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Only reseed the internal RNG when a test is first entered

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This fixes multiple issues with random generators, with the most
important one being that multiple nested generators could return
values from the same sequence, due to internal implementation
details of `GENERATE`, and how they interact with test case
paths.

The cost of doing this is that given this simple `TEST_CASE`,
```cpp
TEST_CASE("foo") {
    auto i = GENERATE(take(10, random(0, 100));
    SECTION("A") {
        auto j = GENERATE(take(10, random(0, 100));
    }
    SECTION("B") {
        auto k = GENERATE(take(10, random(0, 100));
    }
}
```

`k` will have different values between running the test as
a whole, e.g. with `./tests "foo"`, and running only the "B"
section with `./tests "foo" -c "B"`.

I consider this an acceptable cost, because the only alternative
would be very messy to implement, and add a lot of brittle and
complex code for relatively little benefit.

If this calculation changes, we will need to instead walk
the current tracker tree whenever a random generator is being
constructed, check for random generators on the path to root,
and take a seed from them.
This commit is contained in:
Martin Hořeňovský 2022-05-17 21:49:17 +02:00
parent 7a2a6c632f
commit dcafc605f3
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GPG Key ID: DE48307B8B0D381A
1 changed files with 33 additions and 2 deletions
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35
src/catch2/internal/catch_run_context.cpp
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@ -193,6 +193,39 @@ namespace Catch {
assert(rootTracker.isSectionTracker());
static_cast<SectionTracker&>(rootTracker).addInitialFilters(m_config->getSectionsToRun());
// We intentionally only seed the internal RNG once per test case,
// before it is first invoked. The reason for that is a complex
// interplay of generator/section implementation details and the
// Random*Generator types.
//
// The issue boils down to us needing to seed the Random*Generators
// with different seed each, so that they return different sequences
// of random numbers. We do this by giving them a number from the
// shared RNG instance as their seed.
//
// However, this runs into an issue if the reseeding happens each
// time the test case is entered (as opposed to first time only),
// because multiple generators could get the same seed, e.g. in
// ```cpp
// TEST_CASE() {
// auto i = GENERATE(take(10, random(0, 100));
// SECTION("A") {
// auto j = GENERATE(take(10, random(0, 100));
// }
// SECTION("B") {
// auto k = GENERATE(take(10, random(0, 100));
// }
// }
// ```
// `i` and `j` would properly return values from different sequences,
// but `i` and `k` would return the same sequence, because their seed
// would be the same.
// (The reason their seeds would be the same is that the generator
// for k would be initialized when the test case is entered the second
// time, after the shared RNG instance was reset to the same value
// it had when the generator for i was initialized.)
seedRng( *m_config );
uint64_t testRuns = 0;
do {
m_trackerContext.startCycle();
@ -422,8 +455,6 @@ namespace Catch {
m_shouldReportUnexpected = true;
m_lastAssertionInfo = { "TEST_CASE"_sr, testCaseInfo.lineInfo, StringRef(), ResultDisposition::Normal };
seedRng(*m_config);
Timer timer;
CATCH_TRY {
if (m_reporter->getPreferences().shouldRedirectStdOut) {