revlog-native: introduced ABI version in capsule

revlog-native: introduced ABI version in capsule

Concerns that an inconsistency could arise between the actual contents
of the capsule in revlog.c and the Rust consumer have been raised after
the switch to the array of data and function pointers in f384d68d8ea8.

It has been suggested that the `version` from parsers.c could be use for
this. In this change, we introduce instead a separate ABI version number,
which should have the following advantages:

- no need to change the consuming Rust code for changes that have nothing
  to do with the contents of the capsule
- the version number in parsers.c is not explicitely flagged as ABI. It's
  not obvious to me whether an ABI change that would be invisible to Python
  would warrant an increment

The drawback is that developers now have to consider two version numbers.

We expect the added cost of the check to be negligible because it occurs
at instantiation of `CIndex` only, which in turn is tied to instantiation
of Python objects such as `LazyAncestors` and `MixedIndex`. Frequent calls
to `Cindex::new` should also probably hit the CPU branch predictor.

Differential Revision: https://phab.mercurial-scm.org/D7856

--- a/mercurial/cext/revlog.c	Mon Jan 13 19:11:44 2020 -0800
+++ b/mercurial/cext/revlog.c	Tue Jan 14 12:04:12 2020 +0100
@@ -38,6 +38,7 @@
 } nodetreenode;
 
 typedef struct {
+	int abi_version;
 	int (*index_parents)(PyObject *, int, int *);
 } Revlog_CAPI;
 
@@ -3037,6 +3038,9 @@
 #endif /* WITH_RUST */
 
 static Revlog_CAPI CAPI = {
+    /* increment the abi_version field upon each change in the Revlog_CAPI
+       struct or in the ABI of the listed functions */
+    1,
     HgRevlogIndex_GetParents,
 };
 

--- a/rust/hg-cpython/src/cindex.rs	Mon Jan 13 19:11:44 2020 -0800
+++ b/rust/hg-cpython/src/cindex.rs	Tue Jan 14 12:04:12 2020 +0100
@@ -10,12 +10,15 @@
 //! Ideally, we should use an Index entirely implemented in Rust,
 //! but this will take some time to get there.
 
-use cpython::{PyClone, PyObject, PyResult, Python};
+use cpython::{exc::ImportError, PyClone, PyErr, PyObject, PyResult, Python};
 use hg::{Graph, GraphError, Revision, WORKING_DIRECTORY_REVISION};
 use libc::c_int;
 
+const REVLOG_CABI_VERSION: c_int = 1;
+
 #[repr(C)]
 pub struct Revlog_CAPI {
+    abi_version: c_int,
     index_parents: unsafe extern "C" fn(
         index: *mut revlog_capi::RawPyObject,
         rev: c_int,
@@ -66,9 +69,20 @@
 
 impl Index {
     pub fn new(py: Python, index: PyObject) -> PyResult<Self> {
+        let capi = unsafe { revlog_capi::retrieve(py)? };
+        if capi.abi_version != REVLOG_CABI_VERSION {
+            return Err(PyErr::new::<ImportError, _>(
+                py,
+                format!(
+                    "ABI version mismatch: the C ABI revlog version {} \
+                     does not match the {} expected by Rust hg-cpython",
+                    capi.abi_version, REVLOG_CABI_VERSION
+                ),
+            ));
+        }
         Ok(Index {
             index: index,
-            capi: unsafe { revlog_capi::retrieve(py)? },
+            capi: capi,
         })
     }