Add comments re text <-> bytea internal equivalence in convert routines.

diff --git a/src/backend/utils/mb/mbutils.c b/src/backend/utils/mb/mbutils.c
index 4a14f769439848eebf76f9e4bf720fe7ffcb4777..4c7a7afca0138338e845242fe119709b6f39dc08 100644 (file)
--- a/src/backend/utils/mb/mbutils.c
+++ b/src/backend/utils/mb/mbutils.c
@@ -4,7 +4,7 @@
  * (currently mule internal code (mic) is used)
  * Tatsuo Ishii
  *
- * $PostgreSQL: pgsql/src/backend/utils/mb/mbutils.c,v 1.65 2007/09/24 14:59:37 adunstan Exp $
+ * $PostgreSQL: pgsql/src/backend/utils/mb/mbutils.c,v 1.66 2007/09/24 16:38:24 adunstan Exp $
  */
 #include "postgres.h"
 
@@ -305,6 +305,10 @@ pg_convert_to(PG_FUNCTION_ARGS)
                            namein, CStringGetDatum(DatabaseEncoding->name));
    Datum       result;
 
+   /* pg_convert expects a bytea as its first argument. We're passing it
+    * a text argument here, relying on the fact that they are both in fact
+    * varlena types, and thus structurally identical.
+    */
    result = DirectFunctionCall3(
                 pg_convert, string, src_encoding_name, dest_encoding_name);
 
@@ -334,6 +338,12 @@ pg_convert_from(PG_FUNCTION_ARGS)
    /* free memory allocated by namein */
    pfree((void *) src_encoding_name);
 
+   /* pg_convert returns a bytea, which we in turn return as text, relying 
+    * on the fact that they are both in fact varlena types, and thus 
+    * structurally identical. Although not all bytea values are valid text,
+    * in this case it will be because we've told pg_convert to return one
+    * that is valid as text in the current database encoding.
+    */
    PG_RETURN_TEXT_P(result);
 }