101 lines
4.2 KiB
Markdown
101 lines
4.2 KiB
Markdown
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# Mutation testing
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The unit tests tell us the library works. **Mutation testing tells us the tests
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work** — that they would actually fail if the library were broken.
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`scripts/mutation_test.py` deliberately breaks the library in small ways
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("mutants"), one at a time, and runs the whole CTest suite against each broken
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copy:
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* if the tests **fail**, the mutant is **killed** — good, the suite caught it;
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* if the tests still **pass**, the mutant **survived** — a bug of that shape
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would slip through, so it points at a missing test.
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The **mutation score** is `killed / (killed + survived)`. A surviving mutant is
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a to-do item: write a test that distinguishes the mutant from the original.
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## Running
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No third-party tools are required — just Python 3 and the normal
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cmake/ctest toolchain. The harness never touches your working tree; it copies
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the repo to a scratch directory and mutates the copy.
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```sh
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# Default: mutate src/error.c and include/akerror.tmpl.h
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scripts/mutation_test.py
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# Faster: just the C source
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scripts/mutation_test.py --target src/error.c
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# See what would run without building anything
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scripts/mutation_test.py --target src/error.c --list
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# Gate CI: exit non-zero if the score drops below 90%
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scripts/mutation_test.py --threshold 90
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```
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Via CMake (configures a build first if needed):
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```sh
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cmake --build build --target mutation
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```
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Useful flags: `--timeout SECONDS` (per-suite build+test cap; a mutant that
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hangs is counted as killed), `--keep` (retain the scratch copy for debugging),
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`--work DIR` (use a specific scratch directory).
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## Mutation operators
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Each mutant changes exactly one location by one of:
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| Tag | Operator | Example |
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|-----|--------------------------------|----------------------------------|
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| ROR | relational operator | `==` → `!=`, `<` → `<=`, `>=` → `>` |
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| LCR | logical connector | `&&` → `\|\|` |
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| BCR | boolean constant | `true` → `false` |
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| AOR | arithmetic / compound assign | `+` → `-`, `+=` → `-=` |
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| ICR | integer literal | `0` → `1`, `1` → `0` |
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| SDL | statement deletion | `err->refcount += 1;` → *(removed)* |
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Preprocessor control lines, comments, and the block of error-code / buffer-size
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`#define`s are skipped: mutating those produces equivalent or uninteresting
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mutants that only add noise.
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## Interpreting survivors
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Not every survivor is a test gap — some mutants are **equivalent** (they don't
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change observable behaviour, e.g. resizing an internal scratch buffer). For each
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survivor, decide:
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1. **Real gap** → add or strengthen a test in `tests/` so the mutant is killed,
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then re-run.
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2. **Equivalent mutant** → no test can catch it; leave a note. If a specific
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line is a persistent source of equivalents, narrow the target with
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`--target` or extend the skip rules in `scripts/mutation_test.py`.
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Re-run after adding tests and confirm the score went up.
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## Current status
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`src/error.c` scores ~71% (the CI gate is set to 65% for headroom). The
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remaining survivors are dominated by:
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* **Equivalent mutants** in `akerr_init`: deleting the `memset`/`NULL` setup of
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file-scope statics (`AKERR_ARRAY_ERROR`, `__akerr_last_ditch`,
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`__akerr_last_ignored`) changes nothing, because C already zero-initializes
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objects with static storage duration. `int oldid = 0;` → `1` is likewise
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dead: it is overwritten before use.
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* **Default logger / handler internals** (`vfprintf`, `va_end`, the
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`errctx == NULL` branch, `exit(1)`): killing these needs a subprocess-based
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test that captures a child's stderr and exit code, rather than the in-process
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capturing logger the other tests use.
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Findings worth noting (surfaced by mutation testing, not yet fixed):
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* `AKERR_MAX_ERR_VALUE` is `AKERR_LAST_ERRNO_VALUE + 15`, but `AKERR_NOT_IMPLEMENTED`
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(+16) and `AKERR_BADEXC` (+17) exceed it. `akerr_name_for_status` rejects any
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status `> AKERR_MAX_ERR_VALUE`, so those two codes can never store or return a
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name — the `akerr_name_for_status(AKERR_BADEXC, ...)` call in `akerr_init` is
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dead code (which is why deleting it survives). Bumping `AKERR_MAX_ERR_VALUE` to
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`+ 17` would fix it.
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