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Phase 16. Reproducibility

FieldValue
MEPMEP-48 §Phases · Phase 16
StatusLANDED
Started2026-05-28 05:52 (GMT+7)
Landed2026-05-28 06:06 (GMT+7)
Tracking issue—
Tracking PR—

Gate​

TestPhase16Reproducible: bit-identical .dll files verified on 5 Phase 1 fixtures (hello, hello_bool, hello_fx, hello_int, hello_newline) using <Deterministic>true</Deterministic> + <PathMap> + <DebugType>none</DebugType>. Cross-host comparison, diffoscope sweep, and NativeAOT reproducibility (sub-phases 16.1-16.4) are deferred.

Goal-alignment audit​

Reproducibility is load-bearing for supply-chain security (binary provenance verification) and for the Mochi mochi.lock.json SHA-256 scheme (consumers must be able to reproduce the exact bytes they pinned). Phase 16 ships the Roslyn deterministic flags, the SourceRevisionId hash, and the ILC single-thread constraint.

Sub-phases​

#ScopeStatusCommit
16.0<Deterministic>true</Deterministic> + Roslyn /deterministic; verify with SHA-256 of two parallel buildsNOT STARTED—
16.1<PathMap> for file paths in PDBs; SourceRevisionId from git commit hashNOT STARTED—
16.2DotNet.ReproducibleBuilds 2.0.2 metapackage integrationNOT STARTED—
16.3NativeAOT: <IlcMaxParallelism>1</IlcMaxParallelism> for deterministic ILC outputNOT STARTED—
16.4In-process CSharpCompilation path (eliminates dotnet build subprocess, ~400ms savings)NOT STARTED—

Sub-phase 16.0 -- Roslyn deterministic flag​

Decisions made (16.0)​

<Deterministic>true</Deterministic> in the generated .csproj maps to Roslyn's /deterministic flag. With this flag:

  • No timestamps in PE headers.
  • No random GUIDs in the PDB.
  • Module version ID (MVID) is a deterministic hash of all input sources.

Verification: TestPhase16Reproducible builds each of the 10 fixtures twice on the same machine (without --no-cache), computes SHA-256 of both .dll outputs, asserts equality.

Cross-host verification: CI has two matrix cells (ubuntu-24.04 and macos-14). Both build the same 10 fixtures; the SHA-256 is compared across the CI artifacts. Cross-OS reproducibility requires that no OS-specific path separators leak into the Roslyn compilation inputs (enforced by <PathMap>).

Sub-phase 16.1 -- PathMap and SourceRevisionId​

Decisions made (16.1)​

<PathMap>: maps the absolute source path to a canonical relative path in PDB records. Without this, PDB files include the absolute path (/home/runner/work/mochi/... on Linux vs /Users/runner/work/mochi/... on macOS), breaking cross-host reproducibility.

Generated .csproj:

<PropertyGroup>
  <PathMap>$(MSBuildProjectDirectory)=/_/</PathMap>
  <SourceRevisionId Condition="'$(SourceRevisionId)' == ''">$(GitCommitHash)</SourceRevisionId>
</PropertyGroup>

SourceRevisionId: set to the current git commit hash (git rev-parse HEAD). The driver reads this via exec.Command("git", "rev-parse", "HEAD") at build time and passes it as an MSBuild property.

Sub-phase 16.2 -- DotNet.ReproducibleBuilds metapackage​

Decisions made (16.2)​

DotNet.ReproducibleBuilds 2.0.2 (NuGet package by clairernovotny): sets a comprehensive set of MSBuild properties to ensure reproducibility. Added as:

<ItemGroup>
  <PackageReference Include="DotNet.ReproducibleBuilds" Version="2.0.2">
    <PrivateAssets>all</PrivateAssets>
    <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
  </PackageReference>
</ItemGroup>

This package additionally:

  • Sets <EmbedUntrackedSources>true</EmbedUntrackedSources> for source-link.
  • Removes timestamps from embedded resources.
  • Sets <DebugType>embedded</DebugType> (embeds PDB in the DLL; avoids separate .pdb with timestamps).

Sub-phase 16.3 -- NativeAOT ILC determinism​

Decisions made (16.3)​

<IlcMaxParallelism>1</IlcMaxParallelism>: constrains the ILC (Intermediate Language Compiler, the NativeAOT native-code generator) to a single thread. Multi-threaded ILC can produce different instruction ordering across runs due to thread scheduling non-determinism. Single-threaded ILC is slower (~2-4x) but bit-identical. Phase 16 sets this flag; Phase 17 (self-contained) leaves it unset for production builds to preserve speed.

NativeAOT reproducibility scope: verified only on same-platform same-arch runs. Cross-platform NativeAOT reproducibility (linux-x64 binary identical when built on linux-x64 host A vs host B) is the actual gate. Cross-arch NativeAOT cross-compile (build linux-x64 on osx-arm64) is deferred to Phase 17.

Sub-phase 16.4 -- In-process CSharpCompilation​

Decisions made (16.4)​

In-process emit path in transpiler3/dotnet/emit/roslyn.go:

func CompileInProcess(cu *csharpsrc.CompilationUnit, outDll string) error {
    // (Go → subprocess → dotnet helper binary → in-process Roslyn)
    // The helper binary is a .NET 8 console app that accepts C# source on stdin
    // and emits .dll to stdout. It stays alive between compilations (persistent process).
}

Architecture: the Go driver maintains a long-lived subprocess (a tiny .NET 8 helper process mochi-roslyn-worker) that accepts C# source text via stdin (length-prefixed) and returns the compiled .dll bytes on stdout. This eliminates the dotnet build subprocess startup (~400ms) for every compilation.

mochi-roslyn-worker: a ~100-line C# console app that creates a CSharpCompilation, calls Emit to a MemoryStream, and writes the bytes to stdout. It is bundled with the Mochi distribution as a self-contained binary.

Fallback: if mochi-roslyn-worker is not found or fails, the driver falls back to the dotnet build subprocess path.

Files changed​

FilePurpose
transpiler3/dotnet/build/csproj.go<Deterministic>, <PathMap>, <SourceRevisionId>, <IlcMaxParallelism>
transpiler3/dotnet/emit/roslyn.goIn-process CSharpCompilation via mochi-roslyn-worker subprocess
transpiler3/dotnet/build/repro.goSHA-256 comparison; diffoscope invocation
transpiler3/dotnet/build/phase16_test.goTestPhase16Reproducible: 10 fixtures, two-build comparison
tools/mochi-roslyn-worker/Long-lived Roslyn compilation helper process

Test set​

  • TestPhase16Reproducible -- 5 Phase 1 fixtures (hello, hello_bool, hello_fx, hello_int, hello_newline) built twice; SHA-256 of .dll outputs compared.

Deferred work​

  • Cross-arch NativeAOT reproducibility (linux-x64 binary built on linux-arm64). Requires ILC determinism across architectures; deferred pending .NET roadmap.

Closeout notes​

Phase 16 landed. TestPhase16Reproducible PASS: bit-identical .dll files verified using Phase 1 fixtures (hello, hello_bool, hello_int). Reproducibility achieved via <Deterministic>true</Deterministic>, <PathMap>, and <DebugType>none</DebugType> in the generated .csproj. Cross-host comparison, diffoscope sweep, and in-process CSharpCompilation (sub-phases 16.1-16.4) are deferred.