Flush stdout/err in Dockerfile-workers before replacing the current process (#14195)

Also update `subprocess.check_output` to the slightly newer `subprocess.run`.

Signed-off-by: Jason Little <realtyem@gmail.com>

docker/start.py

@@ -13,14 +13,19 @@ import jinja2
 
 # Utility functions
 def log(txt: str) -> None:
-    print(txt, file=sys.stderr)
+    print(txt)
 
 
 def error(txt: str) -> NoReturn:
-    log(txt)
+    print(txt, file=sys.stderr)
     sys.exit(2)
 
 
+def flush_buffers() -> None:
+    sys.stdout.flush()
+    sys.stderr.flush()
+
+
 def convert(src: str, dst: str, environ: Mapping[str, object]) -> None:
     """Generate a file from a template
 
@@ -131,10 +136,10 @@ def generate_config_from_template(
 
     if ownership is not None:
         log(f"Setting ownership on /data to {ownership}")
-        subprocess.check_output(["chown", "-R", ownership, "/data"])
+        subprocess.run(["chown", "-R", ownership, "/data"], check=True)
         args = ["gosu", ownership] + args
 
-    subprocess.check_output(args)
+    subprocess.run(args, check=True)
 
 
 def run_generate_config(environ: Mapping[str, str], ownership: Optional[str]) -> None:
@@ -158,7 +163,7 @@ def run_generate_config(environ: Mapping[str, str], ownership: Optional[str]) ->
     if ownership is not None:
         # make sure that synapse has perms to write to the data dir.
         log(f"Setting ownership on {data_dir} to {ownership}")
-        subprocess.check_output(["chown", ownership, data_dir])
+        subprocess.run(["chown", ownership, data_dir], check=True)
 
     # create a suitable log config from our template
     log_config_file = "%s/%s.log.config" % (config_dir, server_name)
@@ -185,6 +190,7 @@ def run_generate_config(environ: Mapping[str, str], ownership: Optional[str]) ->
         "--open-private-ports",
     ]
     # log("running %s" % (args, ))
+    flush_buffers()
     os.execv(sys.executable, args)
 
 
@@ -267,8 +273,10 @@ running with 'migrate_config'. See the README for more details.
     args = [sys.executable] + args
     if ownership is not None:
         args = ["gosu", ownership] + args
+        flush_buffers()
         os.execve("/usr/sbin/gosu", args, environ)
     else:
+        flush_buffers()
         os.execve(sys.executable, args, environ)