[sql-over-http] Reset session state between pooled connection re-use (#12681)

Session variables can be set during one sql-over-http query and observed
on another when that pooled connection is re-used. To address this we
can use `RESET ALL;` before re-using the connection. LKB-2495

To be on the safe side, we can opt for a full `DISCARD ALL;`, but that
might have performance regressions since it also clears any query plans.
See pgbouncer docs
https://www.pgbouncer.org/config.html#server_reset_query.

`DISCARD ALL` is currently defined as:
```
CLOSE ALL;
SET SESSION AUTHORIZATION DEFAULT;
RESET ALL;
DEALLOCATE ALL;
UNLISTEN *;
SELECT pg_advisory_unlock_all();
DISCARD PLANS;
DISCARD TEMP;
DISCARD SEQUENCES;
```

I've opted to keep everything here except the `DISCARD PLANS`. I've
modified the code so that this query is executed in the background when
a connection is returned to the pool, rather than when taken from the
pool.

This should marginally improve performance for Neon RLS by removing 1
(localhost) round trip. I don't believe that keeping query plans could
be a security concern. It's a potential side channel, but I can't
imagine what you could extract from it.

---

Thanks to
https://github.com/neondatabase/neon/pull/12659#discussion_r2219016205
for probing the idea in my head.

test_runner/fixtures/neon_fixtures.py

@@ -3910,6 +3910,41 @@ class NeonProxy(PgProtocol):
             assert response.status_code == expected_code, f"response: {response.json()}"
         return response.json()
 
+    def http_multiquery(self, *queries, **kwargs):
+        # TODO maybe use default values if not provided
+        user = quote(kwargs["user"])
+        password = quote(kwargs["password"])
+        expected_code = kwargs.get("expected_code")
+        timeout = kwargs.get("timeout")
+
+        json_queries = []
+        for query in queries:
+            if type(query) is str:
+                json_queries.append({"query": query})
+            else:
+                [query, params] = query
+                json_queries.append({"query": query, "params": params})
+
+        queries_str = [j["query"] for j in json_queries]
+        log.info(f"Executing http queries: {queries_str}")
+
+        connstr = f"postgresql://{user}:{password}@{self.domain}:{self.proxy_port}/postgres"
+        response = requests.post(
+            f"https://{self.domain}:{self.external_http_port}/sql",
+            data=json.dumps({"queries": json_queries}),
+            headers={
+                "Content-Type": "application/sql",
+                "Neon-Connection-String": connstr,
+                "Neon-Pool-Opt-In": "true",
+            },
+            verify=str(self.test_output_dir / "proxy.crt"),
+            timeout=timeout,
+        )
+
+        if expected_code is not None:
+            assert response.status_code == expected_code, f"response: {response.json()}"
+        return response.json()
+
     async def http2_query(self, query, args, **kwargs):
         # TODO maybe use default values if not provided
         user = kwargs["user"]

test_runner/regress/test_proxy.py

@@ -17,9 +17,6 @@ if TYPE_CHECKING:
     from typing import Any
 
 
-GET_CONNECTION_PID_QUERY = "SELECT pid FROM pg_stat_activity WHERE state = 'active'"
-
-
 @pytest.mark.asyncio
 async def test_http_pool_begin_1(static_proxy: NeonProxy):
     static_proxy.safe_psql("create user http_auth with password 'http' superuser")
@@ -479,7 +476,7 @@ def test_sql_over_http_pool(static_proxy: NeonProxy):
 
     def get_pid(status: int, pw: str, user="http_auth") -> Any:
         return static_proxy.http_query(
-            GET_CONNECTION_PID_QUERY,
+            "SELECT pg_backend_pid() as pid",
             [],
             user=user,
             password=pw,
@@ -513,6 +510,35 @@ def test_sql_over_http_pool(static_proxy: NeonProxy):
     assert "password authentication failed for user" in res["message"]
 
 
+def test_sql_over_http_pool_settings(static_proxy: NeonProxy):
+    static_proxy.safe_psql("create user http_auth with password 'http' superuser")
+
+    def multiquery(*queries) -> Any:
+        results = static_proxy.http_multiquery(
+            *queries,
+            user="http_auth",
+            password="http",
+            expected_code=200,
+        )
+
+        return [result["rows"] for result in results["results"]]
+
+    [[intervalstyle]] = static_proxy.safe_psql("SHOW IntervalStyle")
+    assert intervalstyle == "postgres", "'postgres' is the default IntervalStyle in postgres"
+
+    result = multiquery("select '0 seconds'::interval as interval")
+    assert result[0][0]["interval"] == "00:00:00", "interval is expected in postgres format"
+
+    result = multiquery(
+        "SET IntervalStyle = 'iso_8601'",
+        "select '0 seconds'::interval as interval",
+    )
+    assert result[1][0]["interval"] == "PT0S", "interval is expected in ISO-8601 format"
+
+    result = multiquery("select '0 seconds'::interval as interval")
+    assert result[0][0]["interval"] == "00:00:00", "interval is expected in postgres format"
+
+
 def test_sql_over_http_urlencoding(static_proxy: NeonProxy):
     static_proxy.safe_psql("create user \"http+auth$$\" with password '%+$^&*@!' superuser")
 
@@ -544,23 +570,37 @@ def test_http_pool_begin(static_proxy: NeonProxy):
     query(200, "SELECT 1;")  # Query that should succeed regardless of the transaction
 
 
-def test_sql_over_http_pool_idle(static_proxy: NeonProxy):
+def test_sql_over_http_pool_tx_reuse(static_proxy: NeonProxy):
     static_proxy.safe_psql("create user http_auth2 with password 'http' superuser")
 
-    def query(status: int, query: str) -> Any:
+    def query(status: int, query: str, *args) -> Any:
         return static_proxy.http_query(
             query,
-            [],
+            args,
             user="http_auth2",
             password="http",
             expected_code=status,
         )
 
-    pid1 = query(200, GET_CONNECTION_PID_QUERY)["rows"][0]["pid"]
+    def query_pid_txid() -> Any:
+        result = query(
+            200,
+            "SELECT pg_backend_pid() as pid, pg_current_xact_id() as txid",
+        )
+
+        return result["rows"][0]
+
+    res0 = query_pid_txid()
+
     time.sleep(0.02)
     query(200, "BEGIN")
-    pid2 = query(200, GET_CONNECTION_PID_QUERY)["rows"][0]["pid"]
-    assert pid1 != pid2
+
+    res1 = query_pid_txid()
+    res2 = query_pid_txid()
+
+    assert res0["pid"] == res1["pid"], "connection should be reused"
+    assert res0["pid"] == res2["pid"], "connection should be reused"
+    assert res1["txid"] != res2["txid"], "txid should be different"
 
 
 @pytest.mark.timeout(60)