[Feature] Replace LibGit2Sharp with a managed Git implementation (managed reads + git CLI writes)

[gittools-bot]

Description

Replace LibGit2Sharp with a hybrid Git backend: a vendored fully-managed reader for all read/history operations plus git CLI shell-out for writes and network. After this, GitVersion ships zero native binaries.

Supersedes the exploration in arturcic#236. Full design document: docs/design/managed-git-migration.md (landing on next/v7).

Status (2026-07, v7.0 prep)

Phases A, B-pre and B are complete and integrating into next/v7 via #5075 (managed backend opt-in behind GITVERSION_GIT_BACKEND=managed, libgit2 the default). C, E and F remain as post-7.0 work; D was dropped.

Phase	Sub-issue	State
A	#5032	✅ done
B-pre	(folded into #5032)	✅ done
B.1 object store	#5033	✅ done
B.2 refs/discovery	#5034	✅ done
B.3 revwalk/merge-base	#5035	✅ done
B.4 diff/status	#5036	✅ done
B.5 adapter + parity	#5037	✅ done
C — v7.1 default flip	#5039	⏳ post-7.0
D — fixture migration	#5038	⛔ not planned (test-only; ~3× slower for zero product benefit — libgit2 stays in the non-shipped GitVersion.Testing)
E — remove libgit2	#5040	⏳ post-7.0
F — commit-graph (optional)	#5041	⏳ optional

Motivation

• Years of native libgit2 load failures: LibGit2Sharp.Core.NativeMethods cannot be initialized: Unable to load shared library 'git2-7ce88e6' or one of its dependencies #1852, LibGit2Sharp.Core.NativeMethods threw an exception.  #2615, GitVersion.Tool does not find LibGit2 library on Ubuntu 19.04 #1744, GitVersion fails on linux (debian) #1203, GitVersion.CommandLine does not work on Linux #1097, [Bug] No error message has been provided by the native library #2884 — RID/libc mismatches (musl vs glibc, ARM, new OpenSSL) and MSBuild assembly-load fragility.
• LibGit2Sharp.NativeBinaries packs ~12 native binaries into runtimes/<rid>/native/ of every artifact — per TFM in GitVersion.MsBuild.
• Platform and feature lag behind git itself (SHA-256, reftable, new RIDs), pinned to LibGit2Sharp 0.31.0.

Key research findings (2026-07)

• No maintained managed .NET library implements git writes or fetch/clone. Every managed option — Nerdbank.GitVersioning's internal ManagedGit, GitReader, ManagedGitLib — is read-only. NBGV itself keeps libgit2 for write paths.
• NBGV's managed engine achieved >10x faster history walks than libgit2 (Consider using a managed Git backend dotnet/Nerdbank.GitVersioning#505); its MIT pack/idx/delta code is the porting source for our vendored reader.
• The seam is already clean: GitVersion.Core has zero LibGit2Sharp references. Only GitVersion.LibGit2Sharp (adapter), GitVersion.Testing (fixtures), and new-cli's source-linked copy touch it. All write/network operations funnel through a single consumer: GitPreparer (CI normalization: clone/fetch/checkout/ls-remote/ref mutation, username/password HTTPS auth only).
• git on PATH will be required only for CI-normalization/dynamic-repo scenarios; pure version calculation on a prepared checkout needs no git binary at all.

Proposed architecture

As shipped (Phase B.5): the vendored reader and the git-CLI executor both live in src/GitVersion.Git.Managed — it implements the GitVersion.Core/Git/ interfaces directly (mirroring GitVersion.LibGit2Sharp), with the CLI executor as its CommandLine/ folder. The interim standalone GitVersion.Git.CommandLine and a separate thin GitVersion.Git adapter project described below were not kept as separate projects. Backend selection lives at the composition roots via GITVERSION_GIT_BACKEND (GitBackendSelector in GitVersion.Core).

• src/GitVersion.Git.Managed — vendored reader (no Core dependency): pack/idx/delta + loose objects ported from NBGV ManagedGit (MIT, attributed); fresh implementations of the parity-critical pieces: revwalk (topo/time/first-parent with libgit2-identical ordering — version output depends on it), merge-base, tree diff (changed paths), .git/index v2–v4 reading for UncommittedChangesCount, discovery/worktrees/shallow. Hash-agnostic object ids (SHA-256-ready); reftable detected with a clear error initially.
• src/GitVersion.Git.CommandLine — git CLI executor: ArgumentList (no shell), plumbing-only parsing, credentials via per-invocation -c http.<url>.extraHeader, stderr mapped onto existing LockedFileException/401/404 handling, git version probe (≥2.30).
• src/GitVersion.Git — thin adapter implementing IGitRepository/IMutatingGitRepository; DI surface unchanged; new-cli later source-links the read-only subset.

Phasing (each shippable)

Phase	Scope	Est.
A	git-CLI mutator replaces GitRepository.mutating.cs (reads stay libgit2) — retires the fragile native network/SSL/credential paths first	2–3 wk
B-pre	Interface cleanups: Core-owned sort enum, typed CommitFilter, remove public ToGitRepository() leak	3–5 d
B	Managed reader behind GITVERSION_GIT_BACKEND=managed|libgit2; dual-backend CI matrix runs the full integration suites both ways + order-sensitive parity tests + real-world corpus JSON diffing	8–12 wk
C	Default flip to managed; libgit2 escape hatch for one release	1 wk + soak
D	GitVersion.Testing fixtures → pure git-CLI writes (already partially shells out for init/pull/gc)	2–3 wk, parallel
E	Delete LibGit2Sharp projects + package; nupkg assertion test: zero runtimes/**/native/*	1–2 wk
F	Optional: commit-graph accelerator (generation numbers for topo/merge-base)	2–3 wk

~4–5 months elapsed to a LibGit2Sharp-free GitVersion.

Top risks (mitigations in the design doc)

• Revwalk ordering parity with libgit2 (port the algorithm precisely; order-sensitive dual-backend tests; corpus diffing gates the default flip)
• Merge-base candidate selection on criss-cross merges
• Shallow-clone boundaries (the CI default) and worktree/commondir resolution
• Fixture suite slowdown from process spawns (batch with git fast-import if needed)