Handle case of incomplete blkio cgroup subsystem

It seems that in Docker Desktop this subsystem exists
but is incomplete (it misses the file that we need).
Work around that by ignoring the subsystem in this case.

Fixes #985.

benchexec/cgroupsv1.py

@@ -26,6 +26,8 @@
 
 CGROUP_NAME_PREFIX = "benchmark_"
 
+BLKIO_BYTES_FILE = "throttle.io_service_bytes"
+
 _PERMISSION_HINT_GROUPS = """
 You need to add your account to the following groups: {0}
 Remember to logout and login again afterwards to make group changes effective."""
@@ -90,6 +92,15 @@ def find_my_cgroups(cgroup_paths=None, fallback=True):
                 and os.path.isdir(fallbackPath)
             ):
                 cgroupPath = fallbackPath
+
+            if subsystem == CgroupsV1.IO and not os.path.exists(
+                os.path.join(cgroupPath, f"{CgroupsV1.IO}.{BLKIO_BYTES_FILE}")
+            ):
+                # At least on Docker Desktop blkio can exist in an incomplete way
+                # (cf. 985). Instead of ignoring this later we can simply skip the
+                # whole subsystem because we only need it for this single file.
+                continue
+
             cgroupsParents[subsystem] = cgroupPath
 
     return cgroupsParents
@@ -539,10 +550,9 @@ def read_allowed_memory_banks(self):
         return util.parse_int_list(self.get_value(self.CPUSET, "mems"))
 
     def read_io_stat(self):
-        blkio_bytes_file = "throttle.io_service_bytes"
         bytes_read = 0
         bytes_written = 0
-        for blkio_line in self.get_file_lines(self.IO, blkio_bytes_file):
+        for blkio_line in self.get_file_lines(self.IO, BLKIO_BYTES_FILE):
             try:
                 dev_no, io_type, bytes_amount = blkio_line.split(" ")
                 if io_type == "Read":