shell: Fix hangs in pytest

For some reason, this now happens all the time on my machine.

This may also be the cause of the random hangs we have had on Jenkins

swh/graph/shell.py

@@ -371,10 +371,13 @@ class Pipe:
     def _run(self, stdin, stdout) -> _RunningPipe:
         read_pipes: List[Any] = [stdin]
         write_pipes: List[Any] = []
+        pipes_to_close: List[int] = []
         for _ in range(len(self.children) - 1):
             (r, w) = os.pipe()
             read_pipes.append(os.fdopen(r, "rb"))
             write_pipes.append(os.fdopen(w, "wb"))
+            pipes_to_close.append(r)
+            pipes_to_close.append(w)
         write_pipes.append(stdout)
 
         running_children = [
@@ -382,6 +385,18 @@ class Pipe:
             for (r, w, child) in zip(read_pipes, write_pipes, self.children)
         ]
 
+        # We need to close these file descriptors in the parent process, so that the
+        # corresponding pipes have an end fully closed when the corresponding process
+        # dies.
+        # On CPython (and when not running in tools like pytest that keep references to
+        # call frames), this would be done automatically when the function exits because
+        # the refcount to all pipes becomes 0, but we cannot rely
+        # on this behavior.
+        # Without it, the other end of the process will hang when trying to read
+        # or write to it.
+        for pipe in pipes_to_close:
+            os.close(pipe)
+
         return _RunningPipe(self, running_children)
 
     def run(self) -> None: