fix: quick remedy for invalid memory access in obuf_create

Tarantool's struct obuf contains one more field in debug builds which is
utilized for additional allocation checks. Before this commit we didn't
mirror it in our rust definition which sometimes resulted in crashes.

This is not a proper fix to the underlying problem but it's certainly
better than leaving it as is.

Author: funbringer

tarantool/src/ffi/sql.rs

@@ -3,6 +3,7 @@
 use libc::{iovec, size_t};
 use std::cmp;
 use std::io::Read;
+use std::mem::MaybeUninit;
 use std::os::raw::{c_char, c_int, c_void};
 
 pub const IPROTO_DATA: u8 = 0x30;
@@ -72,18 +73,11 @@ impl ObufWrapper {
         let inner_buf = unsafe {
             let slab_c = cord_slab_cache();
 
-            let mut buf = Obuf {
-                _slab_cache: std::mem::zeroed(),
-                pos: 0,
-                n_iov: 0,
-                used: 0,
-                start_capacity: start_capacity as size_t,
-                capacity: std::mem::zeroed(),
-                iov: std::mem::zeroed(),
-            };
-            obuf_create(&mut buf as *mut Obuf, slab_c, 1024);
-            buf
+            let mut buf = MaybeUninit::<Obuf>::zeroed();
+            obuf_create(buf.as_mut_ptr(), slab_c, start_capacity);
+            buf.assume_init()
         };
+
         Self {
             inner: inner_buf,
             read_pos: 0,
@@ -135,6 +129,7 @@ impl Read for ObufWrapper {
     }
 }
 
+// TODO: ASan-enabled build has a different layout (obuf_asan.h).
 #[repr(C)]
 pub(crate) struct Obuf {
     _slab_cache: *const c_void,
@@ -144,6 +139,11 @@ pub(crate) struct Obuf {
     pub start_capacity: size_t,
     pub capacity: [size_t; 32],
     pub iov: [iovec; 32],
+    // This flag is only present in debug builds (!NDEBUG),
+    // but it's easier to just add it unconditionally to
+    // prevent illegal memory access in obuf_create.
+    // TODO: prevent this class of errors using a better solution.
+    pub reserved: bool,
 }
 
 impl Drop for Obuf {