libbpf-cargo: Simplify build_btf_mmap() test helper

The build_btf_mmap() function is almost comically complex in what it
does. In order to compile a single .bpf.c file it creates an entire
Cargo Rust project and builds that, instead of simply compiling the file
directly. Now that we have the BpfObjBuilder type, we can just about
accomplish the latter without jumping through dozens of hoops. Do it. As
a pleasant side effect of this reduction in complexity, the btf_dump
test runtime more than halves:

Before:
  $ time target/debug/deps/libbpf_cargo-37cf1fde780326f7 btf_dump
  > running 34 tests
  > [...]
  > ________________________________________________________
  > Executed in    2.09 secs    fish           external

After:
  $ time target/debug/deps/libbpf_cargo-37cf1fde780326f7 btf_dump
  > running 34 tests
  > [...]
  > ________________________________________________________
  > Executed in  810.13 millis    fish           external

Not the motivating factor at this point, but we take it.

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

libbpf-cargo/src/build.rs

@@ -199,7 +199,7 @@ impl BpfObjBuilder {
     /// Build a BPF object file.
     pub fn build(&mut self, src: &Path, dst: &Path) -> Result<CompilationOutput> {
         self.build_many([src], dst).map(|vec| {
-            // SANITY: We pass in a single file we `build_many` is
+            // SANITY: We pass in a single file and `build_many` is
             //         guaranteed to produce as many outputs as input
             //         files; so there must be one.
             vec.into_iter().next().unwrap()

libbpf-cargo/src/test.rs

@@ -21,6 +21,7 @@ use tempfile::TempDir;
 use test_log::test;
 
 use crate::build::build_project;
+use crate::build::BpfObjBuilder;
 use crate::gen::GenBtf;
 use crate::gen::GenStructOps;
 use crate::make::make;
@@ -122,19 +123,23 @@ fn get_libbpf_rs_path() -> PathBuf {
         .expect("failed to canonicalize libbpf-rs")
 }
 
-/// Add vmlinux header into `project`'s src/bpf dir
-fn add_vmlinux_header(project: &Path) {
+fn write_vmlinux_header(dir: &Path) {
     let mut vmlinux = OpenOptions::new()
         .create(true)
         .truncate(true)
         .write(true)
-        .open(project.join("src/bpf/vmlinux.h"))
+        .open(dir.join("vmlinux.h"))
         .expect("failed to open vmlinux.h");
     let () = vmlinux
         .write_all(vmlinux::VMLINUX)
         .expect("failed to write vmlinux.h");
 }
 
+/// Add vmlinux header into `project`'s src/bpf dir
+fn add_vmlinux_header(project: &Path) {
+    write_vmlinux_header(&project.join("src/bpf"))
+}
+
 #[test]
 fn test_build_default() {
     let (_dir, proj_dir, cargo_toml) = setup_temp_project();
@@ -1174,30 +1179,28 @@ macro_rules! find_type_in_btf {
 /// returns struct Btf if able to compile
 /// fails calling test if unable to compile
 fn build_btf_mmap(prog_text: &str) -> Mmap {
-    let (_dir, proj_dir, cargo_toml) = setup_temp_project();
-
-    // Add prog dir
-    create_dir(proj_dir.join("src/bpf")).expect("failed to create prog dir");
-
-    // Add a prog
-    let mut prog = OpenOptions::new()
+    let dir = tempdir().expect("failed to create tempdir");
+    let dir = dir.path();
+    let bpf_c = dir.join("prog.bpf.c");
+    let mut file = OpenOptions::new()
         .write(true)
         .create(true)
         .truncate(true)
-        .open(proj_dir.join("src/bpf/prog.bpf.c"))
+        .open(&bpf_c)
         .expect("failed to open prog.bpf.c");
 
-    write!(prog, "{prog_text}").expect("failed to write prog.bpf.c");
-
+    let () = file
+        .write_all(prog_text.as_bytes())
+        .expect("failed to write prog.bpf.c");
     // Lay down the necessary header files
-    add_vmlinux_header(&proj_dir);
+    let () = write_vmlinux_header(dir);
 
-    // Build the .bpf.o
-    build_project(Some(&cargo_toml), None, Vec::new()).expect("failed to compile");
+    let bpf_o = dir.join("prog.bpf.o");
+    let _output = BpfObjBuilder::default().build(&bpf_c, &bpf_o).unwrap();
 
     let obj = OpenOptions::new()
         .read(true)
-        .open(proj_dir.as_path().join("target/bpf/prog.bpf.o").as_path())
+        .open(bpf_o)
         .expect("failed to open object file");
     unsafe { Mmap::map(&obj) }.expect("Failed to mmap object file")
 }