qubes: Store the process for the spawned qube

Store, in an instance attribute, the process that we have started for
the spawned disposable qube. In subsequent commits, we will use it from
other places as well, aside from the `_convert` method.

Note that this commit does not alter the conversion logic, and only does
the following:
1. Renames `p.` to `self.proc.`
2. Adds an `__init__` method to the Qubes isolation provider, and
   initializes the `self.proc` attribute to `None`.
3. Adds an assert that `self.proc` is not `None` after it's spawned, to
   placate Mypy.

dangerzone/isolation_provider/qubes.py

@@ -61,6 +61,10 @@ def read_debug_text(f: IO[bytes], size: int) -> str:
 class Qubes(IsolationProvider):
     """Uses a disposable qube for performing the conversion"""
 
+    def __init__(self) -> None:
+        self.proc: Optional[subprocess.Popen] = None
+        super().__init__()
+
     def install(self) -> bool:
         return True
 
@@ -89,22 +93,23 @@ class Qubes(IsolationProvider):
                 # Use dz.ConvertDev RPC call instead, if we are in development mode.
                 # Basically, the change is that we also transfer the necessary Python
                 # code as a zipfile, before sending the doc that the user requested.
-                p = subprocess.Popen(
+                self.proc = subprocess.Popen(
                     ["/usr/bin/qrexec-client-vm", "@dispvm:dz-dvm", "dz.ConvertDev"],
                     stdin=subprocess.PIPE,
                     stdout=subprocess.PIPE,
                     stderr=subprocess.PIPE,
                 )
-                assert p.stdin is not None
+                assert self.proc is not None
+                assert self.proc.stdin is not None
 
                 # Send the dangerzone module first.
-                self.teleport_dz_module(p.stdin)
+                self.teleport_dz_module(self.proc.stdin)
 
                 # Finally, send the document, as in the normal case.
-                p.stdin.write(f.read())
-                p.stdin.close()
+                self.proc.stdin.write(f.read())
+                self.proc.stdin.close()
             else:
-                p = subprocess.Popen(
+                self.proc = subprocess.Popen(
                     ["/usr/bin/qrexec-client-vm", "@dispvm:dz-dvm", "dz.Convert"],
                     stdin=f,
                     stdout=subprocess.PIPE,
@@ -115,11 +120,12 @@ class Qubes(IsolationProvider):
             size = os.path.getsize(document.input_filename) / 1024**2
             timeout = calculate_timeout(size) + STARTUP_TIME_SECONDS
 
-            assert p.stdout is not None
-            os.set_blocking(p.stdout.fileno(), False)
+            assert self.proc is not None
+            assert self.proc.stdout is not None
+            os.set_blocking(self.proc.stdout.fileno(), False)
 
             try:
-                n_pages = read_int(p.stdout, timeout)
+                n_pages = read_int(self.proc.stdout, timeout)
             except ValueError:
                 error_code = p.wait()
                 # XXX Reconstruct exception from error code
@@ -139,10 +145,10 @@ class Qubes(IsolationProvider):
             for page in range(1, n_pages + 1):
                 # TODO handle too width > MAX_PAGE_WIDTH
                 # TODO handle too big height > MAX_PAGE_HEIGHT
-                width = read_int(p.stdout, timeout=sw.remaining)
-                height = read_int(p.stdout, timeout=sw.remaining)
+                width = read_int(self.proc.stdout, timeout=sw.remaining)
+                height = read_int(self.proc.stdout, timeout=sw.remaining)
                 untrusted_pixels = read_bytes(
-                    p.stdout,
+                    self.proc.stdout,
                     width * height * 3,
                     timeout=sw.remaining,
                 )  # three color channels
@@ -161,17 +167,17 @@ class Qubes(IsolationProvider):
                 self.print_progress_trusted(document, False, text, percentage)
 
         # Ensure nothing else is read after all bitmaps are obtained
-        p.stdout.close()
+        self.proc.stdout.close()
 
         # TODO handle leftover code input
         text = "Converted document to pixels"
         self.print_progress_trusted(document, False, text, percentage)
 
         if getattr(sys, "dangerzone_dev", False):
-            assert p.stderr is not None
-            os.set_blocking(p.stderr.fileno(), False)
-            untrusted_log = read_debug_text(p.stderr, MAX_CONVERSION_LOG_CHARS)
-            p.stderr.close()
+            assert self.proc.stderr is not None
+            os.set_blocking(self.proc.stderr.fileno(), False)
+            untrusted_log = read_debug_text(self.proc.stderr, MAX_CONVERSION_LOG_CHARS)
+            self.proc.stderr.close()
             log.info(
                 f"Conversion output (doc to pixels)\n{self.sanitize_conversion_str(untrusted_log)}"
             )