integration-test: build-dep on integration-ebpf

Remove the manual dependency tracking machinery in
integration-test/build.rs in favor of a build-dependency on
integration-ebpf. This required adding an empty lib.rs to create the
library target.

This allows integration-test/build.rs to be ignorant of bpf-linker.
Remove that in favor of the logic now in integration-ebpf.

test/integration-ebpf/src/lib.rs

@@ -0,0 +1,3 @@
+#![no_std]
+
+// This file exists to enable the library target.

test/integration-test/Cargo.toml

@@ -25,4 +25,17 @@ tokio = { version = "1.24", default-features = false, features = [
 
 [build-dependencies]
 cargo_metadata = { version = "0.15.4", default-features = false }
+# TODO(https://github.com/rust-lang/cargo/issues/12375): this should be an artifact dependency, but
+# it's not possible to tell cargo to use `-Z build-std` to build it. We cargo-in-cargo in the build
+# script to build this, but we want to teach cargo about the dependecy so that cache invalidation
+# works properly.
+#
+# Note also that https://github.com/rust-lang/cargo/issues/10593 occurs when `target = ...` is added
+# to an artifact dependency; it seems possible to work around that by setting `resolver = "1"` in
+# Cargo.toml in the workspace root.
+#
+# Finally note that *any* usage of `artifact = ...` in *any* Cargo.toml in the workspace breaks
+# workflows with stable cargo; stable cargo outright refuses to load manifests that use unstable
+# features.
+integration-ebpf = { path = "../integration-ebpf" }
 xtask = { path = "../../xtask" }

test/integration-test/build.rs

@@ -1,5 +1,4 @@
 use std::{
-    collections::{HashMap, HashSet},
     env,
     ffi::OsString,
     fmt::Write as _,
@@ -10,9 +9,9 @@ use std::{
 };
 
 use cargo_metadata::{
-    Artifact, CompilerMessage, Dependency, Message, Metadata, MetadataCommand, Package, Target,
+    Artifact, CompilerMessage, Message, Metadata, MetadataCommand, Package, Target,
 };
-use xtask::{create_symlink_to_binary, exec, AYA_BUILD_INTEGRATION_BPF, LIBBPF_DIR};
+use xtask::{exec, AYA_BUILD_INTEGRATION_BPF, LIBBPF_DIR};
 
 fn main() {
     println!("cargo:rerun-if-env-changed={}", AYA_BUILD_INTEGRATION_BPF);
@@ -23,16 +22,11 @@ fn main() {
         .map(Result::unwrap)
         .unwrap_or_default();
 
-    const INTEGRATION_EBPF_PACKAGE: &str = "integration-ebpf";
-
     let Metadata { packages, .. } = MetadataCommand::new().no_deps().exec().unwrap();
-    let packages: HashMap<String, _> = packages
+    let integration_ebpf_package = packages
         .into_iter()
-        .map(|package| {
-            let Package { name, .. } = &package;
-            (name.clone(), package)
-        })
-        .collect();
+        .find(|Package { name, .. }| name == "integration-ebpf")
+        .unwrap();
 
     let manifest_dir = env::var_os("CARGO_MANIFEST_DIR").unwrap();
     let manifest_dir = PathBuf::from(manifest_dir);
@@ -116,27 +110,8 @@ fn main() {
 
         let target = format!("{target}-unknown-none");
 
-        // Teach cargo about our dependencies.
-        let mut visited = HashSet::new();
-        let mut frontier = vec![INTEGRATION_EBPF_PACKAGE];
-        while let Some(package) = frontier.pop() {
-            if !visited.insert(package) {
-                continue;
-            }
-            let Package { dependencies, .. } = packages.get(package).unwrap();
-            for Dependency { name, path, .. } in dependencies {
-                if let Some(path) = path {
-                    println!("cargo:rerun-if-changed={}", path.as_str());
-                    frontier.push(name);
-                }
-            }
-        }
-
-        let bpf_linker_symlink = create_symlink_to_binary(&out_dir, "bpf-linker").unwrap();
-        println!(
-            "cargo:rerun-if-changed={}",
-            bpf_linker_symlink.to_str().unwrap()
-        );
+        let Package { manifest_path, .. } = integration_ebpf_package;
+        let integration_ebpf_dir = manifest_path.parent().unwrap();
 
         let mut cmd = Command::new("cargo");
         cmd.args([
@@ -150,8 +125,6 @@ fn main() {
         ]);
 
         // Workaround to make sure that the rust-toolchain.toml is respected.
-        let Package { manifest_path, .. } = packages.get(INTEGRATION_EBPF_PACKAGE).unwrap();
-        let integration_ebpf_dir = manifest_path.parent().unwrap();
         cmd.env_remove("RUSTUP_TOOLCHAIN")
             .current_dir(integration_ebpf_dir);
 
@@ -209,7 +182,7 @@ fn main() {
             fs::write(&dst, []).unwrap_or_else(|err| panic!("failed to create {dst:?}: {err}"));
         }
 
-        let Package { targets, .. } = packages.get(INTEGRATION_EBPF_PACKAGE).unwrap();
+        let Package { targets, .. } = integration_ebpf_package;
         for Target { name, kind, .. } in targets {
             if *kind != ["bin"] {
                 continue;