Run-Time Error Handling

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Run-Time Error Handling

• Wes Weimer, George Necula

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Motivation

• Static Safety Analyses
• Good
• Often Incomplete
• Or program may actually have bugs
• Could ask the programmers
• Fix it, annotate it, tell us if it is a bug, etc.
  
• Not always feasible
• Loops, heap data structures, preconditions, etc.

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Add Run-Time Checks

• CCured, virtual memory hardware
• Perls Taint Analysis
• Java downcasts
• SLAM / BLAST
• Use predicates to instrument error paths
• Now nothing can possibly go wrong!

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When Good Checks Go Bad

• Most common Java exception handler
• catch (Exception e)
• e.printStackTrace(System.err)
• System.exit(-1)
• Second most common
• catch (Exception e)
• Can we do more than Abort or Ignore?

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Why So Abysmal?

• Ironclad error-handling is
• Repetitive no way to set generic policies
• Path- and flow-sensitive
• Distant handling separate from normal code
• Complicated by exceptions
• Programs have errors in their error-handling
• Lets add language support!

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Plan

• Annotate interfaces and actions with
• Compensations
• Obligations
• Things-to-be-done-before-leaving
• Ways to restore invariants
• Keep dynamic stacks of these at run-time
• As first-class citizens
• Like sagas or compensating xactions

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Guarantees

• Strong static type system run-time checks
• Obligations cannot be ignored
• Dynamic extents (e.g., loops) handled
• Nesting, passing obligations, etc.
• Can enforce (but is more general than)
• t1 tn
• t1 tk ck c1

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Why not use

• Transactions, failure transparency, discount
  checking, system-level undo, linear type systems,
  regions, guide, purely static methods, more
  general exceptions, workflow, flowmark, Barbara
  Liskov, vino,
• All are imperfect fits for this area.

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Conclusion

• Many sources of run-time errors in code
• And were adding new ones with CCured, etc.
• Run-time is only too late if unhandled
• What to do when a run-time error occurs?
• Difficult to specify!
• Lets make it easier to code up.
• Future automatic inference
• For standard libraries and interfaces