Test that a config file pixa may not read aborts startup

Of the places pixa looks for its config file on its own, a file in a
directory pixa may not enter is passed over today and pixa starts
without it. This test puts the config file in such a directory and
expects startup to abort with an error naming the file. It is skipped
when run as root, which may enter any directory.

Model: opus-5-5

internal/config/config_validation_internal_test.go

@@ -564,6 +564,60 @@ func TestMalformedConfigFileAbortsStartup(t *testing.T) {
 	t.Logf("got expected error: %v", err)
 }
 
+// TestConfigFileInDirectoryPixaMayNotEnterAbortsStartup checks that a
+// config file pixa cannot read because it may not enter its directory
+// aborts startup instead of being passed over.
+func TestConfigFileInDirectoryPixaMayNotEnterAbortsStartup(t *testing.T) {
+	if os.Geteuid() == 0 {
+		t.Skip("root may enter any directory")
+	}
+
+	home := t.TempDir()
+	configDir := filepath.Join(home, ".config", "pixa-test-nonexistent-app")
+	configPath := filepath.Join(configDir, "config.yml")
+
+	err := os.MkdirAll(configDir, 0o700)
+	if err != nil {
+		t.Fatalf("failed to create config directory: %v", err)
+	}
+
+	err = os.WriteFile(configPath, []byte(signingKeyLine), 0o600)
+	if err != nil {
+		t.Fatalf("failed to write config: %v", err)
+	}
+
+	err = os.Chmod(configDir, 0)
+	if err != nil {
+		t.Fatalf("failed to remove the config directory's permissions: %v", err)
+	}
+
+	// Give the directory back its permissions so t.TempDir can remove it.
+	t.Cleanup(func() {
+		//nolint:gosec // G302: a directory needs its execute bit to be removed
+		_ = os.Chmod(configDir, 0o700)
+	})
+
+	// The ~/.config candidate is the only one that exists: the appname
+	// rules out /etc, and the working directory is empty.
+	t.Setenv("PIXA_CONFIG_PATH", "")
+	t.Setenv("HOME", home)
+	t.Chdir(t.TempDir())
+
+	log := slog.New(slog.DiscardHandler)
+
+	sc, err := loadConfigFile(log, "pixa-test-nonexistent-app")
+	if err == nil {
+		t.Fatalf("config file pixa cannot read must abort startup, got config: %v",
+			sc)
+	}
+
+	t.Logf("got expected error: %v", err)
+
+	if !strings.Contains(err.Error(), configPath) {
+		t.Errorf("error %q does not name the config file %s", err.Error(), configPath)
+	}
+}
+
 func TestEnsureStateDirCreatesDirectory(t *testing.T) {
 	t.Parallel()