MINOR: global: Add an option to disable the zero-copy forwarding

The zero-copy forwarding or the mux-to-mux forwarding is a way to
fast-forward data without using the channels buffers. Data are transferred
from a mux to the other one. The kernel splicing is an optimization of the
zero-copy forwarding. But it can also use normal buffers (but not channels
ones). This way, it could be possible to fast-forward data with muxes not
supporting the kernel splicing (H2 and H3 muxes) but also with applets.

However, this mode can introduce regressions or bugs in future (just like
the kernel splicing). Thus, It could be usefull to disable this optim. To do
so, in configuration, the global tune settting
'tune.disable-zero-copy-forwarding' may be set in a global section or the
'-dZ' command line parameter may be used to start HAProxy. Of course, this
also disables the kernel splicing.

doc/configuration.txt

@@ -1175,6 +1175,7 @@ The following keywords are supported in the "global" section :
    - tune.bufsize
    - tune.comp.maxlevel
    - tune.disable-fast-forward
+   - tune.disable-zero-copy-forwarding
    - tune.fail-alloc
    - tune.fd.edge-triggered
    - tune.h2.be.initial-window-size
@@ -2892,11 +2893,18 @@ tune.disable-fast-forward [ EXPERIMENTAL ]
   Disables the data fast-forwarding. It is a mechanism to optimize the data
   forwarding by passing data directly from a side to the other one without
   waking the stream up. Thanks to this directive, it is possible to disable
-  this optimization. Note it also disable any kernel tcp splicing. This command
-  is not meant for regular use, it will generally only be suggested by
-  developers along complex debugging sessions. For this reason it is internally
-  marked as experimental, meaning that "expose-experimental-directives" must
-  appear on a line before this directive.
+  this optimization. Note it also disable any kernel tcp splicing but also the
+  zero-copy forwarding. This command is not meant for regular use, it will
+  generally only be suggested by developers along complex debugging
+  sessions. For this reason it is internally marked as experimental, meaning
+  that "expose-experimental-directives" must appear on a line before this
+  directive.
+
+tune.zero-copy-forwarding
+  Disables the zero-copy forwarding of data. It is a mechanism to optimize the
+  data fast-forwarding by avoiding to use the channel's buffer. Thanks to this
+  directive, it is possible to disable this optimization. Note it also disable
+  any kernel tcp splicing.
 
 tune.fail-alloc
   If compiled with DEBUG_FAIL_ALLOC or started with "-dMfail", gives the

include/haproxy/global-t.h

@@ -66,7 +66,7 @@
 #define GTUNE_USE_SYSTEMD        (1<<10)
 
 #define GTUNE_BUSY_POLLING       (1<<11)
-/* unused: (1<<12) */
+#define GTUNE_USE_ZERO_COPY_FWD  (1<<12)
 #define GTUNE_SET_DUMPABLE       (1<<13)
 #define GTUNE_USE_EVPORTS        (1<<14)
 #define GTUNE_STRICT_LIMITS      (1<<15)

src/cfgparse-global.c

@@ -517,6 +517,11 @@ int cfg_parse_global(const char *file, int linenum, char **args, int kwm)
 			goto out;
 		global.tune.options &= GTUNE_USE_FAST_FWD;
 	}
+	else if (strcmp(args[0], "tune.disable-zero-copy-forwarding") == 0) {
+		if (alertif_too_many_args(0, file, linenum, args, &err_code))
+			goto out;
+		global.tune.options &= GTUNE_USE_ZERO_COPY_FWD;
+	}
 	else if (strcmp(args[0], "cluster-secret") == 0) {
 		blk_SHA_CTX sha1_ctx;
 		unsigned char sha1_out[20];

src/haproxy.c

@@ -649,6 +649,7 @@ static void usage(char *name)
 		"        -dW fails if any warning is emitted\n"
 		"        -dD diagnostic mode : warn about suspicious configuration statements\n"
 		"        -dF disable fast-forward\n"
+		"        -dZ disable zero-copy forwarding\n"
 		"        -sf/-st [pid ]* finishes/terminates old pids.\n"
 		"        -x <unix_socket> get listening sockets from a unix socket\n"
 		"        -S <bind>[,<bind options>...] new master CLI\n"
@@ -1609,6 +1610,7 @@ static void init_args(int argc, char **argv)
 	global.tune.options |= GTUNE_STRICT_LIMITS;
 
 	global.tune.options |= GTUNE_USE_FAST_FWD; /* Use fast-forward by default */
+	global.tune.options |= GTUNE_USE_ZERO_COPY_FWD; /* Use zero-copy forwarding by default */
 
 	/* keep a copy of original arguments for the master process */
 	old_argv = copy_argv(argc, argv);
@@ -1664,6 +1666,8 @@ static void init_args(int argc, char **argv)
 				global.tune.options &= ~GTUNE_USE_FAST_FWD;
 			else if (*flag == 'd' && flag[1] == 'V')
 				global.ssl_server_verify = SSL_SERVER_VERIFY_NONE;
+			else if (*flag == 'd' && flag[1] == 'Z')
+				global.tune.options &= ~GTUNE_USE_ZERO_COPY_FWD;
 			else if (*flag == 'V')
 				arg_mode |= MODE_VERBOSE;
 			else if (*flag == 'd' && flag[1] == 'C') {

src/stconn.c

@@ -1271,7 +1271,8 @@ static int sc_conn_recv(struct stconn *sc)
 	/* First, let's see if we may fast-forward data from a side to the other
 	 * one without using the channel buffer.
 	 */
-	if (sc_ep_test(sc, SE_FL_MAY_FASTFWD) && ic->to_forward) {
+	if ((global.tune.options & GTUNE_USE_ZERO_COPY_FWD) &&
+	    sc_ep_test(sc, SE_FL_MAY_FASTFWD) && ic->to_forward) {
 		if (c_data(ic)) {
 			/* We're embarrassed, there are already data pending in
 			 * the buffer and we don't want to have them at two