Commits on Jul 29, 2023

1. build: LIBGIT2_NO_VENDOR to force to use system libgit2 …

   Specify `LIBGIT2_NO_VENDOR` to force to use system libgit2.
   
   Due to the additive nature of Cargo features, if some crate in the
   dependency graph activates `vendored` feature, there is no way to
   revert it back. This env var serves as a workaround for this purpose.
   
   An alternative is having `no_vendored` feature. We still need to
   modify `build.rs` to make both `vendored` and `no_vendored` play
   nice with each other (or bail out as they are mutual-exclusive).
   However, there is no way to activate a Cargo feature via environment
   variable (see rust-lang/cargo#4829). Altering environment variables
   may be the only way to interact with some external build systems.
   
   It is also pretty common that people don't want to vendor anything and
   rather to see the build fail than vendoring.
   
   Some prior arts:
   
   * `OPENSSL_NO_VENDOR` in `openssl` crate, which serves the exact
     purpose of `LIBGIT2_NO_VENDOR` [^1].
   * Crate `ssh2` has a similar `LIBSSH2_SYS_USE_PKG_CONFIG`, but it
     doesn't force to do so. It still falls back to vendored lib [^2].
   * Crate `curl` has a feature `force-system-lib-on-osx` to trump all
     others features. It was created primarily for Rust releases [^3].
   
   More motivations why this is something we want:
   
   * We have no control over transitive dependencies that accidentally
     activate the vendored feature. And there is no way to turn it off.
   * If the build environment has the wrong system libgit2, we want
     build to always fail because that is a real mistake. For example,
     libgit2 expects a range 1.6.4..1.7.0 and it silently goes vendoring
     if nothing found. We may want an explicit error like "your system
     needs a newer libgit2 to proceed".
   
   [^1]: https://github.com/sfackler/rust-openssl/blob/50787ed35bf9efa9dd3cbb1993a2564014b67489/openssl/src/lib.rs#L65
   [^2]: https://github.com/alexcrichton/ssh2-rs/blob/d9a1dfac4b8c09c5437eb477606b82aa4f67b092/libssh2-sys/build.rs#L22-L33
   [^3]: https://github.com/alexcrichton/curl-rust/blob/431babf1dffe205641793353d3d57fdd36fe8534/curl-sys/build.rs#L15-L20

   

   weihanglo committed Jul 29, 2023
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