BUILD: Re-enable -Wimplicit-fallthrough

Getting rid of this warning is cleaner solved using a 'fall through' comment,
because it clarifies intent to a human reader.

This patch adjust a few places that cause -Wimplicit-fallthrough to trigger:

- Fix typos in the comment.
- Remove redundant 'no break' that trips up gcc from comment.
- Move the comment out of the block when the 'case' is completely surrounded
  by braces.
- Add comments where I could determine that the fall through was intentional.

Changes tested on

    gcc (Debian 9.3.0-13) 9.3.0
    Copyright (C) 2019 Free Software Foundation, Inc.
    This is free software; see the source for copying conditions.  There is NO
    warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

using

    make -j4 all TARGET=linux-glibc USE_OPENSSL=1 USE_LUA=1 USE_ZLIB=1 USE_PCRE2=1 USE_PCRE2_JIT=1 USE_GETADDRINFO=1

Makefile

@@ -185,7 +185,6 @@ SPEC_CFLAGS += $(call cc-nowarn,sign-compare)
 SPEC_CFLAGS += $(call cc-nowarn,unused-parameter)
 SPEC_CFLAGS += $(call cc-nowarn,clobbered)
 SPEC_CFLAGS += $(call cc-nowarn,missing-field-initializers)
-SPEC_CFLAGS += $(call cc-nowarn,implicit-fallthrough)
 SPEC_CFLAGS += $(call cc-nowarn,stringop-overflow)
 SPEC_CFLAGS += $(call cc-nowarn,cast-function-type)
 SPEC_CFLAGS += $(call cc-nowarn,string-plus-int)

src/acl.c

@@ -607,6 +607,7 @@ struct acl_expr *parse_acl_expr(const char **args, char **err, struct arg_list *
 
 				case STD_OP_GT:
 					value++; /* gt = ge + 1 */
+					/* fall through */
 
 				case STD_OP_GE:
 					if (expr->pat.parse == pat_parse_int)
@@ -619,6 +620,7 @@ struct acl_expr *parse_acl_expr(const char **args, char **err, struct arg_list *
 
 				case STD_OP_LT:
 					value--; /* lt = le - 1 */
+					/* fall through */
 
 				case STD_OP_LE:
 					if (expr->pat.parse == pat_parse_int)

src/check.c

@@ -445,7 +445,7 @@ void __health_adjust(struct server *s, short status)
 			if (s->check.health > s->check.rise)
 				s->check.health = s->check.rise + 1;
 
-			/* no break - fall through */
+			/* fall through */
 
 		case HANA_ONERR_FAILCHK:
 		/* simulate a failed health check */

src/cli.c

@@ -1191,6 +1191,7 @@ static int cli_io_handler_show_cli_sock(struct appctx *appctx)
 				return 0;
 			}
 			appctx->st2 = STAT_ST_LIST;
+			/* fall through */
 
 		case STAT_ST_LIST:
 			if (global.stats_fe) {
@@ -1267,6 +1268,7 @@ static int cli_io_handler_show_cli_sock(struct appctx *appctx)
 					appctx->ctx.cli.p0 = &bind_conf->by_fe; /* store the latest list node dumped */
 				}
 			}
+			/* fall through */
 		default:
 			appctx->st2 = STAT_ST_FIN;
 			return 1;

src/hlua.c

@@ -6255,6 +6255,7 @@ static int hlua_sample_conv_wrapper(const struct arg *arg_p, struct sample *smp,
 	case HLUA_E_ERR:
 		/* Display log. */
 		SEND_ERR(stream->be, "Lua converter '%s' returns an unknown error.\n", fcn->name);
+		/* fall through */
 
 	default:
 		return 0;
@@ -6388,6 +6389,7 @@ static int hlua_sample_fetch_wrapper(const struct arg *arg_p, struct sample *smp
 	case HLUA_E_ERR:
 		/* Display log. */
 		SEND_ERR(smp->px, "Lua sample-fetch '%s' returns an unknown error.\n", fcn->name);
+		/* fall through */
 
 	default:
 		return 0;

src/peers.c

@@ -2281,8 +2281,8 @@ switchstate:
 				appctx->ctx.peers.ptr = curpeer;
 				appctx->st0 = PEER_SESS_ST_SENDSUCCESS;
 				_HA_ATOMIC_ADD(&active_peers, 1);
-				/* fall through */
 			}
+			/* fall through */
 			case PEER_SESS_ST_SENDSUCCESS: {
 				prev_state = appctx->st0;
 				if (!curpeer) {
@@ -2328,8 +2328,8 @@ switchstate:
 
 				/* switch to the waiting statuscode state */
 				appctx->st0 = PEER_SESS_ST_GETSTATUS;
-				/* fall through */
 			}
+			/* fall through */
 			case PEER_SESS_ST_GETSTATUS: {
 				prev_state = appctx->st0;
 				if (!curpeer) {
@@ -2370,8 +2370,8 @@ switchstate:
 				}
 				_HA_ATOMIC_ADD(&connected_peers, 1);
 				appctx->st0 = PEER_SESS_ST_WAITMSG;
-				/* fall through */
 			}
+			/* fall through */
 			case PEER_SESS_ST_WAITMSG: {
 				uint32_t msg_len = 0;
 				char *msg_cur = trash.area;
@@ -2456,8 +2456,8 @@ send_msgs:
 					goto out;
 				appctx->st0 = PEER_SESS_ST_END;
 				prev_state = appctx->st0;
-				/* fall through */
 			}
+			/* fall through */
 			case PEER_SESS_ST_END: {
 				if (prev_state == PEER_SESS_ST_WAITMSG)
 					_HA_ATOMIC_SUB(&connected_peers, 1);

src/stream_interface.c

@@ -232,6 +232,7 @@ static void stream_int_shutw(struct stream_interface *si)
 	case SI_ST_TAR:
 		/* Note that none of these states may happen with applets */
 		si->state = SI_ST_DIS;
+		/* fall through */
 	default:
 		si->flags &= ~SI_FL_NOLINGER;
 		si_rx_shut_blk(si);
@@ -1667,6 +1668,7 @@ static void stream_int_shutw_applet(struct stream_interface *si)
 		/* Note that none of these states may happen with applets */
 		si_applet_release(si);
 		si->state = SI_ST_DIS;
+		/* fall through */
 	default:
 		si->flags &= ~SI_FL_NOLINGER;
 		si_rx_shut_blk(si);

src/tcpcheck.c

@@ -3565,7 +3565,7 @@ static int check_proxy_tcpcheck(struct proxy *px)
 		case TCPCHK_ACT_CONNECT:
 			if (!chk->comment && comment)
 				chk->comment = strdup(comment);
-			/* fall though */
+			/* fall through */
 		case TCPCHK_ACT_ACTION_KW:
 			free(comment);
 			comment = NULL;