MINOR: server: implement "add server help"

Implement "help" as a sub-command for "add server" CLI. The objective is
to list all the keywords that are supported for dynamic servers. CLI IO
handler and add_srv_ctx are used to support reentrancy on full output
buffer.

Now that this command is implemented, the outdated keyword list on "add
server" from management documentation can be removed.

doc/management.txt

@@ -1724,8 +1724,9 @@ add server <backend>/<server> [args]*
   The <server> name must not be already used in the backend. A special
   restriction is put on the backend which must used a dynamic load-balancing
   algorithm. A subset of keywords from the server config file statement can be
-  used to configure the server behavior. Also note that no settings will be
-  reused from an hypothetical 'default-server' statement in the same backend.
+  used to configure the server behavior (see "add server help" to list them).
+  Also note that no settings will be reused from an hypothetical
+  'default-server' statement in the same backend.
 
   Currently a dynamic server is statically initialized with the "none"
   init-addr method. This means that no resolution will be undertaken if a FQDN
@@ -1755,78 +1756,10 @@ add server <backend>/<server> [args]*
   servers. Please refer to the "u-limit" global keyword documentation in this
   case.
 
-  Here is the list of the currently supported keywords :
-
-  - agent-addr
-  - agent-check
-  - agent-inter
-  - agent-port
-  - agent-send
-  - allow-0rtt
-  - alpn
-  - addr
-  - backup
-  - ca-file
-  - check
-  - check-alpn
-  - check-proto
-  - check-send-proxy
-  - check-sni
-  - check-ssl
-  - check-via-socks4
-  - ciphers
-  - ciphersuites
-  - cookie
-  - crl-file
-  - crt
-  - disabled
-  - downinter
-  - error-limit
-  - fall
-  - fastinter
-  - force-sslv3/tlsv10/tlsv11/tlsv12/tlsv13
-  - id
-  - init-state
-  - inter
-  - maxconn
-  - maxqueue
-  - minconn
-  - no-ssl-reuse
-  - no-sslv3/tlsv10/tlsv11/tlsv12/tlsv13
-  - no-tls-tickets
-  - npn
-  - observe
-  - on-error
-  - on-marked-down
-  - on-marked-up
-  - pool-low-conn
-  - pool-max-conn
-  - pool-purge-delay
-  - port
-  - proto
-  - proxy-v2-options
-  - rise
-  - send-proxy
-  - send-proxy-v2
-  - send-proxy-v2-ssl
-  - send-proxy-v2-ssl-cn
-  - slowstart
-  - sni
-  - source
-  - ssl
-  - ssl-max-ver
-  - ssl-min-ver
-  - tfo
-  - tls-tickets
-  - track
-  - usesrc
-  - verify
-  - verifyhost
-  - weight
-  - ws
-
-  Their syntax is similar to the server line from the configuration file,
-  please refer to their individual documentation for details.
+add server help
+  List the keywords supported for dynamic servers by the current haproxy
+  version. Keyword syntax is similar to the server line from the configuration
+  file, please refer to their individual documentation for details.
 
 add ssl ca-file <cafile> <payload>
    Add a new certificate to a ca-file. This command is useful when you reached

src/server.c

@@ -5816,6 +5816,7 @@ int srv_init_per_thr(struct server *srv)
 /* Distinguish between "add server" default usage or one of its sub-commands. */
 enum add_srv_mode {
 	ADD_SRV_MODE_DEF,  /* default mode, IO handler should be skipped by parser. */
+	ADD_SRV_MODE_HELP, /* help mode to list supported keywords */
 };
 
 /* Context for "add server" CLI. */
@@ -5825,12 +5826,45 @@ struct add_srv_ctx {
 	void *obj2;
 };
 
-/* Handler for "add server" command. Should be reserved to extra sub-commands. */
+/* Handler for "add server" command. Should be reserved to extra sub-commands
+ * such as "help".
+ */
 int cli_io_handler_add_server(struct appctx *appctx)
 {
 	struct add_srv_ctx *ctx = appctx->svcctx;
+	struct srv_kw_list *kwl = ctx->obj1;
+	struct srv_kw *kw;
 
 	switch (ctx->mode) {
+	case ADD_SRV_MODE_HELP:
+		if (!kwl) {
+			/* first invocation */
+			if (applet_putstr(appctx, "List of keywords supported for dynamic server:\n") < 0)
+				return cli_err(appctx, "output error");
+
+			kwl = LIST_NEXT(&srv_keywords.list, struct srv_kw_list *, list);
+			ctx->obj1 = kwl;
+			ctx->obj2 = kwl->kw;
+		}
+
+		while (kwl != &srv_keywords) {
+			for (kw = ctx->obj2; kw->kw; ++kw) {
+				if (!kw->dynamic_ok)
+					continue;
+
+				ctx->obj2 = kw;
+				chunk_reset(&trash);
+				chunk_printf(&trash, "%s\n", kw->kw);
+				if (applet_putchk(appctx, &trash) == -1)
+					goto full;
+			}
+
+			kwl = LIST_NEXT(&kwl->list, struct srv_kw_list *, list);
+			ctx->obj1 = kwl;
+			ctx->obj2 = kwl->kw;
+		}
+		break;
+
 	case ADD_SRV_MODE_DEF:
 		/* Add srv parser must return 1 to prevent I/O handler execution in default mode. */
 		ABORT_NOW();
@@ -5838,6 +5872,9 @@ int cli_io_handler_add_server(struct appctx *appctx)
 	}
 
 	return 1;
+
+ full:
+	return 0;
 }
 
 /* Parse a "add server" command.
@@ -5861,6 +5898,12 @@ static int cli_parse_add_server(char **args, char *payload, struct appctx *appct
 
 	++args;
 
+	if (strcmp(args[1], "help") == 0) {
+		ctx->mode = ADD_SRV_MODE_HELP;
+		ctx->obj2 = ctx->obj1 = NULL;
+		return 0;
+	}
+
 	ctx->mode = ADD_SRV_MODE_DEF;
 	sv_name = be_name = args[1];
 	/* split backend/server arg */