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debug-patterns

Common defect patterns and debugging strategies.

Common Defect Patterns

1. Null/Undefined Reference Errors

Pattern: Accessing properties or methods on null/undefined objects Indicators:

  • TypeError: Cannot read property 'X' of undefined
  • NullPointerException
  • Segmentation fault

Common Causes:

  • Missing null checks
  • Asynchronous data not yet loaded
  • Optional dependencies not injected
  • Incorrect initialization order

Detection Strategy:

  • Add defensive null checks
  • Use optional chaining (?.)
  • Initialize with safe defaults
  • Validate inputs at boundaries

2. Race Conditions

Pattern: Multiple threads/processes accessing shared resources Indicators:

  • Intermittent failures
  • Works in debug but fails in production
  • Order-dependent behavior
  • Data corruption

Common Causes:

  • Missing synchronization
  • Incorrect lock ordering
  • Shared mutable state
  • Async operations without proper await

Detection Strategy:

  • Add logging with timestamps
  • Use thread-safe data structures
  • Implement proper locking mechanisms
  • Review async/await usage

3. Memory Leaks

Pattern: Memory usage grows over time without release Indicators:

  • Increasing memory consumption
  • Out of memory errors
  • Performance degradation over time
  • GC pressure

Common Causes:

  • Event listeners not removed
  • Circular references
  • Large objects in closures
  • Cache without eviction

Detection Strategy:

  • Profile memory usage
  • Review object lifecycle
  • Check event listener cleanup
  • Implement cache limits

4. Off-by-One Errors

Pattern: Incorrect loop boundaries or array indexing Indicators:

  • ArrayIndexOutOfBounds
  • Missing first/last element
  • Infinite loops
  • Fence post errors

Common Causes:

  • Confusion between length and last index
  • Inclusive vs exclusive ranges
  • Loop condition errors
  • Zero-based vs one-based indexing

Detection Strategy:

  • Review loop conditions carefully
  • Test boundary cases
  • Use forEach/map when possible
  • Add assertions for array bounds

5. Type Mismatches

Pattern: Incorrect data types passed or compared Indicators:

  • Type errors at runtime
  • Unexpected coercion behavior
  • Failed validations
  • Serialization errors

Common Causes:

  • Weak typing assumptions
  • Missing type validation
  • Incorrect type conversions
  • API contract violations

Detection Strategy:

  • Add runtime type checking
  • Use TypeScript/type hints
  • Validate at API boundaries
  • Review type coercion rules

6. Resource Exhaustion

Pattern: Running out of system resources Indicators:

  • Too many open files
  • Connection pool exhaustion
  • Thread pool starvation
  • Disk space errors

Common Causes:

  • Resources not properly closed
  • Missing connection pooling
  • Unbounded growth
  • Inadequate limits

Detection Strategy:

  • Implement try-finally blocks
  • Use connection pooling
  • Set resource limits
  • Monitor resource usage

7. Concurrency Deadlocks

Pattern: Threads waiting for each other indefinitely Indicators:

  • Application hangs
  • Threads in BLOCKED state
  • No progress being made
  • Timeout errors

Common Causes:

  • Circular wait conditions
  • Lock ordering violations
  • Nested synchronized blocks
  • Resource starvation

Detection Strategy:

  • Always acquire locks in same order
  • Use lock-free data structures
  • Implement timeout mechanisms
  • Avoid nested locks

8. SQL Injection Vulnerabilities

Pattern: Unvalidated input in SQL queries Indicators:

  • Unexpected database errors
  • Data breaches
  • Malformed query errors
  • Authorization bypasses

Common Causes:

  • String concatenation for queries
  • Missing input validation
  • Inadequate escaping
  • Dynamic query construction

Detection Strategy:

  • Use parameterized queries
  • Validate all inputs
  • Review dynamic SQL
  • Implement least privilege

9. Infinite Recursion

Pattern: Function calling itself without termination Indicators:

  • Stack overflow errors
  • Maximum call stack exceeded
  • Application crashes
  • Memory exhaustion

Common Causes:

  • Missing base case
  • Incorrect termination condition
  • Circular dependencies
  • Mutual recursion errors

Detection Strategy:

  • Review base cases
  • Add recursion depth limits
  • Test edge cases
  • Use iteration when possible

10. Cache Invalidation Issues

Pattern: Stale data served from cache Indicators:

  • Outdated information displayed
  • Inconsistent state
  • Changes not reflected
  • Data synchronization issues

Common Causes:

  • Missing invalidation logic
  • Incorrect cache keys
  • Race conditions in updates
  • TTL too long

Detection Strategy:

  • Review invalidation triggers
  • Implement cache versioning
  • Use appropriate TTLs
  • Add cache bypass for testing

Anti-Patterns to Avoid

1. Shotgun Debugging

Making random changes hoping something works

2. Blame the Compiler

Assuming the problem is in the framework/language

3. Programming by Coincidence

Not understanding why a fix works

4. Copy-Paste Solutions

Using solutions without understanding them

5. Ignoring Warnings

Dismissing compiler/linter warnings

Debugging Best Practices

1. Systematic Approach

  • Reproduce consistently
  • Isolate the problem
  • Form hypotheses
  • Test systematically

2. Use Scientific Method

  • Observe symptoms
  • Form hypothesis
  • Design experiment
  • Test and validate

3. Maintain Debug Log

  • Document what you tried
  • Record what worked/failed
  • Note patterns observed
  • Track time spent

4. Leverage Tools

  • Debuggers
  • Profilers
  • Static analyzers
  • Log aggregators

5. Collaborate

  • Pair debugging
  • Code reviews
  • Knowledge sharing
  • Post-mortems