Use rust memoffset crate to replace C offsetof().

Cherry-picked from Eric's PR #208

Cargo.lock

@@ -928,6 +928,15 @@ version = "2.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b16bd47d9e329435e309c58469fe0791c2d0d1ba96ec0954152a5ae2b04387dc"
 
+[[package]]
+name = "memoffset"
+version = "0.6.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "59accc507f1338036a0477ef61afdae33cde60840f4dfe481319ce3ad116ddf9"
+dependencies = [
+ "autocfg",
+]
+
 [[package]]
 name = "mime"
 version = "0.3.16"
@@ -1364,6 +1373,7 @@ dependencies = [
  "hex",
  "lazy_static",
  "log",
+ "memoffset",
  "rand",
  "regex",
  "thiserror",

postgres_ffi/Cargo.toml

@@ -17,6 +17,7 @@ crc32c = "0.6.0"
 hex = "0.4.3"
 lazy_static = "1.4"
 log = "0.4.14"
+memoffset = "0.6.2"
 thiserror = "1.0"
 workspace_hack = { path = "../workspace_hack" }
 

postgres_ffi/build.rs

@@ -25,7 +25,6 @@ fn main() {
         //
         .whitelist_type("ControlFileData")
         .whitelist_var("PG_CONTROL_FILE_SIZE")
-        .whitelist_var("PG_CONTROLFILEDATA_OFFSETOF_CRC")
         .whitelist_type("DBState")
         //
         // Path the server include dir. It is in tmp_install/include/server, if you did

postgres_ffi/pg_control_ffi.h

@@ -6,10 +6,3 @@
  */
 #include "c.h"
 #include "catalog/pg_control.h"
-
-/*
- * PostgreSQL uses "offsetof(ControlFileData, crc)" in multiple places to get the
- * size of the control file up to the CRC, which is the last field, but there is
- * no constant for it. We also need it in the Rust code.
- */
-const uint32 PG_CONTROLFILEDATA_OFFSETOF_CRC = offsetof(ControlFileData, crc);

postgres_ffi/src/controlfile_utils.rs

@@ -23,15 +23,20 @@
 //! information. You can use PostgreSQL's pg_controldata utility to view its
 //! contents.
 //!
-use crate::{ControlFileData, PG_CONTROLFILEDATA_OFFSETOF_CRC, PG_CONTROL_FILE_SIZE};
+use crate::{ControlFileData, PG_CONTROL_FILE_SIZE};
 
 use anyhow::{bail, Result};
 use bytes::{Bytes, BytesMut};
 
-// sizeof(ControlFileData)
+/// Equivalent to sizeof(ControlFileData) in C
 const SIZEOF_CONTROLDATA: usize = std::mem::size_of::<ControlFileData>();
-// offsetof(ControlFileData, crc)
-const OFFSETOF_CRC: usize = PG_CONTROLFILEDATA_OFFSETOF_CRC as usize;
+
+/// Compute the offset of the `crc` field within the `ControlFileData` struct.
+/// Equivalent to offsetof(ControlFileData, crc) in C.
+// Someday this can be const when the right compiler features land.
+fn pg_control_crc_offset() -> usize {
+    memoffset::offset_of!(ControlFileData, crc)
+}
 
 
 ///
@@ -47,6 +52,7 @@ pub fn decode_pg_control(buf: &[u8]) -> Result<ControlFileData> {
     }
 
     // Compute the expected CRC of the content.
+    let OFFSETOF_CRC = pg_control_crc_offset();
     let expectedcrc = crc32c::crc32c(&buf[0..OFFSETOF_CRC]);
 
     // Convert the slice into an array of the right size, and use `transmute` to
@@ -104,9 +110,8 @@ pub fn encode_pg_control(controlfile: &ControlFileData) -> Bytes {
         unsafe { std::mem::transmute::<ControlFileData, [u8; SIZEOF_CONTROLDATA]>(*controlfile) };
 
     // Recompute the CRC
-    let mut data_without_crc: [u8; OFFSETOF_CRC] = [0u8; OFFSETOF_CRC];
-    data_without_crc.copy_from_slice(&b[0..OFFSETOF_CRC]);
-    let newcrc = crc32c::crc32c(&data_without_crc);
+    let OFFSETOF_CRC = pg_control_crc_offset();
+    let newcrc = crc32c::crc32c(&b[0..OFFSETOF_CRC]);
 
     let mut buf = BytesMut::with_capacity(PG_CONTROL_FILE_SIZE as usize);
     buf.extend_from_slice(&b[0..OFFSETOF_CRC]);