Add hooks for session start and session end, take two

These hooks can be used in loadable modules.  A simple test module is
included.

The first attempt was done with cd8ce3a but we lacked handling for
NO_INSTALLCHECK in the MSVC scripts (problem solved afterwards by
431f1599) so the buildfarm got angry.  This also fixes a couple of
issues noticed upon review compared to the first attempt, so the code
has slightly changed, resulting in a more simple test module.

Author: Fabrízio de Royes Mello, Yugo Nagata
Reviewed-by: Andrew Dunstan, Michael Paquier, Aleksandr Parfenov
Discussion: https://postgr.es/m/20170720204733.40f2b7eb.nagata@sraoss.co.jp
Discussion: https://postgr.es/m/20190823042602.GB5275@paquier.xyz

1 files changed, 146 insertions, 0 deletions

diff --git a/src/test/modules/test_session_hooks/test_session_hooks.c b/src/test/modules/test_session_hooks/test_session_hooks.c
new file mode 100644
index 00000000000..d047c5d219a
--- /dev/null
+++ b/src/test/modules/test_session_hooks/test_session_hooks.c
@@ -0,0 +1,146 @@
+/* -------------------------------------------------------------------------
+ *
+ * test_session_hooks.c
+ * 		Code for testing start and end session hooks.
+ *
+ * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *		src/test/modules/test_session_hooks/test_session_hooks.c
+ *
+ * -------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "access/xact.h"
+#include "commands/dbcommands.h"
+#include "executor/spi.h"
+#include "lib/stringinfo.h"
+#include "miscadmin.h"
+#include "tcop/tcopprot.h"
+#include "utils/snapmgr.h"
+#include "utils/builtins.h"
+
+PG_MODULE_MAGIC;
+
+/* Entry point of library loading/unloading */
+void		_PG_init(void);
+void		_PG_fini(void);
+
+/* GUC variables */
+static char *session_hook_username = "postgres";
+
+/* Previous hooks on stack */
+static session_start_hook_type prev_session_start_hook = NULL;
+static session_end_hook_type prev_session_end_hook = NULL;
+
+static void
+register_session_hook(const char *hook_at)
+{
+	const char *username;
+
+	StartTransactionCommand();
+	SPI_connect();
+	PushActiveSnapshot(GetTransactionSnapshot());
+
+	/* Check the current user validity */
+	username = GetUserNameFromId(GetUserId(), false);
+
+	/* Register log just for configured username */
+	if (strcmp(username, session_hook_username) == 0)
+	{
+		const char *dbname;
+		int			ret;
+		StringInfoData buf;
+
+		dbname = get_database_name(MyDatabaseId);
+
+		initStringInfo(&buf);
+
+		appendStringInfo(&buf, "INSERT INTO session_hook_log (dbname, username, hook_at) ");
+		appendStringInfo(&buf, "VALUES (%s, %s, %s);",
+						 quote_literal_cstr(dbname),
+						 quote_literal_cstr(username),
+						 quote_literal_cstr(hook_at));
+
+		ret = SPI_exec(buf.data, 0);
+		if (ret != SPI_OK_INSERT)
+			elog(ERROR, "SPI_execute failed: error code %d", ret);
+	}
+
+	SPI_finish();
+	PopActiveSnapshot();
+	CommitTransactionCommand();
+}
+
+/* sample session start hook function */
+static void
+sample_session_start_hook(void)
+{
+	if (prev_session_start_hook)
+		prev_session_start_hook();
+
+	/* consider only normal backends */
+	if (MyBackendId == InvalidBackendId)
+		return;
+
+	/* consider backends connected to a database */
+	if (!OidIsValid(MyDatabaseId))
+		return;
+
+	register_session_hook("START");
+}
+
+/* sample session end hook function */
+static void
+sample_session_end_hook(void)
+{
+	if (prev_session_end_hook)
+		prev_session_end_hook();
+
+	/* consider only normal backends */
+	if (MyBackendId == InvalidBackendId)
+		return;
+
+	/* consider backends connected to a database */
+	if (!OidIsValid(MyDatabaseId))
+		return;
+
+	register_session_hook("END");
+}
+
+/*
+ * Module load callback
+ */
+void
+_PG_init(void)
+{
+	/* Save previous hooks */
+	prev_session_start_hook = session_start_hook;
+	prev_session_end_hook = session_end_hook;
+
+	/* Set new hooks */
+	session_start_hook = sample_session_start_hook;
+	session_end_hook = sample_session_end_hook;
+
+	/* Load GUCs */
+	DefineCustomStringVariable("test_session_hooks.username",
+							   "Username to register log on session start or end",
+							   NULL,
+							   &session_hook_username,
+							   "postgres",
+							   PGC_SIGHUP,
+							   0, NULL, NULL, NULL);
+}
+
+/*
+ * Module unload callback
+ */
+void
+_PG_fini(void)
+{
+	/* Uninstall hooks */
+	session_start_hook = prev_session_start_hook;
+	session_end_hook = prev_session_end_hook;
+}