Lets see if resetting the broken control connection fixes the test on Ubuntu as well

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ jobs:
         runs-on: ${{ matrix.os }}
         strategy:
             matrix:
-                os: [macos-latest, windows-latest]
+                os: [ubuntu-latest, macos-latest, windows-latest]
         steps:
             - name: Checkout source code
               uses: actions/checkout@master

src/calibre/srv/loop.py

@@ -652,6 +652,9 @@ class ServerLoop(object):
                 self.control_in.write(what)
                 self.control_in.flush()
             except BrokenPipeError:
+                # this started happening in the CI environment for the ajax_book test
+                # in early Dec 2020
+                self.log.error('Control pipe broken, resetting')
                 self.create_control_connection()
                 self.control_in.write(what)
                 self.control_in.flush()