Overhaul vendoring/linking features

This change overhauls the vendoring & static linking features that the
library exposes. Please refer to the larger discussion [0] for
additional context, but in short we introduce separate features for
vendoring/linking each of the three system library dependencies: libbpf,
libelf, zlib. That is to say, conceptually we fully decouple

[0] libbpf#64 (comment)

Signed-off-by: Daniel Müller <deso@posteo.net>

Cargo.toml

@@ -36,6 +36,25 @@ num_cpus = "^1.16.0"
 crate-type = ["lib", "staticlib"]
 
 [features]
+default = ["vendored-libbpf"]
+# Don't vendor anything. This feature is deprecated. Simply build without
+# any features instead.
 novendor = []
-static = []
-vendored = ["static"]
+# Meta-feature to use vendored versions of all dependencies.
+vendored = ["vendored-libbpf", "vendored-libelf", "vendored-zlib"]
+# Use vendored `libbpf`. Implies linking it statically.
+vendored-libbpf = ["static-libbpf"]
+# Use vendored `libelf`. Implies linking it statically.
+vendored-libelf = ["static-libelf"]
+# Use vendored `zlib`. Implies linking it statically.
+vendored-zlib = ["static-zlib"]
+# Meta-feature to link against all dependencies statically.
+static = ["static-libbpf", "static-libelf", "static-zlib"]
+# Link libbpf statically.
+static-libbpf = []
+# Link libelf statically. Implies linking libbpf statically, because libbpf is
+# the libelf consumer.
+static-libelf = ["static-libbpf"]
+# Link zlib statically. Implies linking libbpf statically, because libbpf is
+# the libelf consumer.
+static-zlib = ["static-libbpf"]

build.rs

@@ -80,20 +80,10 @@ fn generate_bindings(src_dir: path::PathBuf) {
 #[cfg(not(feature = "bindgen"))]
 fn generate_bindings(_: path::PathBuf) {}
 
-#[cfg(feature = "static")]
-fn library_prefix() -> String {
-    "static=".to_string()
-}
-
-#[cfg(not(feature = "static"))]
-fn library_prefix() -> String {
-    "".to_string()
-}
-
 fn pkg_check(pkg: &str) {
     if process::Command::new(pkg).status().is_err() {
         panic!(
-            "{} is required to compile libbpf-sys using the vendored copy of libbpf",
+            "{} is required to compile libbpf-sys with the selected set of features",
             pkg
         );
     }
@@ -104,17 +94,42 @@ fn main() {
 
     generate_bindings(src_dir.clone());
 
+    let vendored_libbpf = cfg!(feature = "vendored-libbpf");
+    let vendored_libelf = cfg!(feature = "vendored-libelf");
+    let vendored_zlib = cfg!(feature = "vendored-zlib");
+    println!("Using feature vendored-libbpf={}", vendored_libbpf);
+    println!("Using feature vendored-libelf={}", vendored_libelf);
+    println!("Using feature vendored-zlib={}", vendored_zlib);
+
+    let static_libbpf = cfg!(feature = "static-libbpf");
+    let static_libelf = cfg!(feature = "static-libelf");
+    let static_zlib = cfg!(feature = "static-zlib");
+    println!("Using feature static-libbpf={}", static_libbpf);
+    println!("Using feature static-libelf={}", static_libelf);
+    println!("Using feature static-zlib={}", static_zlib);
+
     if cfg!(feature = "novendor") {
-        println!("cargo:rustc-link-lib={}bpf\n", library_prefix());
+        println!("cargo:warning=the `novendor` feature of `libbpf-sys` is deprecated; build without features instead");
+        println!(
+            "cargo:rustc-link-lib={}bpf",
+            if cfg!(feature = "static-libbpf") {
+                "static="
+            } else {
+                ""
+            }
+        );
         return;
     }
 
     let out_dir = path::PathBuf::from(env::var_os("OUT_DIR").unwrap());
 
     // check for all necessary compilation tools
-    pkg_check("make");
-    pkg_check("pkg-config");
-    if cfg!(feature = "vendored") {
+    // TODO: Not all packages are required for all builds. Dependencies should
+    //       be clarified and these checks be pushed into individual `make_*`
+    //       functions.
+    if vendored_libbpf || vendored_libelf || vendored_zlib {
+        pkg_check("make");
+        pkg_check("pkg-config");
         pkg_check("autoreconf");
         pkg_check("autopoint");
         pkg_check("flex");
@@ -129,32 +144,38 @@ fn main() {
         ),
     };
 
-    if cfg!(feature = "vendored") {
+    let mut cflags = compiler.cflags_env();
+
+    if vendored_zlib {
         make_zlib(&compiler, &src_dir, &out_dir);
-        make_elfutils(&compiler, &src_dir, &out_dir);
+        cflags.push(&format!(" -I{}/zlib/", src_dir.display()));
     }
 
-    let cflags = if cfg!(feature = "vendored") {
-        // make sure that the headerfiles from libelf and zlib
-        // for libbpf come from the vendorized version
-
-        let mut cflags = compiler.cflags_env();
+    if vendored_libelf {
+        make_elfutils(&compiler, &src_dir, &out_dir);
         cflags.push(&format!(" -I{}/elfutils/libelf/", src_dir.display()));
-        cflags.push(&format!(" -I{}/zlib/", src_dir.display()));
-        cflags
-    } else {
-        compiler.cflags_env()
-    };
+    }
 
-    make_libbpf(&compiler, &cflags, &src_dir, &out_dir);
+    if vendored_libbpf {
+        make_libbpf(&compiler, &cflags, &src_dir, &out_dir);
+    }
 
     println!(
         "cargo:rustc-link-search=native={}",
         out_dir.to_string_lossy()
     );
-    println!("cargo:rustc-link-lib={}elf", library_prefix());
-    println!("cargo:rustc-link-lib={}z", library_prefix());
-    println!("cargo:rustc-link-lib=static=bpf");
+    println!(
+        "cargo:rustc-link-lib={}elf",
+        if static_libelf { "static=" } else { "" }
+    );
+    println!(
+        "cargo:rustc-link-lib={}z",
+        if static_zlib { "static=" } else { "" }
+    );
+    println!(
+        "cargo:rustc-link-lib={}bpf",
+        if static_libbpf { "static=" } else { "" }
+    );
     println!("cargo:include={}/include", out_dir.to_string_lossy());
 
     if let Ok(ld_path) = env::var("LD_LIBRARY_PATH") {

src/lib.rs

@@ -6,6 +6,7 @@
 
 include!("bindings.rs");
 
+#[cfg(feature = "vendor-libbpf")]
 macro_rules! header {
     ($file:literal) => {
         ($file, include_str!(concat!("../libbpf/src/", $file)))
@@ -15,7 +16,7 @@ macro_rules! header {
 /// Vendored libbpf headers
 ///
 /// Tuple format is: (header filename, header contents)
-#[cfg(not(feature = "novendor"))]
+#[cfg(feature = "vendor-libbpf")]
 pub const API_HEADERS: [(&str, &str); 10] = [
     header!("bpf.h"),
     header!("libbpf.h"),