Re: BUG #17170: Invalid collation created with provider icu and initdb' locale C

I wrote:
> The immediate issue is that DROP COLLATION uses the same lookup
> rule as other code paths, so it fails to see inapplicable
> pg_collation entries at all.  So one approach to a fix is to
> relax its lookup rule.  But I wonder if we should have prevented
> the CREATE in the first place, instead.

After a bit of experimentation, I'm inclined to the idea that
we should have rejected the CREATE COLLATION in the first place.
It's not very useful to create a collation that you're not going
to be able to use, and I think it's more user-friendly to say so
up front than to leave the user guessing about why subsequent
references don't work.

There is arguably one use-case that this fix shuts off: manually
adding ICU-based collations to template0, in the expectation
that they could get cloned into other databases where they'd be
useful.  But that seems like a mighty thin argument, especially
since you could initdb with an ICU-compatible encoding if you
want to do that.

            regards, tom lane

diff --git a/src/backend/commands/collationcmds.c b/src/backend/commands/collationcmds.c
index 5a2ba56cec..9b8a259eb2 100644
--- a/src/backend/commands/collationcmds.c
+++ b/src/backend/commands/collationcmds.c
@@ -215,7 +215,19 @@ DefineCollation(ParseState *pstate, List *names, List *parameters, bool if_not_e
     if (!fromEl)
     {
         if (collprovider == COLLPROVIDER_ICU)
+        {
+            /*
+             * While an ICU collation can support various encodings, only
+             * allow one to be created when the current database's encoding is
+             * supported.  Otherwise we get surprising behaviors like not
+             * being able to drop the collation.
+             */
+            if (!is_encoding_supported_by_icu(GetDatabaseEncoding()))
+                ereport(ERROR,
+                        (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+                         errmsg("current database's encoding is not supported with this provider")));
             collencoding = -1;
+        }
         else
         {
             collencoding = GetDatabaseEncoding();