This change addresses a panic that can occur in the control plane client's DNS resolution backoff logic within linkerd/app/core/src/control.rs.

Problem:

When resolving the control plane address, if a dns::ResolveError occurs and provides a negative TTL via negative_ttl(), this TTL is used to schedule the next resolution attempt using tokio::time::interval(ttl).

However, if the negative_ttl() returns Some(Duration::ZERO), passing a zero duration to time::interval causes a panic:

thread 'main' panicked at linkerd/app/core/src/control.rs:87:49:
`period` must be non-zero.
   0:     0x563f974dd323 - <unknown>
   1:     0x563f9680ed2c - <unknown>
   2:     0x563f974b030d - <unknown>
   3:     0x563f974de99e - <unknown>
   4:     0x563f974de564 - <unknown>
   5:     0x563f974df5c5 - <unknown>
   6:     0x563f974eecc9 - <unknown>
   7:     0x563f974eec96 - <unknown>
   8:     0x563f96737dfa - <unknown>
   9:     0x563f974f116c - <unknown>
  10:     0x563f97233597 - <unknown>
  11:     0x563f9715cc1a - <unknown>
  12:     0x563f9727a651 - <unknown>
  13:     0x563f971a91af - <unknown>
  14:     0x563f96c5a01c - <unknown>
  15:     0x563f967902b3 - <unknown>
  16:     0x563f96c56ba5 - <unknown>
  17:     0x7fc1ccd2624a - <unknown>
  18:     0x7fc1ccd26305 - __libc_start_main
  19:     0x563f9674f711 - <unknown>
  20:                0x0 - <unknown>

This panic was observed in production environments, particularly during restarts or issues with the linkerd-destination service. When the proxy sidecar panicked due to this error, it resulted in service unavailability for meshed applications, requiring manual restarts of deployments to recover connectivity.

Solution:

This commit introduces a minimum backoff duration (min_duration = 100ms) for cases where a negative TTL is provided by the DNS resolver. It uses std::cmp::max(ttl, min_duration) to ensure that the duration passed to time::interval is never zero.

This prevents the panic and ensures the proxy gracefully handles zero TTLs by applying a minimal delay before the next resolution attempt, improving resilience during control plane discovery issues.