A test host on CI is killed by its own memory watchdog when its resident size crosses 6 GiB. On 2026-09-12 that happened to two shards of one pull request, in two different test assemblies, with almost no managed heap in use — and the watchdog's message named the cause as a cumulative container leak from the mesh test classes.
The trace it writes says something else. Every mesh class on the shard released what it took; the
growth was two steps, of 1.8 and 3.9 GB, each inside a window where no mesh class was running at
all. What was running were plain test classes compiling small assemblies with Roslyn — and each
compile built its reference list from scratch, MetadataReference.CreateFromFile over every one of
the ~520 assemblies the process can see, so every compile re-read the whole platform into memory
that only a finalizer gives back. Nineteen compiles in four seconds is 3.4 GB.
Those classes now share the reference set the platform's own NodeType compiler uses, built once per process. Peak memory of the affected classes fell by two thirds and they run three to five times faster. The watchdog's message no longer names a cause: it prints what it measured — now including how much the garbage collector has committed, so a small managed number beside a large resident one can be read for what it is — and points at the trace and the rule for reading it.