Syncing account sizes between c and rust (#197)

* defined size in oracle.h and asserted it in rust

* added docstring for the test

* updated the doc

* updated the error message

* Update comment in program/c/src/oracle/oracle.h

Co-authored-by: Tom Pointon <tom@teepeestudios.net>

* updated time machine size name

Co-authored-by: Tom Pointon <tom@teepeestudios.net>

Author: majabbour

program/c/src/oracle/oracle.h

@@ -13,6 +13,10 @@ extern "C" {
 // defines to u32 (even with ULL suffix)
 const uint64_t SUCCESSFULLY_UPDATED_AGGREGATE = 1000ULL;
 
+// The size of the "time machine" account defined in the 
+// Rust portion of the codebase.
+const uint64_t TIME_MACHINE_STRUCT_SIZE = 1864ULL;
+
 // magic number at head of account
 #define PC_MAGIC 0xa1b2c3d4
 

program/rust/src/lib.rs

@@ -1,6 +1,7 @@
 //c_oracle_header is auto generated by build_bpf.sh
 //to reflect the current status of oracle.h
 mod c_oracle_header;
+mod time_machine_types;
 
 //do not link with C during unit tests (which are built in native architecture, unlike libpyth.o) 
 #[cfg(target_arch = "bpf")]

program/rust/src/time_machine_types.rs

@@ -0,0 +1,20 @@
+use super::c_oracle_header;
+#[derive(Debug, Clone)]
+#[repr(C)]
+/// this wraps multiple SMA and tick trackers, and includes all the state 
+/// used by the time machine 
+pub struct TimeMachineWrapper{
+    //Place holder with the size of the fields I am planning to add
+    place_holder: [u8;1864],
+}
+
+#[test]
+///test that the size defined in C matches that
+///defined in Rust
+fn c_time_machine_size_is_correct() {
+    assert_eq!(
+        ::std::mem::size_of::<TimeMachineWrapper>(),
+        c_oracle_header::TIME_MACHINE_STRUCT_SIZE.try_into().unwrap(),
+        "expected TIME_MACHINE_STRUCT_SIZE in oracle.h to the same as the size of TimeMachineWrapper"
+    );
+}