git: Always use OpenSSL on win32

By default, libssh2 uses Windows Cryptography Next Generation when targeting
win32. The wincng backend does not support ED25519, which is a widely-used
algorithm among git hosting services, and in some cases may be the only option
on remotes in certain configurations.

This change necessitates Windows users building jj to install openssl via vcpkg
for the target triple x64-windows-static-md when building for MSVC, or otherwise
have OpenSSL available at build time. This should also generally work when
building for MinGW, though I did not try it.

Fixes martinvonz#3322

CHANGELOG.md

@@ -15,6 +15,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * The default template alias `builtin_log_root(change_id: ChangeId, commit_id: CommitId)` was replaced by `format_root_commit(root: Commit)`.
 
+* When targeting Windows, libssh2 will use OpenSSL rather than wincng. This fixes
+  interacting with Git+SSH hosts using ED25519 keys, but Windows developers will
+  need to provide OpenSSL at build time.
+
 ### New features
 
 * The list of conflicted paths is printed whenever the working copy changes.
@@ -64,6 +68,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   literals. This means that `snapshot.max-new-file-size="1"` and
   `snapshot.max-new-file-size=1` are now equivalent.
 
+* ED25519 host keys are now supported correctly when connecting to Git+SSH hosts
+  on Windows.
+
 ## [0.16.0] - 2024-04-03
 
 ### Deprecations

Cargo.toml

@@ -60,6 +60,7 @@ indoc = "2.0.4"
 insta = { version = "1.38.0", features = ["filters"] }
 itertools = "0.12.1"
 libc = { version = "0.2.153" }
+libssh2-sys = { version = "0.3.0", features = ["openssl-on-win32"] }
 maplit = "1.0.2"
 minus = { version = "5.6.1", features = ["dynamic_output", "search"] }
 num_cpus = "1.16.0"

docs/windows.md

@@ -92,4 +92,20 @@ Mode. If those conditions are not satisfied, `jj` will materialize symlinks as
 ordinary files.
 
 For colocated repositories, Git support must also be enabled using the
-`git config` option `core.symlinks=true`.
+`git config` option `core.symlinks=true`.
+
+## Building jj on Windows
+
+By default, `jj` will build with `libssh2` using OpenSSL as its cryptography
+backend. This is because libssh2's wincng backend does not support the ED25519
+algorithm, preventing Git+SSH remotes using ED25519 keys from working.
+
+The easiest way to provide this is to install it via [vcpkg].
+
+```bash
+VCPKG_ROOT=path/to/your/vcpkg_installation
+vcpkg install --triplet=x64-windows-static-md openssl
+cargo build --release
+```
+
+[vcpkg]: https://vcpkg.io/

lib/Cargo.toml

@@ -49,6 +49,7 @@ hex = { workspace = true }
 ignore = { workspace = true }
 itertools = { workspace = true }
 jj-lib-proc-macros = { workspace = true }
+libssh2-sys = { workspace = true }
 maplit = { workspace = true }
 once_cell = { workspace = true }
 pest = { workspace = true }