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 @@ class IsolationProvider(ABC):
     ) -> 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 @@ class Container(IsolationProvider):
             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 @@ class Dummy(IsolationProvider):
         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 @@ class Qubes(IsolationProvider):
             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 @@ class DummyWait(Dummy):
             stdin=subprocess.PIPE,
             stdout=subprocess.PIPE,
             stderr=subprocess.PIPE,
+            start_new_session=True,
         )
 
     def terminate_doc_to_pixels_proc(