HTTP listener cannot be
+// established or dies for a reason other than a requested shutdown, or
+// error reporting is configured and cannot be started. It must stay
+// non-zero: systemd `Restart=on-failure` and Docker's restart policies
+// key off it, and a zero exit would read as a deliberate stop.
+const StartupFailureExitCode = 1

 // SentryFlushBudget reports how long the Sentry flush may run when
 // remaining is the time left on the fx stop context after the HTTP
@@ -135,11 +136,25 @@ func New(lc fx.Lifecycle, params ServerParams) (*Server, error) {
 }

 // Run configures Sentry and starts serving HTTP requests.
+//
+// A Sentry failure ends the application instead of listening. It runs
+// before the listener rather than after it so that the process never
+// binds a port it is about to give up.
 func (s *Server) Run() {
 	s.configure()

 	// logging before sentry, because sentry logs
-	s.enableSentry()
+	err := s.enableSentry()
+	if err != nil {
+		s.log.Error(
+			"SENTRY_DSN is set but error reporting could not be "+
+				"started; refusing to serve with it off",
+			"error", err,
+		)
+		s.shutdownWithFailure()
+
+		return
+	}

 	s.serve()
 }
@@ -150,11 +165,23 @@ func (s *Server) MaintenanceMode() bool {
 	return s.params.Config.MaintenanceMode
 }

-func (s *Server) enableSentry() {
+// enableSentry initialises the Sentry SDK when error reporting is
+// configured, and reports the failure when it is configured and cannot
+// be initialised. A DSN that is not set is not a failure: reporting
+// stays off and the server starts normally.
+//
+// There is no fallback to running with reporting off. An operator who
+// set SENTRY_DSN asked for failures to be visible, and serving traffic
+// with reporting quietly off is the one state nothing can ever tell
+// them about — the DSN is still set, so every later signal says it is
+// on. Config already refused a DSN the SDK cannot parse, which is what
+// a typo produces, so reaching this branch means the SDK refused
+// something that parsed: not a condition to guess at either.
+func (s *Server) enableSentry() error {
 	s.sentryEnabled.Store(false)

-	if s.params.Config.SentryDSN == "" {
-		return
+	if !s.params.Config.SentryEnabled() {
+		return nil
 	}

 	err := sentry.Init(sentryClientOptions(
@@ -166,19 +193,19 @@ func (s *Server) enableSentry() {
 		),
 	))
 	if err != nil {
-		s.log.Error("sentry init failure", "error", err)
-		// Don't use fatal since we still want the service to run
-		return
+		return fmt.Errorf("initialising sentry: %w", err)
 	}

 	s.log.Info("sentry error reporting activated")
 	s.sentryEnabled.Store(true)
+
+	return nil
 }

 // serve installs the signal watcher, starts the listener and blocks
 // until the server's context is cancelled. The process exit status is
 // fx's to decide — from a signal, or from the code
-// shutdownOnListenFailure hands the Shutdowner — so this reports
+// shutdownWithFailure hands the Shutdowner — so this reports
 // nothing back to its caller.
 func (s *Server) serve() {
 	ctx, cancelFunc := context.WithCancel(context.Background())
@@ -208,20 +235,24 @@ func (s *Server) serve() {
 	// Do not call cleanShutdown() here to avoid double invocation.
 }

-// shutdownOnListenFailure ends the application after the HTTP
-// listener failed. The fx OnStart hook returns as soon as the serving
-// goroutine is spawned, so nothing downstream of it ever learns that
-// the listen failed: fx reports RUNNING and the process sits alive
-// with nothing bound, which is invisible to systemd and Docker
-// restart policies. Asking the Shutdowner to stop the app with a
-// non-zero code is what turns that into a visible failure.
+// shutdownWithFailure ends the application non-zero from the serving
+// goroutine. It is how anything on that goroutine fails fatally: the
+// fx OnStart hook returns as soon as the goroutine is spawned, so
+// nothing downstream of it ever learns that the goroutine gave up. fx
+// reports RUNNING and the process sits alive having done neither what
+// it was asked nor anything visible instead, which systemd and
+// Docker restart policies cannot see. Asking the Shutdowner to stop
+// the app with a non-zero code is what turns that into a visible
+// failure, and it is the whole of "fatal" here — no panic, no
+// os.Exit, and every stop hook still runs.
 //
-// The context cancel that follows only unwinds serve()'s own wait.
-// The shutdown itself runs through fx's normal stop sequence, so the
-// clean-shutdown drain in cleanShutdown is reached unchanged.
-func (s *Server) shutdownOnListenFailure() {
+// The context cancel that follows only unwinds serve()'s own wait,
+// and is skipped before serve has installed one. The shutdown itself
+// runs through fx's normal stop sequence, so the clean-shutdown drain
+// in cleanShutdown is reached unchanged.
+func (s *Server) shutdownWithFailure() {
 	err := s.params.Shutdowner.Shutdown(
-		fx.ExitCode(ListenFailureExitCode),
+		fx.ExitCode(StartupFailureExitCode),
 	)
 	if err != nil {
 		s.log.Error("shutdown request failed", "error", err)