Add check to determine if current thread is main

This commit introduces a check to determine if the current
running thread is the main thread. If True, then we continue to block
interactive mode on macOS (it is not possible to share the main thread
with GUI event loop needed for the ImageJ GUI). If False, then the
current thread is *not* the main thread (e.g. a jaunched session) and
interactive mode for macOS can proceed.

src/imagej/__init__.py

@@ -38,6 +38,7 @@
 import sys
 import threading
 import time
+from ctypes import cdll
 from enum import Enum
 from functools import lru_cache
 from pathlib import Path
@@ -1206,7 +1207,11 @@ def init(
     macos = sys.platform == "darwin"
 
     if macos and mode == Mode.INTERACTIVE:
-        raise EnvironmentError("Sorry, the interactive mode is not available on macOS.")
+        # check for main thread only on macOS
+        if _macos_is_main_thread():
+            raise EnvironmentError(
+                "Sorry, the interactive mode is not available on macOS."
+            )
 
     if not sj.jvm_started():
         success = _create_jvm(ij_dir_or_version_or_endpoint, mode, add_legacy)
@@ -1517,6 +1522,24 @@ def _includes_imagej_legacy(items: list):
     return any(item.startswith("net.imagej:imagej-legacy") for item in items)
 
 
+def _macos_is_main_thread():
+    """Detect if the current thread is the main thread on macOS.
+
+    :return: Boolean indicating if the current thread is the main thread.
+    """
+    # try to load the pthread library
+    try:
+        pthread = cdll.LoadLibrary("libpthread.dylib")
+    except OSError as e:
+        print("No pthread library found.", e)
+
+    # detect if main thread
+    if pthread.pthread_main_np() == 1:
+        return True
+    else:
+        return False
+
+
 def _set_ij_env(ij_dir):
     """
     Create a list of required jars and add to the java classpath.