parking_lot_core::parking_lot::HashTable::new crash in [@ <T>::from_entropy::{{closure}}]

[cpeterso]

Firefox on Android and Linux is hitting a rare parking_lot panic (just 590 crash reports over the last six months): https://bugzilla.mozilla.org/show_bug.cgi?id=1584043

FromEntropy::from_entropy() failed: All entropy sources failed (permanently unavailable); cause: OS RNG not yet seeded (not ready yet); cause: Try again (os error 11)

We've seen similar RNG errors in Firefox on Android and Linux tests when was unable to open /dev/urandom because we hit some fd limit. 95% of these crash reports are from Android. 5% are from Linux.

To avoid Windows RNG error crashes, @Manishearth changed Stylo's hashmaps use FNV hash instead of an RNG/cryptographically secure hasher since hashmap DOS is not a problem for browsers:

rust-random/rand#180 (comment)

parking_lot could probably do the same for its HashTables. We probably don't have enough threads in the parking_lot for hashtable DOS to be a risk. Threads are heavier weight than a big hashtable.

OTOH, if the OS RNG is failing in parking_lot code and we sidestep this panic, some other code will probably fail reading the OS RNG soon after.

Partial stack trace from a Firefox for Android crash report: https://crash-stats.mozilla.com/report/index/5742d966-7264-4325-90e2-3f0760190926#tab-details

0 libxul.so GeckoCrash toolkit/xre/nsAppRunner.cpp:5162
1 libxul.so gkrust_shared::panic_hook toolkit/library/rust/shared/lib.rs:246
2 libxul.so core::ops::function::Fn::call src/libcore/ops/function.rs:69
3 libxul.so std::panicking::rust_panic_with_hook src/libstd/panicking.rs:478
4 libxul.so std::panicking::continue_panic_fmt src/libstd/panicking.rs:381
5 libxul.so std::panicking::begin_panic_fmt src/libstd/panicking.rs:336
6 libxul.so <R as rand::FromEntropy>::from_entropy::{{closure}} hg:hg.mozilla.org/releases/mozilla-beta:third_party/rust/parking_lot_core/<::std::macros::panic macros>:fbd37d14e8976c336816d21137e8239811265630:8
7 libxul.so parking_lot_core::parking_lot::HashTable::new src/libcore/result.rs:764
8 libxul.so parking_lot_core::parking_lot::ThreadData::new third_party/rust/parking_lot_core/src/parking_lot.rs:143
9 libxul.so parking_lot::raw_rwlock::RawRwLock::lock_upgradable_slow third_party/rust/parking_lot/src/raw_rwlock.rs:740
10 libxul.so style::rule_tree::StrongRuleNode::ensure_child servo/components/style/rule_tree/mod.rs:1152
...

[Amanieu]

Have you tried upgrading parking_lot? Newer versions don't use rand any more.

[cpeterso]

Firefox was using parking_lot 0.8.0 and just updated to 0.9.0 last month, which includes your rand changes. I'll close this bug. Thanks!

https://bugzilla.mozilla.org/show_bug.cgi?id=1575008