lduchosal / ipnetwork
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coverage: 94.069% (+0.06%) from 94.011%
1009

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fix: publish the broadcast cache atomically (#429) (#436)

Contains() could silently return a wrong result under concurrency.

The broadcast cache was a BigInteger?, a 24 byte struct spanning three
fields (hasValue, BigInteger._sign and BigInteger._bits), read on a
lock-free fast path. Neither the read nor the write of such a struct is
atomic, so a reader could observe hasValue == true while the magnitude
was still half written, and compute a broadcast address of 0 or 1
instead of the real one. Contains() then answered false for addresses
that are in the network.

Measured on arm64: the torn state stayed observable for 7 to 19 us, not
a few nanoseconds, because the publishing thread can be descheduled
between the individual stores. Every reader taking the fast path during
that window gets a wrong answer.

The cache is now a volatile StrongBox<BigInteger>: a reference is
published by a single atomic store, and volatile orders that store
after the box is fully initialised. The double-checked lock is dropped
because CreateBroadcast is pure, so a racing duplicate computation is
harmless and cheaper than serialising every first access. That also
removes the per-instance sync object, one allocation less for every
IPNetwork2 ever created.

Tests: two race detectors plus one deterministic structural guard. The
race detectors only reproduce on weakly ordered multi-core machines, so
the structural guard is what protects the invariant on CI: it fails if
the cache ever goes back to a multi-word value type.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

2284 of 2428 relevant lines covered (94.07%)

487916.24 hits per line

Relevant lines Covered
2428 RELEVANT LINES 2284 COVERED LINES
487916.24 HITS PER LINE
Source Files on master

Recent builds

Builds Branch Commit Type Ran Committer Via Coverage
1009 master fix: publish the broadcast cache atomically (#429) (#436) Contains() could silently return a wrong result under concurrency. The broadcast cache was a BigInteger?, a 24 byte struct spanning three fields (hasValue, BigInteger._sign and BigInteger... push web-flow appveyor
94.07
32107531712 master fix: publish the broadcast cache atomically (#429) (#436) Contains() could silently return a wrong result under concurrency. The broadcast cache was a BigInteger?, a 24 byte struct spanning three fields (hasValue, BigInteger._sign and BigInteger... push web-flow travis-pro
93.58
1008 fix/429-broadcast-cache-atomic-publication Merge 2f3d26a00 into 5b1456b0a Pull #436 web-flow appveyor
94.07
32105822453 fix/429-broadcast-cache-atomic-publication Merge 2f3d26a00 into 5b1456b0a Pull #436 web-flow travis-pro
93.58
1007 master Merge pull request #435 from lduchosal/dependabot/nuget/src/TestProject/multi-40646b1c30 Bump Microsoft.CodeCoverage and Microsoft.NET.Test.Sdk push web-flow appveyor
94.01
32005216573 master Merge pull request #435 from lduchosal/dependabot/nuget/src/TestProject/multi-40646b1c30 Bump Microsoft.CodeCoverage and Microsoft.NET.Test.Sdk push web-flow travis-pro
93.52
1006 dependabot/nuget/src/TestProject/multi-40646b1c30 Merge ecb73bb06 into 383cdcaea Pull #435 web-flow appveyor
94.01
32004739097 dependabot/nuget/src/TestProject/multi-40646b1c30 Merge ecb73bb06 into 383cdcaea Pull #435 web-flow travis-pro
93.52
32004684711 dependabot/nuget/src/TestProject/Microsoft.CodeCoverage-18.9.0 Merge acff1b798 into 383cdcaea Pull #434 web-flow travis-pro
93.52
1004 master Merge pull request #433 from lduchosal/dependabot/nuget/dot-config/dotnet-coverage-18.10.0 Bump dotnet-coverage from 18.9.0 to 18.10.0 push web-flow appveyor
94.01
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