[tor-bugs] #19476 [Core Tor/Tor]: Use conditional intervals in heartbeat_callback

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#19476: Use conditional intervals in heartbeat_callback
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 Reporter:  cypherpunks                          |          Owner:
     Type:  defect                               |         Status:
                                                 |  needs_review
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  0.3.2.x-final
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  tor-relay, heartbeat, usability,     |  Actual Points:
  review-group-20                                |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:
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Comment (by isis):

 Hi! I applied cypherpunks' patch in my `bug19476` [ branch]. It mostly
 LGTM.

 I made the following small changes:
  - Instead of rescheduling for the next second—if the writing the
 heartbeat log message failed, retry `MIN_HEARTBEAT_PERIOD` seconds later.
 (The documentation for `MIN_HEARTBEAT_PERIOD` states that it is the
 "Lowest allowable value for HeartbeatPeriod; if this is too low, we might
 expose more information than we're comfortable with.")
  - Documented this behaviour in the `heartbeat_callback()` docstring.
  - Added a changes file.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19476#comment:10>
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