Abort startup on a config file pixa cannot read (closes #176)

Of the places pixa looks for its config file on its own, it passed over
any place where os.Stat failed, so a file in a directory pixa may not
enter was skipped without a word and pixa started on a later file or on
the environment and defaults. Now only a path that does not exist, or
that runs through a file (such as under a HOME of /dev/null), is passed
over; any other error aborts startup naming the file, as a file that
does not parse already did. README.md says so where it gives the search
order. A config.yml that links to itself tests this as root too.

Model: opus-5-5

internal/config/config_validation_internal_test.go

@@ -564,6 +564,116 @@ 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)
+	}
+}
+
+// TestConfigFileLinkingToItselfAbortsStartup checks that a config file
+// pixa cannot read for a reason other than not existing aborts startup,
+// as root too: a symbolic link to itself fails with "too many levels of
+// symbolic links".
+func TestConfigFileLinkingToItselfAbortsStartup(t *testing.T) {
+	workDir := t.TempDir()
+
+	err := os.Symlink("config.yml", filepath.Join(workDir, "config.yml"))
+	if err != nil {
+		t.Fatalf("failed to create symbolic link: %v", err)
+	}
+
+	// Only the working directory's config.yml is there: the appname rules
+	// out /etc, and HOME is empty.
+	t.Setenv("PIXA_CONFIG_PATH", "")
+	t.Setenv("HOME", t.TempDir())
+	t.Chdir(workDir)
+
+	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(), "config.yml") {
+		t.Errorf("error %q does not name the config file config.yml", err.Error())
+	}
+}
+
+// TestConfigPathThroughFileIsPassedOver checks that a config file path
+// that runs through a file, such as one under a HOME of /dev/null, is
+// passed over like one that does not exist, since no file can be there.
+func TestConfigPathThroughFileIsPassedOver(t *testing.T) {
+	// No config file is there: the appname rules out /etc, HOME is
+	// /dev/null, and the working directory is empty.
+	t.Setenv("PIXA_CONFIG_PATH", "")
+	t.Setenv("HOME", os.DevNull)
+	t.Chdir(t.TempDir())
+
+	log := slog.New(slog.DiscardHandler)
+
+	sc, err := loadConfigFile(log, "pixa-test-nonexistent-app")
+	if err != nil {
+		t.Fatalf("a config path through a file must be passed over, got error: %v",
+			err)
+	}
+
+	if sc != nil {
+		t.Errorf("expected no config file, got config: %v", sc)
+	}
+}
+
 func TestEnsureStateDirCreatesDirectory(t *testing.T) {
 	t.Parallel()