fix: killpg permission error on macos

Behavior of `killpg` slightly differs in Mac and Linux. For some reason,
`killpg` returns error EPERM when sending a signal to a zomibie process.
And that is the reason of `test_process_management` failure on mac -
there's a small gap between killing child and and subreaper calls
`waitpid`.

Now pytest handles this exception properly.

Close https://git.picodata.io/picodata/picodata/picodata/-/issues/70

See also:

- Stackoverflow: Why would `killpg` return "not permitted" when ownership is correct?
  https://stackoverflow.com/questions/12521705/why-would-killpg-return-not-permitted-when-ownership-is-correct

- Linux `man 2 killpg`:
  https://linux.die.net/man/2/killpg#Notes

  > Notes
  >
  > There are various differences between the permission checking in
  > BSD-type systems and System V-type systems. See the POSIX rationale
  > for kill(). A difference not mentioned by POSIX concerns the return
  > value EPERM: BSD documents that no signal is sent and EPERM returned
  > when the permission check failed for **at least one** target process,
  > while POSIX documents EPERM only when the permission check failed for
  > **all** target processes.

- MacOS `man 2 killpg`:
  https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/killpg.2.html

  > [EPERM] The sending process is not the super-user and
  >         **one or more** of the target processes has an effective
  >         user ID different from that of the sending process.

- Linux `man 2 kill`:
  https://linux.die.net/man/2/kill

  >  EPERM  The process does not have permission to send the signal
  >         *to any* of the target processes.
  >

- Process states in Linux:
  https://kerneltalks.com/linux/process-states-in-linux/

- Reproduce killpg returning EPERM on MacOS:
  https://git.picodata.io/picodata/picodata/picodata/-/snippets/7

test/int/test_basics.py

@@ -141,6 +141,12 @@ def test_process_management(instance: Instance):
             os.killpg(pgrp, 0)
         except ProcessLookupError:
             return True
+        except PermissionError:
+            # According to `man 2 kill`, MacOS raises it if at least one process
+            # in the process group has insufficient permissions. In fact, it also
+            # returns EPERM if the targed process is a zombie.
+            # See https://git.picodata.io/picodata/picodata/picodata/-/snippets/7
+            raise StillAlive
         else:
             raise StillAlive
 
@@ -162,16 +168,24 @@ def test_process_management(instance: Instance):
     assert exc.value.errno == errno.ECONNRESET
     with pytest.raises(StillAlive):
         waitpg(pgrp)
+    print(f"{instance} is still alive")
 
     # Kill the remaining child in the process group
     instance.killpg()
 
-    # When the supervisor is killed, the orphaned child is reparented to
-    # a subreaper. Pytest isn't the one, and therefore it can't do `waitpid`.
-    # Instead, the test retries `killpg` until it succeeds.
+    # When the supervisor is killed, the orphaned child is reparented
+    # to a subreaper. Pytest isn't the one, and therefore it can't do
+    # `waitpid` directly. Instead, the test retries `killpg` until
+    # it succeeds.
+    #
+    # Also, note, that after the child is killed, it remains
+    # a zombie for a while. The child is removed from the process
+    # table when a supreaper calls `waitpid`.
+    #
+    waitpg(pgrp)
+    print(f"{instance} is finally dead")
 
     # Ensure the child is dead
-    waitpg(pgrp)
     with pytest.raises(ProcessLookupError):
         os.killpg(pgrp, 0)
 
@@ -181,7 +195,6 @@ def test_process_management(instance: Instance):
     instance.start()
     pid2 = instance.process.pid
     assert pid1 == pid2
-
     instance.terminate()
     instance.terminate()
     instance.killpg()