51 lines
2.0 KiB
Markdown
51 lines
2.0 KiB
Markdown
# Advanced Debugging Techniques
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## When the Program Hangs
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When a program runs but never returns, that *is* information — something is stuck. Don't guess; interrupt
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and observe.
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```
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Bug: program hangs (infinite loop or deadlock)
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→ dap pause ← interrupt wherever it is (returns OK immediately)
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[the already-blocking debug/continue/step call returns auto-context: location + locals]
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Stopped at process() · worker.py:55, locals: i=99999
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→ dap threads ← are other threads blocked too?
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→ dap eval "lock.locked()" ← test deadlock hypothesis
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Root cause: lock never released. Fix → dap stop.
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```
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Check: is it one thread stuck (infinite loop, blocking I/O), or are multiple threads waiting on each
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other (deadlock)? The location where `pause` stops is your first clue.
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## Digging Into Complex State
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When a variable is opaque or deeply nested, expand it: `dap inspect data --depth 2`.
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## Concurrency Bugs
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If state is wrong but the code path looks correct, consider: is another thread modifying state
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concurrently?
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**First move at any concurrent crash or hang:** run `dap threads`, then inspect every thread's stack with
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`dap thread <id>` — the thread causing the problem is often not the one currently stopped.
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- **Deadlock pattern**: two or more threads each waiting for a resource the other holds. Check thread
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states to confirm.
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- **Race condition**: unexpected values that change between stops. Look for shared mutable state
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accessed without synchronization.
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## Bisecting Loops (Wolf Fence)
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A loop goes wrong at an unknown iteration. Binary search it:
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```
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dap debug app.py --break "app.py:45:i == 500" # midpoint of 1000
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→ dap eval "is_valid(result)" # True → bug is after 500
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→ dap break add "app.py:45:i == 750" # update the condition
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→ dap restart # restart preserving new breakpoint
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```
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~10 iterations to find the bug in 1000. Not 1000 step commands.
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