build: Add the parallel feature, set -j arg based on NUM_JOBS

protobuf-src/Cargo.toml

@@ -31,3 +31,7 @@ links = "protobuf-src"
 
 [build-dependencies]
 cmake = "0.1.50"
+
+[features]
+default = ["parallel"]
+parallel = []

protobuf-src/build.rs

@@ -16,7 +16,8 @@
 use std::error::Error;
 
 fn main() -> Result<(), Box<dyn Error>> {
-    let install_dir = cmake::Config::new("protobuf")
+    let mut build_config = cmake::Config::new("protobuf");
+    build_config
         .define("ABSL_PROPAGATE_CXX_STD", "ON")
         .define("protobuf_BUILD_TESTS", "OFF")
         .define("protobuf_DEBUG_POSTFIX", "")
@@ -25,8 +26,26 @@ fn main() -> Result<(), Box<dyn Error>> {
         // want a stable location that we can add to the linker search path.
         // Since we're not actually installing to /usr or /usr/local, there's no
         // harm to always using "lib" here.
-        .define("CMAKE_INSTALL_LIBDIR", "lib")
-        .build();
+        .define("CMAKE_INSTALL_LIBDIR", "lib");
+
+    #[cfg(feature = "parallel")]
+    {
+        // The `cmake` crate does not enable parallelism for Makefile backed builds, instead it
+        // tries to parallelize the build jobs with Cargo's jobserver. This doesn't seem to work
+        // very well and manually specifying the `-j` flag based on the value from `NUM_JOBS` more
+        // effectively parallelizes the build. Also, using `NUM_JOBS` is recommended by the Cargo
+        // documentation for this purpose.
+        //
+        // Cargo Docs: <https://doc.rust-lang.org/cargo/reference/environment-variables.html>
+        let maybe_num_jobs: Result<_, Box<dyn Error>> = std::env::var("NUM_JOBS")
+            .map_err(|err| Box::new(err).into())
+            .and_then(|val| val.parse::<usize>().map_err(|err| Box::new(err).into()));
+        if let Ok(num_jobs) = maybe_num_jobs {
+            build_config.build_arg(format!("-j{num_jobs}"));
+        }
+    }
+
+    let install_dir = build_config.build();
 
     println!("cargo:rustc-env=INSTALL_DIR={}", install_dir.display());
     println!("cargo:CXXBRIDGE_DIR0={}/include", install_dir.display());