Fix issue #1032 (#1033)

Blindly use the `FMGR_ABI_EXTRA` constant as part of the magic block.
Hardcoding to "PostgreSQL" isn't exactly correct when using bindings
from, for example, a Postgres fork that has purposely changed its ABI
"name".

* On pg15, detect if we're being compiled against something with a different ABI than "PostgreSQL" and fail to compile.  Unless the `--unsafe-postgres` feature flag is present

Co-authored-by: Smittyvb <[email protected]>

README.md

@@ -242,6 +242,26 @@ this feature is now considered deprecated in favor of a lower-overhead interop.
 You may still request implementations of `TryFrom<time::Type> for pgx::MatchingType`
 and `From<time::Type> for pgx::MatchingType` by enabling the `"time-crate"` feature.
 
+### "unsafe-postgres": Allow compilation for Postgres forks that have a different ABI
+
+As of Postgres v15, forks are allowed to specify they use a different ABI than canonical Postgres.
+Since pgx makes countless assumptions about Postgres' internal ABI it is not possible for it to 
+guarantee that a compiled pgx extension will probably execute within such a Postgres fork.  You,
+dear compiler runner, can make this guarantee for yourself by specifying the `unsafe-postgres` 
+feature flag.  Otherwise, a pgx extension will fail to compile with an error similar to:
+
+```
+error[E0080]: evaluation of constant value failed
+   --> pgx/src/lib.rs:151:5
+    |
+151 | /     assert!(
+152 | |         same_slice(pg_sys::FMGR_ABI_EXTRA, b"xPostgreSQL\0"),
+153 | |         "Unsupported Postgres ABI. Perhaps you need `--features unsafe-postgres`?",
+154 | |     );
+    | |_____^ the evaluated program panicked at 'Unsupported Postgres ABI. Perhaps you need `--features unsafe-postgres`?', pgx/src/lib.rs:151:5
+    |
+```
+
 ### Experimental Features
 
 Adding `pgx = { version = "0.5.0", features = ["postgrestd"] }` to your Cargo.toml

pgx/Cargo.toml

@@ -27,6 +27,7 @@ pg14 = [ "pgx-pg-sys/pg14" ]
 pg15 = [ "pgx-pg-sys/pg15" ]
 time-crate = ["dep:time"]
 no-schema-generation = ["pgx-macros/no-schema-generation", "pgx-sql-entity-graph/no-schema-generation"]
+unsafe-postgres = []     # when trying to compile against something that looks like Postgres but claims to be diffent
 
 [package.metadata.docs.rs]
 features = ["pg14", "cshim"]

pgx/src/lib.rs

@@ -128,6 +128,33 @@ pub use pg_sys::{
 #[doc(hidden)]
 pub use pgx_sql_entity_graph;
 
+// Postgres v15 has the concept of an ABI "name".  The default is `b"PostgreSQL\0"` and this is the
+// ABI that pgx extensions expect to be running under.  We will refuse to compile if it is detected
+// that we're trying to be built against some other kind of "postgres" that has its own ABI name.
+//
+// Unless the compiling user explicitly told us that they're aware of this via `--features unsafe-postgres`.
+#[cfg(all(feature = "pg15", not(feature = "unsafe-postgres")))]
+const _: () = {
+    // to appease `const`
+    const fn same_slice(a: &[u8], b: &[u8]) -> bool {
+        if a.len() != b.len() {
+            return false;
+        }
+        let mut i = 0;
+        while i < a.len() {
+            if a[i] != b[i] {
+                return false;
+            }
+            i += 1;
+        }
+        true
+    }
+    assert!(
+        same_slice(pg_sys::FMGR_ABI_EXTRA, b"PostgreSQL\0"),
+        "Unsupported Postgres ABI. Perhaps you need `--features unsafe-postgres`?",
+    );
+};
+
 /// A macro for marking a library compatible with [`pgx`][crate].
 ///
 /// <div class="example-wrap" style="display:inline-block">
@@ -220,8 +247,9 @@ macro_rules! pg_magic_func {
                 namedatalen: pgx::pg_sys::NAMEDATALEN as i32,
                 float8byval: cfg!(target_pointer_width = "64") as i32,
                 abi_extra: {
-                    // array::from_fn isn't const yet, boohoo, so const-copy a bstr
-                    let magic = b"PostgreSQL";
+                    // we'll use what the bindings tell us, but if it ain't "PostgreSQL" then we'll
+                    // raise a compilation error unless the `unsafe-postgres` feature is set
+                    let magic = pgx::pg_sys::FMGR_ABI_EXTRA;
                     let mut abi = [0 as ::pgx::ffi::c_char; 32];
                     let mut i = 0;
                     while i < magic.len() {