Testability Skill | Agent Skills

Testability

Philosophy

Code that's hard to test is telling you something about its design. Coverage gaps are opportunities to improve architecture, not problems to silence.

Fix the design, don't silence the messenger.

Refactoring Patterns

1. Extract Pure Logic from I/O

// BEFORE: Hard to test
res_t process_config(const char *path) {
    char *content = read_file(path);
    // ... parsing logic ...
}

// AFTER: Pure logic extracted
res_t parse_config(const char *content, config_t *out);  // Testable
res_t load_config(const char *path, config_t *out) {     // Thin wrapper
    char *content = read_file(path);
    return parse_config(content, out);
}

2. Infallible Functions → void

If a function cannot fail, don't pretend it can:

// BEFORE: Fake error path (untestable)
res_t init_defaults(config_t *c) {
    c->timeout = 30;
    return OK_RES;
}

// AFTER: Honest signature
void init_defaults(config_t *c) {
    c->timeout = 30;
}

3. OOM Checks → Single-Line PANIC

When allocations PANIC on failure, downstream checks become unreachable:

// BEFORE: 3 lines, 2 exclusions
result_t res = allocate_something();
if (is_err(&res)) {                // LCOV_EXCL_LINE
    return res;                     // LCOV_EXCL_LINE
}

// AFTER: 1 line, 1 exclusion
result_t res = allocate_something();
if (is_err(&res)) PANIC("allocation failed"); // LCOV_EXCL_BR_LINE

4. Unreachable Else → PANIC

// BEFORE: Unreachable else
if (condition_always_true) {
    handle();
} else {
    return ERR(...);  // LCOV_EXCL_LINE
}

// AFTER: Assert invariant
if (condition_always_true) {
    handle();
} else {
    PANIC("Invariant violated");  // LCOV_EXCL_BR_LINE
}

5. Reduce Conditional Complexity

// BEFORE: Deep nesting
if (a) {
    if (b) {
        if (c) { /* action */ }
    }
}

// AFTER: Early returns
if (!a) return;
if (!b) return;
if (!c) return;
// action

6. Parameterize Behavior

// BEFORE: Hardcoded, can't test timeout
void wait_for_response(void) {
    sleep(30);
}

// AFTER: Testable
void wait_for_response(int timeout_sec) {
    sleep(timeout_sec);
}

7. Wrap Vendor Functions

When vendor functions have inline branches we can't cover:

// BEFORE: Can't test yyjson_doc_get_root returning NULL
yyjson_val *root = yyjson_doc_get_root(doc);
if (!root) return ERR(...);  // "Can't happen"

// AFTER: Wrapper allows mocking
yyjson_val *root = yyjson_doc_get_root_(doc);  // Wrapped
if (!root) return ERR(...);  // Testable with mock

When to Refactor vs Mock

| Situation | Action | |-----------|--------| | Our code is complex | Refactor | | External library call | Mock via wrapper | | System call | Mock via wrapper |

Rule: Refactor our code, mock external dependencies.

Related Skills

coverage - Policy (90% requirement)
lcov - Finding specific gaps
mocking - Testing external dependencies

Agent Skills: Testability

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