Allow postgres_fdw to ship extension funcs/operators for remote execution.

The user can whitelist specified extension(s) in the foreign server's
options, whereupon we will treat immutable functions and operators of those
extensions as candidates to be sent for remote execution.

Whitelisting an extension in this way basically promises that the extension
exists on the remote server and behaves compatibly with the local instance.
We have no way to prove that formally, so we have to rely on the user to
get it right.  But this seems like something that people can usually get
right in practice.

We might in future allow functions and operators to be whitelisted
individually, but extension granularity is a very convenient special case,
so it got done first.

The patch as-committed lacks any regression tests, which is unfortunate,
but introducing dependencies on other extensions for testing purposes
would break "make installcheck" scenarios, which is worse.  I have some
ideas about klugy ways around that, but it seems like material for a
separate patch.  For the moment, leave the problem open.

Paul Ramsey, hacked up a bit more by me

1 files changed, 54 insertions, 0 deletions

diff --git a/contrib/postgres_fdw/option.c b/contrib/postgres_fdw/option.c
index 7547ec28172..380ac80ab3a 100644
--- a/contrib/postgres_fdw/option.c
+++ b/contrib/postgres_fdw/option.c
@@ -19,6 +19,8 @@
 #include "catalog/pg_foreign_table.h"
 #include "catalog/pg_user_mapping.h"
 #include "commands/defrem.h"
+#include "commands/extension.h"
+#include "utils/builtins.h"
 
 
 /*
@@ -124,6 +126,11 @@ postgres_fdw_validator(PG_FUNCTION_ARGS)
 						 errmsg("%s requires a non-negative numeric value",
 								def->defname)));
 		}
+		else if (strcmp(def->defname, "extensions") == 0)
+		{
+			/* check list syntax, warn about uninstalled extensions */
+			(void) ExtractExtensionList(defGetString(def), true);
+		}
 	}
 
 	PG_RETURN_VOID();
@@ -150,6 +157,8 @@ InitPgFdwOptions(void)
 		/* cost factors */
 		{"fdw_startup_cost", ForeignServerRelationId, false},
 		{"fdw_tuple_cost", ForeignServerRelationId, false},
+		/* shippable extensions */
+		{"extensions", ForeignServerRelationId, false},
 		/* updatable is available on both server and table */
 		{"updatable", ForeignServerRelationId, false},
 		{"updatable", ForeignTableRelationId, false},
@@ -293,3 +302,48 @@ ExtractConnectionOptions(List *defelems, const char **keywords,
 	}
 	return i;
 }
+
+/*
+ * Parse a comma-separated string and return a List of the OIDs of the
+ * extensions named in the string.  If any names in the list cannot be
+ * found, report a warning if warnOnMissing is true, else just silently
+ * ignore them.
+ */
+List *
+ExtractExtensionList(const char *extensionsString, bool warnOnMissing)
+{
+	List	   *extensionOids = NIL;
+	List	   *extlist;
+	ListCell   *lc;
+
+	/* SplitIdentifierString scribbles on its input, so pstrdup first */
+	if (!SplitIdentifierString(pstrdup(extensionsString), ',', &extlist))
+	{
+		/* syntax error in name list */
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("parameter \"%s\" must be a list of extension names",
+						"extensions")));
+	}
+
+	foreach(lc, extlist)
+	{
+		const char *extension_name = (const char *) lfirst(lc);
+		Oid			extension_oid = get_extension_oid(extension_name, true);
+
+		if (OidIsValid(extension_oid))
+		{
+			extensionOids = lappend_oid(extensionOids, extension_oid);
+		}
+		else if (warnOnMissing)
+		{
+			ereport(WARNING,
+					(errcode(ERRCODE_UNDEFINED_OBJECT),
+					 errmsg("extension \"%s\" is not installed",
+							extension_name)));
+		}
+	}
+
+	list_free(extlist);
+	return extensionOids;
+}