Fix rerun directive issuance for build inputs

561158b changed build.rs to use CargoCallbacks with the option to issue
the rerun directive for all input headers. This caused unconditional
rebuilds of foundations because one of the input headers is generated in
build.rs.

While 9479fad prevents unconditional rebuilds, it also prevents the
rerun directive from being issued for any input header, which can be
annoying during development.

This change fine tunes rerun directive issuance, allowing it to be
issued for input headers but excluding all generated files (which we
assume are all files under `out_dir`).
We also now issue the rerun directive for the `seccomp.h` header template.

Author: abread

foundations/build.rs

@@ -122,7 +122,8 @@ mod security {
     }
 
     fn render_header(out_dir: &Path, include_dir: &Path) -> PathBuf {
-        let rendered = fs::read_to_string(include_dir.join("seccomp.h.in"))
+        let template = include_dir.join("seccomp.h.in");
+        let rendered = fs::read_to_string(&template)
             .unwrap()
             .replace(
                 "@VERSION_MAJOR@",
@@ -141,6 +142,10 @@ mod security {
 
         fs::write(&header_file, rendered).unwrap();
 
+        // We don't emit cargo:rerun-if-changed for the generated header to avoid unconditionally
+        // recompiling foundations, but we should emit it for the template its generated from.
+        println!("cargo:rerun-if-changed={}", template.display());
+
         header_file
     }
 
@@ -175,9 +180,9 @@ mod security {
             .allowlist_var("PR_GET_SECCOMP")
             .allowlist_var("PR_SET_NAME")
             .derive_default(true)
-            .parse_callbacks(Box::new(
-                bindgen::CargoCallbacks::new().rerun_on_header_files(false),
-            ))
+            .parse_callbacks(Box::new(CargoCallbacksIgnoreGenHeaders::new(
+                out_dir.to_owned(),
+            )))
             .generate()
             .unwrap()
             .write_to_file(out_dir.join("security_sys.rs"))
@@ -196,4 +201,39 @@ mod security {
             .try_compile("check_have_linux_seccomp_h")
             .is_ok()
     }
+
+    /// Customized version of bindgen::CargoCallbacks that ignores files under out_dir.
+    ///
+    /// We cannot emit cargo:rerun-if-changed for generated files because it leads to
+    /// unconditional recompilation. This assumes all files under out_dir are generated.
+    #[derive(Debug)]
+    struct CargoCallbacksIgnoreGenHeaders {
+        out_dir: PathBuf,
+    }
+
+    impl CargoCallbacksIgnoreGenHeaders {
+        fn new(out_dir: PathBuf) -> Self {
+            Self { out_dir }
+        }
+
+        fn is_generated_file(&self, filename: &str) -> bool {
+            Path::new(filename).starts_with(&self.out_dir)
+        }
+    }
+
+    impl bindgen::callbacks::ParseCallbacks for CargoCallbacksIgnoreGenHeaders {
+        fn header_file(&self, filename: &str) {
+            self.include_file(filename);
+        }
+
+        fn include_file(&self, filename: &str) {
+            if !self.is_generated_file(filename) {
+                println!("cargo:rerun-if-changed={filename}");
+            }
+        }
+
+        fn read_env_var(&self, key: &str) {
+            println!("cargo:rerun-if-env-changed={key}");
+        }
+    }
 }