feat: enforce minimal required alignment for pool allocations

Refuse to compile if the NGX_ALIGNMENT value detected or specified while
building nginx is insufficient.

Author: bavshin-f5

nginx-sys/build/main.rs

@@ -216,6 +216,8 @@ fn generate_binding(nginx: &NginxSource) {
         // Bindings will not compile on Linux without block listing this item
         // It is worth investigating why this is
         .blocklist_item("IPPORT_RESERVED")
+        // will be restored later in build.rs
+        .blocklist_item("NGX_ALIGNMENT")
         .generate_cstr(true)
         // The input header we would like to generate bindings for.
         .header("build/wrapper.h")

nginx-sys/build/wrapper.h

@@ -20,6 +20,10 @@
 
 const char *NGX_RS_MODULE_SIGNATURE = NGX_MODULE_SIGNATURE;
 
+// NGX_ALIGNMENT could be defined as a constant or an expression, with the
+// latter being unsupported by bindgen.
+const size_t NGX_RS_ALIGNMENT = NGX_ALIGNMENT;
+
 // `--prefix=` results in not emitting the declaration
 #ifndef NGX_PREFIX
 #define NGX_PREFIX ""

nginx-sys/src/lib.rs

@@ -36,6 +36,14 @@ pub use queue::*;
 #[cfg(ngx_feature = "stream")]
 pub use stream::*;
 
+/// Default alignment for pool allocations.
+pub const NGX_ALIGNMENT: usize = NGX_RS_ALIGNMENT;
+
+// Check if the allocations made with ngx_palloc are properly aligned.
+// If the check fails, objects allocated from `ngx_pool` can violate Rust pointer alignment
+// requirements.
+const _: () = assert!(core::mem::align_of::<ngx_str_t>() <= NGX_ALIGNMENT);
+
 impl ngx_command_t {
     /// Creates a new empty [`ngx_command_t`] instance.
     ///