pmb.helpers.run_core: Properly type pipe_read() (MR 2464)

I think the "these types make no sense" comment were about
output_return_buffer, which defaulted to False but was to be set to
list[bytes] if used. I changed it so that it defaults to None, which
probably is more conventional. Other than that I didn't notice anything
weird about the types here.

Also check for None as necessary to appease mypy.

pmb/helpers/run_core.py

@@ -14,6 +14,7 @@ import sys
 import threading
 import time
 from collections.abc import Sequence
+from typing import overload, Literal
 import pmb.helpers.run
 
 """For a detailed description of all output modes, read the description of
@@ -94,14 +95,33 @@ def pipe(cmd, working_dir=None):
     return ret
 
 
-# FIXME (#2324): These types make no sense at all
+@overload
 def pipe_read(
-    process,
-    output_to_stdout=False,
-    output_log=True,
-    output_return=False,
-    output_return_buffer=False,
-):
+    process: subprocess.Popen,
+    output_to_stdout: bool = ...,
+    output_log: bool = ...,
+    output_return: Literal[False] = ...,
+    output_return_buffer: None = ...,
+) -> None: ...
+
+
+@overload
+def pipe_read(
+    process: subprocess.Popen,
+    output_to_stdout: bool = ...,
+    output_log: bool = ...,
+    output_return: Literal[True] = ...,
+    output_return_buffer: list[bytes] = ...,
+) -> None: ...
+
+
+def pipe_read(
+    process: subprocess.Popen,
+    output_to_stdout: bool = False,
+    output_log: bool = True,
+    output_return: bool = False,
+    output_return_buffer: list[bytes] | None = None,
+) -> None:
     """Read all output from a subprocess, copy it to the log and optionally stdout and a buffer variable.
 
     This is only meant to be called by foreground_pipe() below.
@@ -115,13 +135,18 @@ def pipe_read(
     """
     while True:
         # Copy available output
-        out = process.stdout.readline()
+        process_stdout = process.stdout
+        if process_stdout is None:
+            raise RuntimeError("subprocess has no stdout?")
+        out = process_stdout.readline()
         if len(out):
             if output_log:
                 pmb.helpers.logging.logfd.buffer.write(out)
             if output_to_stdout:
                 sys.stdout.buffer.write(out)
             if output_return:
+                if output_return_buffer is None:
+                    raise AssertionError
                 output_return_buffer.append(out)
             continue