Review of the #50531 salvage found the cross-session HERMES_SESSION_* leak also
survives on the non-terminal spawn helper hermes_subprocess_env (added by #56202
after #50531 was written), which does os.environ.copy() without the guard. Of
its six callers, five re-bind the session identity explicitly (slash_worker/ACP
via --session-key argv) and are safe by accident; but tui_gateway cli.exec
(server.py) spawns a fresh CLI with NO --session-key under the engaged TUI host,
so it inherits a possibly-foreign HERMES_SESSION_* from the last-writer-wins
global and would stamp Kanban rows / telemetry with another session's id.
Route hermes_subprocess_env through the same _inject_session_context_env
chokepoint, restoring the single-uniform-policy-across-every-spawn-surface
invariant the codebase already claims for the internal-secret filter. Safe for
all six callers: bound ContextVars win (re-binders unaffected), _UNSET strips
(closes cli.exec). Adds 3 guard tests; mutation-checked.
Session vars (HERMES_SESSION_*) have a process-global os.environ mirror written
last-writer-wins as a CLI/cron fallback and never cleared. Under a concurrent
multi-session host (messaging gateway, ACP adapter, API server, TUI) that global
belongs to whichever turn wrote it last. A subprocess spawned from a task whose
session ContextVar is _UNSET (a sibling task that never bound, or one that
inherited another session's context) inherited the FOREIGN global and acted on
another session's identity.
Add a session_context_engaged() latch (set once any host calls set_session_vars)
and route both terminal spawn paths through a single _inject_session_context_env
chokepoint: once engaged, a bound ContextVar (incl. "") is authoritative and an
_UNSET var is STRIPPED rather than inheriting the possibly-foreign global. Pure
single-process CLI/one-shot (never engaged) keeps the inherited fallback.
Salvaged from #50531 (supersedes #49922). local.py hunk re-applied by intent
onto the current hermes_subprocess_env refactor.
Co-authored-by: PolyphonyRequiem <3107779+PolyphonyRequiem@users.noreply.github.com>
