mingw: optionally disable side-band-64k for transport

Since commit 0c499ea (send-pack: demultiplex a sideband stream with
status data, 2010-02-05) the built-in send-pack uses the side-band-64k
capability if advertised by the server.

Unfortunately this breaks pushing over the dump git protocol if used
over a network connection when using MinGW (but *not* when using
mingw-w64).

The detailed reasons for this, are courtesy of Jeff Preshing, quoted
from https://groups.google.com/d/msg/msysgit/at8D7J-h7mw/eaLujILGUWoJ:

	MinGW wraps Windows sockets in CRT file descriptors in order to
	mimic the functionality of POSIX sockets. This causes msvcrt.dll
	to treat sockets as Installable File System (IFS) handles,
	calling ReadFile, WriteFile, DuplicateHandle and CloseHandle on
	them. This approach works well in simple cases on recent
	versions of Windows, but does not support all usage patterns.
	In particular, using this approach, any attempt to read & write
	concurrently on the same socket (from one or more processes)
	will deadlock in a scenario where the read waits for a response
	from the server which is only invoked after the write. This is
	what send_pack currently attempts to do in the use_sideband
	codepath.

The new config option "sendpack.sideband" allows to override the
side-band-64k capability of the server, and thus makes the plain git://
protocol work.

As this only affects builds against MinGW, the default is still to use
side band.

[jes: split out the documentation into Documentation/config/, touched up
the commit message.]

Signed-off-by: Thomas Braun <thomas.braun@byte-physics.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Documentation/config.txt

@@ -406,6 +406,8 @@ include::config/reset.txt[]
 
 include::config/sendemail.txt[]
 
+include::config/sendpack.txt[]
+
 include::config/sequencer.txt[]
 
 include::config/showbranch.txt[]

Documentation/config/sendpack.txt

@@ -0,0 +1,3 @@
+sendpack.sideband::
+	Allows to disable the side-band-64k capability for send-pack even
+	when it is advertised by the server. Defaults to true.

send-pack.c

@@ -38,6 +38,16 @@ int option_parse_push_signed(const struct option *opt,
 	die("bad %s argument: %s", opt->long_name, arg);
 }
 
+static int config_use_sideband = 1;
+
+static int send_pack_config(const char *var, const char *value, void *unused)
+{
+	if (!strcmp("sendpack.sideband", var))
+		config_use_sideband = git_config_bool(var, value);
+
+	return 0;
+}
+
 static void feed_object(const struct object_id *oid, FILE *fh, int negative)
 {
 	if (negative && !has_object_file(oid))
@@ -390,14 +400,16 @@ int send_pack(struct send_pack_args *args,
 	const char *push_cert_nonce = NULL;
 	struct packet_reader reader;
 
+	git_config(send_pack_config, NULL);
+
 	/* Does the other end support the reporting? */
 	if (server_supports("report-status"))
 		status_report = 1;
 	if (server_supports("delete-refs"))
 		allow_deleting_refs = 1;
 	if (server_supports("ofs-delta"))
 		args->use_ofs_delta = 1;
-	if (server_supports("side-band-64k"))
+	if (config_use_sideband && server_supports("side-band-64k"))
 		use_sideband = 1;
 	if (server_supports("quiet"))
 		quiet_supported = 1;