Add ABI extra field to fmgr magic block

This allows derived products to intentionally make their fmgr ABI
incompatible, with a clean error message.

Discussion: https://www.postgresql.org/message-id/flat/55215fda-db31-a045-d6b7-d6f2d2dc9920%40enterprisedb.com

src/backend/utils/fmgr/dfmgr.c

@@ -330,6 +330,21 @@ incompatible_module_error(const char *libname,
 						   magic_data.version / 100, library_version)));
 	}
 
+	/*
+	 * Similarly, if the ABI extra field doesn't match, error out.  Other
+	 * fields below might also mismatch, but that isn't useful information if
+	 * you're using the wrong product altogether.
+	 */
+	if (strcmp(module_magic_data->abi_extra, magic_data.abi_extra) != 0)
+	{
+		ereport(ERROR,
+				(errmsg("incompatible library \"%s\": ABI mismatch",
+						libname),
+				 errdetail("Server has ABI \"%s\", library has \"%s\".",
+						   magic_data.abi_extra,
+						   module_magic_data->abi_extra)));
+	}
+
 	/*
 	 * Otherwise, spell out which fields don't agree.
 	 *

src/include/fmgr.h

@@ -458,6 +458,7 @@ typedef struct
 	int			indexmaxkeys;	/* INDEX_MAX_KEYS */
 	int			namedatalen;	/* NAMEDATALEN */
 	int			float8byval;	/* FLOAT8PASSBYVAL */
+	char		abi_extra[32];	/* see pg_config_manual.h */
 } Pg_magic_struct;
 
 /* The actual data block contents */
@@ -468,9 +469,13 @@ typedef struct
 	FUNC_MAX_ARGS, \
 	INDEX_MAX_KEYS, \
 	NAMEDATALEN, \
-	FLOAT8PASSBYVAL \
+	FLOAT8PASSBYVAL, \
+	FMGR_ABI_EXTRA, \
 }
 
+StaticAssertDecl(sizeof(FMGR_ABI_EXTRA) <= sizeof(((Pg_magic_struct*)0)->abi_extra),
+				 "FMGR_ABI_EXTRA too long");
+
 /*
  * Declare the module magic function.  It needs to be a function as the dlsym
  * in the backend is only guaranteed to work on functions, not data

src/include/pg_config_manual.h

@@ -42,6 +42,23 @@
  */
 #define FUNC_MAX_ARGS		100
 
+/*
+ * When creating a product derived from PostgreSQL with changes that cause
+ * incompatibilities for loadable modules, it is recommended to change this
+ * string so that dfmgr.c can refuse to load incompatible modules with a clean
+ * error message.  Typical examples that cause incompatibilities are any
+ * changes to node tags or node structures.  (Note that dfmgr.c already
+ * detects common sources of incompatibilities due to major version
+ * differences and due to some changed compile-time constants.  This setting
+ * is for catching anything that cannot be detected in a straightforward way.)
+ *
+ * There is no prescribed format for the string.  The suggestion is to include
+ * product or company name, and optionally any internally-relevant ABI
+ * version.  Example: "ACME Postgres/1.2".  Note that the string will appear
+ * in a user-facing error message if an ABI mismatch is detected.
+ */
+#define FMGR_ABI_EXTRA		"PostgreSQL"
+
 /*
  * Maximum number of columns in an index.  There is little point in making
  * this anything but a multiple of 32, because the main cost is associated