9406: Explain why there are two separate CgroupParent fields.

diff --git a/services/crunch-run/crunchrun.go b/services/crunch-run/crunchrun.go
index 40e9fc11eac528965f978f4b0472249759b42cbe..f99beff83e47cb49bd242c1cc4ae28ecd0cf0113 100644 (file)
--- a/services/crunch-run/crunchrun.go
+++ b/services/crunch-run/crunchrun.go
@@ -93,12 +93,23 @@ type ContainerRunner struct {
        ArvMountExit   chan error
        finalState     string
 
-       statLogger         io.WriteCloser
-       statReporter       *crunchstat.Reporter
-       statInterval       time.Duration
-       cgroupRoot         string
+       statLogger   io.WriteCloser
+       statReporter *crunchstat.Reporter
+       statInterval time.Duration
+       cgroupRoot   string
+       // What we expect the container's cgroup parent to be.
        expectCgroupParent string
-       setCgroupParent    string
+       // What we tell docker to use as the container's cgroup
+       // parent. Note: Ideally we would use the same field for both
+       // expectCgroupParent and setCgroupParent, and just make it
+       // default to "docker". However, when using docker < 1.10 with
+       // systemd, specifying a non-empty cgroup parent (even the
+       // default value "docker") hits a docker bug
+       // (https://github.com/docker/docker/issues/17126). Using two
+       // separate fields makes it possible to use the "expect cgroup
+       // parent to be X" feature even on sites where the "specify
+       // cgroup parent" feature breaks.
+       setCgroupParent string
 }
 
 // SetupSignals sets up signal handling to gracefully terminate the underlying