Always start conversion process in new session

Start the conversion process in a new session, so that we can later on
kill the process group, without killing the controlling script (i.e.,
the Dangezone UI). This should not affect the conversion process in any
other way.

dangerzone/isolation_provider/base.py

@@ -259,6 +259,11 @@ def doc_to_pixels_proc(
     ) -> Iterator[subprocess.Popen]:
         """Start a conversion process, pass it to the caller, and then clean it up."""
         p = self.start_doc_to_pixels_proc(document)
+        if platform.system() != "Windows":
+            assert os.getpgid(p.pid) != os.getpgid(
+                os.getpid()
+            ), "Parent shares same PGID with child"
+
         try:
             yield p
         except errors.ConverterProcException as e:

dangerzone/isolation_provider/container.py

@@ -267,6 +267,9 @@ def exec(
             stdout=subprocess.PIPE,
             stderr=self.proc_stderr,
             startupinfo=startupinfo,
+            # Start the conversion process in a new session, so that we can later on
+            # kill the process group, without killing the controlling script.
+            start_new_session=True,
         )
 
     def exec_container(

dangerzone/isolation_provider/dummy.py

@@ -73,7 +73,7 @@ def pixels_to_pdf(
         pass
 
     def start_doc_to_pixels_proc(self, document: Document) -> subprocess.Popen:
-        return subprocess.Popen("True")
+        return subprocess.Popen("True", start_new_session=True)
 
     def terminate_doc_to_pixels_proc(
         self, document: Document, p: subprocess.Popen

dangerzone/isolation_provider/qubes.py

@@ -67,6 +67,9 @@ def start_doc_to_pixels_proc(self, document: Document) -> subprocess.Popen:
             stdin=subprocess.PIPE,
             stdout=subprocess.PIPE,
             stderr=stderr,
+            # Start the conversion process in a new session, so that we can later on
+            # kill the process group, without killing the controlling script.
+            start_new_session=True,
         )
 
         if dev_mode:

tests/isolation_provider/test_dummy.py

@@ -25,6 +25,7 @@ def start_doc_to_pixels_proc(self, document: Document) -> subprocess.Popen:
             stdin=subprocess.PIPE,
             stdout=subprocess.PIPE,
             stderr=subprocess.PIPE,
+            start_new_session=True,
         )
 
     def terminate_doc_to_pixels_proc(