chore(rust): add opt_get! and expose some FMODE_* as Rust const

In an effort to rewrite `bch2_read_super` from C to Rust,
it is neccessary to have `opt_get!` macro defined, and some
FMODE_* consts (defined as macro in `include/linux/blkdev.h`)
defined as Rust const.

Bindgen is currently unable to exapnd C functional macro [1],
this this commit use the workaround as introduced in [2].

[1] https://github.com/rust-lang/rust-bindgen/issues/753
[2] https://github.com/rust-lang/rust-bindgen/issues/753#issuecomment-608546390

Signed-off-by: TruongSinh Tran-Nguyen <i@truongsinh.pro>

rust-src/bch_bindgen/build.rs

@@ -1,3 +1,12 @@
+
+#[derive(Debug)]
+pub struct Fix753 {}
+impl bindgen::callbacks::ParseCallbacks for Fix753 {
+    fn item_name(&self, original_item_name: &str) -> Option<String> {
+        Some(original_item_name.trim_start_matches("Fix753_").to_owned())
+    }
+}
+
 fn main() {
     use std::path::PathBuf;
 
@@ -49,6 +58,7 @@ fn main() {
         .blocklist_type("srcu_struct")
         .allowlist_var("BCH_.*")
         .allowlist_var("KEY_SPEC_.*")
+        .allowlist_var("Fix753_FMODE_.*")
         .allowlist_var("bch.*")
         .allowlist_var("__BTREE_ITER.*")
         .allowlist_var("BTREE_ITER.*")
@@ -69,6 +79,7 @@ fn main() {
         .no_partialeq("bkey")
         .no_partialeq("bpos")
         .generate_inline_functions(true)
+        .parse_callbacks(Box::new(Fix753 {}))
         .generate()
         .expect("BindGen Generation Failiure: [libbcachefs_wrapper]");
     bindings

rust-src/bch_bindgen/src/libbcachefs_wrapper.h

@@ -10,4 +10,11 @@
 #include "../libbcachefs.h"
 #include "../crypto.h"
 #include "../include/linux/bio.h"
+#include "../include/linux/blkdev.h"
 
+
+#define MARK_FIX_753(req_name) const fmode_t Fix753_##req_name = req_name;
+
+MARK_FIX_753(FMODE_READ);
+MARK_FIX_753(FMODE_WRITE);
+MARK_FIX_753(FMODE_EXCL);

rust-src/bch_bindgen/src/opts.rs

@@ -3,7 +3,33 @@ macro_rules! opt_set {
     ($opts:ident, $n:ident, $v:expr) => {
         bch_bindgen::paste! {
             $opts.$n = $v;
-            $opts.[<set_ $n _defined>](1);
+            $opts.[<set_ $n _defined>](1)
+        }
+    };
+}
+
+#[macro_export]
+macro_rules! opt_defined {
+    ($opts:ident, $n:ident) => {
+        bch_bindgen::paste! {
+            $opts.[< $n _defined>]()
+        }
+    };
+}
+
+#[macro_export]
+macro_rules! opt_get {
+    ($opts:ident, $n:ident) => {
+        if bch_bindgen::opt_defined!($opts, $n) == 0 {
+            bch_bindgen::paste! {
+                unsafe {
+                    bch_bindgen::bcachefs::bch2_opts_default.$n
                 }
             }
+        } else {
+            bch_bindgen::paste! {
+                $opts.$n
+            }
+        }
+    };
 }