libbpf-cargo: return stderr from clang

Update `build_and_generate` to return a `CompilationOutput` owning the stderr
of the compiler process. This gives consumers access to the stderr to output as
they see fit.

Provides a `stderr` method that calls `String::from_utf8_lossy`. The primary
consumer is `sched-ext/scx` which outputs each line with
`println!("cargo:warning={}", l)`, showing the warnings quite nicely in the
Cargo output when there isn't an error.

Test plan:
- Pointed that Cargo.lock at this repo and built.

libbpf-cargo/src/build.rs

@@ -1,3 +1,4 @@
+use std::borrow::Cow;
 use std::collections::HashSet;
 use std::env;
 use std::env::consts::ARCH;
@@ -19,6 +20,20 @@ use tempfile::tempdir;
 use crate::metadata;
 use crate::metadata::UnprocessedObj;
 
+/// Contains information about a successful compilation.
+pub struct CompilationOutput {
+    stderr: Vec<u8>,
+}
+
+impl CompilationOutput {
+    // Only used in libbpf-cargo library
+    #[allow(dead_code)]
+    /// Read the stderr from the compilation as a Copy-on-Write String
+    pub fn stderr(&self) -> Cow<'_, str> {
+        String::from_utf8_lossy(&self.stderr)
+    }
+}
+
 fn check_progs(objs: &[UnprocessedObj]) -> Result<()> {
     let mut set = HashSet::with_capacity(objs.len());
     for obj in objs {
@@ -171,7 +186,7 @@ fn compile_one(
     out: &Path,
     clang: &Path,
     clang_args: &[OsString],
-) -> Result<()> {
+) -> Result<CompilationOutput> {
     if debug {
         println!("Building {}", source.display());
     }
@@ -234,7 +249,10 @@ fn compile_one(
     // system specific and temporary paths. That can render our generated
     // skeletons unstable, potentially rendering them unsuitable for inclusion
     // in version control systems. So strip this information.
-    strip_dwarf_info(out).with_context(|| format!("Failed to strip object file {}", out.display()))
+    strip_dwarf_info(out)
+        .with_context(|| format!("Failed to strip object file {}", out.display()))?;
+
+    Ok(CompilationOutput{ stderr: output.stderr })
 }
 
 fn compile(
@@ -243,31 +261,31 @@ fn compile(
     clang: &Path,
     mut clang_args: Vec<OsString>,
     target_dir: &Path,
-) -> Result<()> {
+) -> Result<Vec<CompilationOutput>> {
     let header_dir = extract_libbpf_headers_to_disk(target_dir)?;
     if let Some(dir) = header_dir {
         clang_args.push(OsString::from("-I"));
         clang_args.push(dir.into_os_string());
     }
 
-    for obj in objs {
-        let stem = obj.path.file_stem().with_context(|| {
-            format!(
-                "Could not calculate destination name for obj={}",
-                obj.path.display()
-            )
-        })?;
-
-        let mut dest_name = stem.to_os_string();
-        dest_name.push(".o");
-
-        let mut dest_path = obj.out.to_path_buf();
-        dest_path.push(&dest_name);
-        fs::create_dir_all(&obj.out)?;
-        compile_one(debug, &obj.path, &dest_path, clang, &clang_args)?;
-    }
+    objs.iter()
+        .map(|obj| -> Result<_> {
+            let stem = obj.path.file_stem().with_context(|| {
+                format!(
+                    "Could not calculate destination name for obj={}",
+                    obj.path.display()
+                )
+            })?;
 
-    Ok(())
+            let mut dest_name = stem.to_os_string();
+            dest_name.push(".o");
+
+            let mut dest_path = obj.out.to_path_buf();
+            dest_path.push(&dest_name);
+            fs::create_dir_all(&obj.out)?;
+            compile_one(debug, &obj.path, &dest_path, clang, &clang_args)
+        })
+        .collect::<Result<_, _>>()
 }
 
 fn extract_clang_or_default(clang: Option<&PathBuf>) -> PathBuf {
@@ -317,7 +335,7 @@ pub fn build_single(
     clang: Option<&PathBuf>,
     skip_clang_version_checks: bool,
     mut clang_args: Vec<OsString>,
-) -> Result<()> {
+) -> Result<CompilationOutput> {
     let clang = extract_clang_or_default(clang);
     check_clang(debug, &clang, skip_clang_version_checks)?;
     let header_parent_dir = tempdir()?;
@@ -332,9 +350,7 @@ pub fn build_single(
     // BPF. See https://lkml.org/lkml/2020/2/21/1000.
     clang_args.push(OsString::from("-fno-stack-protector"));
 
-    compile_one(debug, source, out, &clang, &clang_args)?;
-
-    Ok(())
+    compile_one(debug, source, out, &clang, &clang_args)
 }
 
 #[test]

libbpf-cargo/src/lib.rs

@@ -64,6 +64,8 @@
 )]
 #![deny(unsafe_op_in_unsafe_fn)]
 
+pub use build::CompilationOutput;
+
 use std::ffi::OsStr;
 use std::ffi::OsString;
 use std::path::Path;
@@ -214,17 +216,17 @@ impl SkeletonBuilder {
     }
 
     /// Build BPF programs and generate the skeleton at path `output`
-    pub fn build_and_generate<P: AsRef<Path>>(&mut self, output: P) -> Result<()> {
-        self.build()?;
+    pub fn build_and_generate<P: AsRef<Path>>(&mut self, output: P) -> Result<CompilationOutput> {
+        let comp_output = self.build()?;
         self.generate(output)?;
 
-        Ok(())
+        Ok(comp_output)
     }
 
     // Build BPF programs without generating a skeleton.
     //
     // [`SkeletonBuilder::source`] must be set for this to succeed.
-    pub fn build(&mut self) -> Result<()> {
+    pub fn build(&mut self) -> Result<CompilationOutput> {
         let source = self
             .source
             .as_ref()
@@ -261,9 +263,7 @@ impl SkeletonBuilder {
             self.skip_clang_version_check,
             self.clang_args.clone(),
         )
-        .with_context(|| format!("failed to build `{}`", source.display()))?;
-
-        Ok(())
+        .with_context(|| format!("failed to build `{}`", source.display()))
     }
 
     // Generate a skeleton at path `output` without building BPF programs.

libbpf-cargo/src/test.rs

@@ -785,7 +785,8 @@ fn test_skeleton_builder_clang_opts() {
         .debug(true)
         .clang("clang")
         .build_and_generate(skel.path())
-        .unwrap_err();
+        .err()
+        .unwrap();
 
     // Should succeed b/c we defined the macro
     SkeletonBuilder::new()