t0051: test GIT_TRACE to a windows named pipe

Create a test-tool helper to create the server side of
a windows named pipe, wait for a client connection, and
copy data written to the pipe to stdout.

Create t0051 test to route GIT_TRACE output of a command
to a named pipe using the above test-tool helper.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-windows-named-pipe.c

@@ -0,0 +1,72 @@
+#include "test-tool.h"
+#include "git-compat-util.h"
+#include "strbuf.h"
+
+#ifdef GIT_WINDOWS_NATIVE
+static const char *usage_string = "<pipe-filename>";
+
+#define TEST_BUFSIZE (4096)
+
+int cmd__windows_named_pipe(int argc, const char **argv)
+{
+	const char *filename;
+	struct strbuf pathname = STRBUF_INIT;
+	int err;
+	HANDLE h;
+	BOOL connected;
+	char buf[TEST_BUFSIZE + 1];
+
+	if (argc < 2)
+		goto print_usage;
+	filename = argv[1];
+	if (strchr(filename, '/') || strchr(filename, '\\'))
+		goto print_usage;
+	strbuf_addf(&pathname, "//./pipe/%s", filename);
+
+	/*
+	 * Create a single instance of the server side of the named pipe.
+	 * This will allow exactly one client instance to connect to it.
+	 */
+	h = CreateNamedPipeA(
+		pathname.buf,
+		PIPE_ACCESS_INBOUND | FILE_FLAG_FIRST_PIPE_INSTANCE,
+		PIPE_TYPE_MESSAGE | PIPE_READMODE_MESSAGE | PIPE_WAIT,
+		PIPE_UNLIMITED_INSTANCES,
+		TEST_BUFSIZE, TEST_BUFSIZE, 0, NULL);
+	if (h == INVALID_HANDLE_VALUE) {
+		err = err_win_to_posix(GetLastError());
+		fprintf(stderr, "CreateNamedPipe failed: %s\n",
+			strerror(err));
+		return err;
+	}
+
+	connected = ConnectNamedPipe(h, NULL)
+		? TRUE
+		: (GetLastError() == ERROR_PIPE_CONNECTED);
+	if (!connected) {
+		err = err_win_to_posix(GetLastError());
+		fprintf(stderr, "ConnectNamedPipe failed: %s\n",
+			strerror(err));
+		CloseHandle(h);
+		return err;
+	}
+
+	while (1) {
+		DWORD nbr;
+		BOOL success = ReadFile(h, buf, TEST_BUFSIZE, &nbr, NULL);
+		if (!success || nbr == 0)
+			break;
+		buf[nbr] = 0;
+
+		write(1, buf, nbr);
+	}
+
+	DisconnectNamedPipe(h);
+	CloseHandle(h);
+	return 0;
+
+print_usage:
+	fprintf(stderr, "usage: %s %s\n", argv[0], usage_string);
+	return 1;
+}
+#endif