Take the master key from KEEPORG_MASTER_KEY_FILE

Why: the usual way to hand a secret to a container is to mount it as a file
and name the path in the environment. Compose, swarm and Kubernetes all do
it that way, and it keeps the key out of the process environment and off any
disk the container can write.

What: the master key is now looked for in KEEPORG_MASTER_KEY, then
KEEPORG_MASTER_KEY_FILE, then the configured master_key_file. A secret file
may hold base64, with or without padding or a trailing newline, or exactly
32 raw bytes, because secret stores produce both.

Only the configured path is ever created, which is what keeps a plain
package install working with no setup. The two environment variables name a
key something else owns, so a missing one is reported and vault creation
refused. Minting a replacement there would be worse than failing: it opens
none of the existing vaults, and the mistake would otherwise surface one
failed merge at a time.

The compose example now provisions the key as a real secret rather than an
environment variable, and the Dockerfile and README show the same. Neither
names a version any more, so they stop drifting as the server is bumped.

Proved: unit tests for every source and precedence, all four accepted
encodings, and that a missing secret writes nothing anywhere. In real
containers: a podman secret carrying base64 survives a stop and start with
"1 vault keys available for merging" and merges a stale push afterwards; a
secret holding 32 raw bytes creates a vault; and a secret that failed to
mount is named in the startup log, with vault creation answering with the
three places a key may come from.

internal/secret/keyring_test.go

@@ -4,6 +4,7 @@ import (
 	"encoding/base64"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -90,3 +91,140 @@ func TestLoadKeyringRejectsAShortKey(t *testing.T) {
 		t.Fatal("a short master key was accepted")
 	}
 }
+
+// wrapWith is a helper: build a keyring from a known key and wrap a secret
+// with it, so another keyring can be checked for being the same key.
+func wrapWith(t *testing.T, key []byte) string {
+	t.Helper()
+	k, err := NewKeyring(key)
+	if err != nil {
+		t.Fatal(err)
+	}
+	wrapped, err := k.Wrap("v", "secret")
+	if err != nil {
+		t.Fatal(err)
+	}
+	return wrapped
+}
+
+// sameKey reports whether k is the keyring that produced wrapped.
+func sameKey(t *testing.T, k *Keyring, wrapped string) bool {
+	t.Helper()
+	got, err := k.Unwrap("v", wrapped)
+	return err == nil && got == "secret"
+}
+
+func TestMasterKeyFileEnvReadsAProvisionedSecret(t *testing.T) {
+	key := randBytes(32)
+	wrapped := wrapWith(t, key)
+
+	for _, tc := range []struct {
+		name    string
+		content []byte
+	}{
+		{"base64", []byte(base64.StdEncoding.EncodeToString(key))},
+		{"base64 with a trailing newline", []byte(base64.StdEncoding.EncodeToString(key) + "\n")},
+		{"base64 without padding", []byte(base64.RawStdEncoding.EncodeToString(key))},
+		{"32 raw bytes", key},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			path := filepath.Join(t.TempDir(), "secret")
+			if err := os.WriteFile(path, tc.content, 0o400); err != nil {
+				t.Fatal(err)
+			}
+			t.Setenv(MasterKeyEnv, "")
+			t.Setenv(MasterKeyFileEnv, path)
+			k, err := LoadKeyring("")
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !sameKey(t, k, wrapped) {
+				t.Fatal("the key read from the secret file is not the one written")
+			}
+		})
+	}
+}
+
+// A secret that was supposed to be mounted and is not must be reported. A
+// new key minted in its place would open none of the existing vaults.
+func TestMissingSecretFileIsReportedAndMintsNothing(t *testing.T) {
+	dir := t.TempDir()
+	missing := filepath.Join(dir, "not-mounted")
+	configured := filepath.Join(dir, "master.key")
+	t.Setenv(MasterKeyEnv, "")
+	t.Setenv(MasterKeyFileEnv, missing)
+
+	_, err := LoadKeyring(configured)
+	if err == nil {
+		t.Fatal("a missing secret file was accepted")
+	}
+	if !strings.Contains(err.Error(), MasterKeyFileEnv) {
+		t.Fatalf("the error does not name the variable: %v", err)
+	}
+	if _, err := os.Stat(configured); !os.IsNotExist(err) {
+		t.Fatal("a replacement key was minted at the configured path")
+	}
+	if _, err := os.Stat(missing); !os.IsNotExist(err) {
+		t.Fatal("a key was written where the secret should have been mounted")
+	}
+}
+
+func TestMasterKeyEnvWinsOverTheSecretFile(t *testing.T) {
+	envKey, fileKey := randBytes(32), randBytes(32)
+	wrappedByEnv := wrapWith(t, envKey)
+
+	path := filepath.Join(t.TempDir(), "secret")
+	if err := os.WriteFile(path, fileKey, 0o400); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv(MasterKeyEnv, base64.StdEncoding.EncodeToString(envKey))
+	t.Setenv(MasterKeyFileEnv, path)
+
+	k, err := LoadKeyring("")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !sameKey(t, k, wrappedByEnv) {
+		t.Fatal("the secret file took precedence over the environment key")
+	}
+}
+
+func TestSecretFileWinsOverTheConfiguredPath(t *testing.T) {
+	secretKey := randBytes(32)
+	wrapped := wrapWith(t, secretKey)
+	dir := t.TempDir()
+
+	secretPath := filepath.Join(dir, "provisioned")
+	if err := os.WriteFile(secretPath, secretKey, 0o400); err != nil {
+		t.Fatal(err)
+	}
+	configured := filepath.Join(dir, "master.key")
+	if err := os.WriteFile(configured,
+		[]byte(base64.StdEncoding.EncodeToString(randBytes(32))), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv(MasterKeyEnv, "")
+	t.Setenv(MasterKeyFileEnv, secretPath)
+
+	k, err := LoadKeyring(configured)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !sameKey(t, k, wrapped) {
+		t.Fatal("the configured path took precedence over the provisioned secret")
+	}
+}
+
+func TestSecretFileContentIsValidated(t *testing.T) {
+	for _, content := range []string{"", "not base64 at all !!", "c2hvcnQ="} {
+		path := filepath.Join(t.TempDir(), "secret")
+		if err := os.WriteFile(path, []byte(content), 0o400); err != nil {
+			t.Fatal(err)
+		}
+		t.Setenv(MasterKeyEnv, "")
+		t.Setenv(MasterKeyFileEnv, path)
+		if _, err := LoadKeyring(""); err == nil {
+			t.Fatalf("content %q was accepted as a master key", content)
+		}
+	}
+}