Update client varcache with canonical values from server

Since PG14, Postgres doesn't send ParameterStatus packets anymore when a
setting has not actually changed. This could cause the client varcache
and server varcache to become out of sync. Which then in turn resulted
in unnecessary `SET` commands to be sent to the server.

Fixes #776

include/varcache.h

@@ -23,3 +23,4 @@ void varcache_fill_unset(VarCache *src, PgSocket *dst);
 void varcache_clean(VarCache *cache);
 void varcache_add_params(PktBuf *pkt, VarCache *vars);
 void varcache_deinit(void);
+void varcache_set_canonical(PgSocket *server, PgSocket *client);

src/server.c

@@ -611,6 +611,18 @@ bool server_proto(SBuf *sbuf, SBufEvent evtype, struct MBuf *data)
 			PgSocket *client = server->link;
 			Assert(client);
 
+			/*
+			 * It's possible that the client vars and server vars have
+			 * different string representations, but still Postgres did not
+			 * send a ParamaterStatus packet. This happens when the server
+			 * variable is the canonical version of the client variable, i.e.
+			 * they mean the same just written slightly different. To make sure
+			 * that the canonical version is also stored in the client, we now
+			 * copy the server variables over to the client variables.
+			 * See issue #776 for an example of this.
+			 */
+			varcache_set_canonical(server, client);
+
 			server->setting_vars = false;
 			sbuf_continue(&client->sbuf);
 			break;

src/varcache.c

@@ -238,6 +238,23 @@ bool varcache_apply(PgSocket *server, PgSocket *client, bool *changes_p)
 	return pktbuf_send_immediate(pkt, server);
 }
 
+void varcache_set_canonical(PgSocket *server, PgSocket *client)
+{
+	struct PStr *server_val, *client_val;
+	const struct var_lookup *lk;
+	for (lk = lookup; lk->name; lk++) {
+		server_val = server->vars.var_list[lk->idx];
+		client_val = client->vars.var_list[lk->idx];
+		if (client_val && server_val && client_val != server_val) {
+			slog_debug(client, "varcache_set_canonical: setting %s to its canonical version %s -> %s",
+					lk->name, client_val->str, server_val->str);
+			strpool_incref(server_val);
+			strpool_decref(client_val);
+			client->vars.var_list[lk->idx] = server_val;
+		}
+	}
+}
+
 void varcache_fill_unset(VarCache *src, PgSocket *dst)
 {
 	struct PStr *srcval, *dstval;

test/test_misc.py

@@ -5,7 +5,7 @@
 import psycopg
 import pytest
 
-from .utils import HAVE_IPV6_LOCALHOST, WINDOWS
+from .utils import HAVE_IPV6_LOCALHOST, PG_MAJOR_VERSION, WINDOWS
 
 
 def test_connect_query(bouncer):
@@ -203,3 +203,18 @@ def test_options_startup_param(bouncer):
         match="unsupported startup parameter in options: enable_seqscan",
     ):
         bouncer.test(options="-c enable_seqscan=true")
+
+
+def test_equivalent_startup_param(bouncer):
+    bouncer.admin("set verbose=2")
+
+    canonical_expected_times = 1 if PG_MAJOR_VERSION >= 14 else 0
+    with bouncer.cur(options="-c DateStyle=ISO") as cur:
+        with bouncer.log_contains(
+            "varcache_apply: .*SET DateStyle='ISO'", times=1
+        ), bouncer.log_contains(
+            "varcache_set_canonical: setting DateStyle to its canonical version ISO -> ISO, MDY",
+            times=canonical_expected_times,
+        ):
+            cur.execute("SELECT 1")
+            cur.execute("SELECT 1")