When a new login arrives while the previous connection's async cleanup
is still running, LookupISCSISession finds the stale session but
LookupConnection returns nil (old connection already closed). This
caused a nil pointer dereference when accessing existConn.session
during session reinstatement.
Fix by checking existConn != nil before reinstatement. If the old
connection is already gone, unbind the stale session and create
a fresh one instead.
Also add sync.Once to removeConnectionFromSession to prevent
concurrent goroutine and main-path cleanup from racing.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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