BUG/MEDIUM: lua: defer Lua VM initialisation to the first Lua config keyword

HAProxy used to call hlua_init() unconditionally from step_init_1(),
before any configuration file was parsed.  As a consequence, Lua states
0 and 1 were always created with hlua_openlibs_flags set to its default
value (HLUA_OPENLIBS_ALL), regardless of any tune.lua.openlibs directive
that appeared later in the global section.  With multiple threads, states
2..N were created correctly in hlua_post_init() after the config had been
parsed, while states 0 and 1 retained the full standard-library set.
This produced the observable bug reported in GitHub issue #3396: a script
loaded with lua-load-per-thread could see require() as a function on
thread 1 but nil on thread 2 when tune.lua.openlibs was used to restrict
the available libraries.

The initialisation is now lazy.  hlua_init() is idempotent: it returns
immediately if the states already exist (hlua_states[0] != NULL).  It is
called explicitly from the three config keyword handlers that need the
Lua states to be live before they can do their work (lua-load,
lua-load-per-thread, lua-prepend-path) and from tune.lua.openlibs, after
the hlua_openlibs_flags variable has been updated, so that the states are
always created with the correct library set.

hlua_post_init() calls hlua_init() unconditionally as a safety net,
covering the case where no Lua directive appeared in the configuration at
all (no global section, or only pure-tuning directives such as timeouts
and memory limits), and ensuring correct behaviour with multiple
consecutive global sections.

As a result of this change, tune.lua.openlibs must now appear before
lua-load, lua-load-per-thread, and lua-prepend-path in the configuration;
if any of those keywords is encountered first, the Lua states will already
be initialised and tune.lua.openlibs with a non-default value will return
a parse error.

No backport needed.

doc/configuration.txt

@@ -4940,8 +4940,8 @@ tune.lua.openlibs [all | none | <lib>[,<lib>...]]
     tune.lua.openlibs string,math,table,utf8  # safe subset, no I/O or OS
     tune.lua.openlibs all                     # default, load everything
 
-  This setting must be set before any "lua-load" or "lua-load-per-thread"
+  This setting must be set before any "lua-load", "lua-load-per-thread" or
-  directive, otherwise a parse error is returned.
+  "lua-prepend-path" directive, otherwise a parse error is returned.
 
 tune.lua.service-timeout <timeout>
   This is the execution timeout for the Lua services. This is useful for

src/haproxy.c

@@ -2139,9 +2139,6 @@ static void step_init_1()
 	if (init_acl() != 0)
 		exit(1);
 
-	/* Initialise lua. */
-	hlua_init();
-
 	/* set modes given from cmdline */
 	global.mode |= (arg_mode & (MODE_DAEMON | MODE_MWORKER | MODE_FOREGROUND | MODE_VERBOSE
 				    | MODE_QUIET | MODE_CHECK | MODE_DEBUG | MODE_ZERO_WARNING

src/hlua.c

@@ -13392,6 +13392,16 @@ static int hlua_cfg_parse_openlibs(char **args, int section_type, struct proxy *
 		return -1;
 	}
 
+	/* Reject a non-default restriction if the Lua VM is already initialised,
+	 * which happens when lua-load, lua-load-per-thread or lua-prepend-path
+	 * appeared before this directive.
+	 */
+	if (flags != HLUA_OPENLIBS_ALL && hlua_states[0]) {
+		memprintf(err, "'%s' must appear before any 'lua-load', 'lua-load-per-thread' or 'lua-prepend-path' directive",
+		           args[0]);
+		return -1;
+	}
+
 	hlua_openlibs_flags = flags;
 	return 0;
 }
@@ -13492,6 +13502,8 @@ static int hlua_load(char **args, int section_type, struct proxy *curpx,
 		return -1;
 	}
 
+	hlua_init();
+
 	/* loading for global state */
 	hlua_state_id = 0;
 	ha_set_thread(NULL);
@@ -13510,6 +13522,8 @@ static int hlua_load_per_thread(char **args, int section_type, struct proxy *cur
 		return -1;
 	}
 
+	hlua_init();
+
 	if (per_thread_load == NULL) {
 		/* allocate the first entry large enough to store the final NULL */
 		per_thread_load = calloc(1, sizeof(*per_thread_load));
@@ -13598,6 +13612,8 @@ static int hlua_config_prepend_path(char **args, int section_type, struct proxy
 	struct prepend_path *p = NULL;
 	size_t i;
 
+	hlua_init();
+
 	if (too_many_args(2, args, err, NULL)) {
 		goto err;
 	}
@@ -14006,6 +14022,11 @@ int hlua_post_init()
 
 	hlua_body = 0;
 
+	/* Ensure the Lua VM is initialised even if no Lua directive appeared
+	 * in the configuration (e.g. no global section at all).
+	 */
+	hlua_init();
+
 #if defined(USE_OPENSSL)
 	/* Initialize SSL server. */
 	if (socket_ssl->xprt->prepare_srv) {
@@ -14833,6 +14854,9 @@ void hlua_init(void) {
 	};
 #endif
 
+	if (hlua_states[0])
+		return; /* already initialised */
+
 	/* Init post init function list head */
 	for (i = 0; i < MAX_THREADS + 1; i++)
 		LIST_INIT(&hlua_init_functions[i]);