Test a config file that links to itself and a HOME that is a file

The test for a directory pixa may not enter is skipped as root, where
the gate runs the tests. A config.yml in the working directory that is
a symbolic link to itself fails os.Stat as root too, and must abort
startup. A config path that runs through a file, such as one under a
HOME of /dev/null, cannot hold a file and must be passed over like one
that does not exist; today it aborts startup.

Model: opus-5-5

internal/config/config_validation_internal_test.go

@@ -618,6 +618,62 @@ func TestConfigFileInDirectoryPixaMayNotEnterAbortsStartup(t *testing.T) {
 	}
 }
 
+// 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()