Merge branch 'jk/pipe-command-nonblock' into maint

Fix deadlocks between main Git process and subprocess spawned via
the pipe_command() API, that can kill "git add -p" that was
reimplemented in C recently.

* jk/pipe-command-nonblock:
  pipe_command(): mark stdin descriptor as non-blocking
  pipe_command(): handle ENOSPC when writing to a pipe
  pipe_command(): avoid xwrite() for writing to pipe
  git-compat-util: make MAX_IO_SIZE define globally available
  nonblock: support Windows
  compat: add function to enable nonblocking pipes

compat/nonblock.c

@@ -0,0 +1,50 @@
+#include "git-compat-util.h"
+#include "nonblock.h"
+
+#ifdef O_NONBLOCK
+
+int enable_pipe_nonblock(int fd)
+{
+	int flags = fcntl(fd, F_GETFL);
+	if (flags < 0)
+		return -1;
+	flags |= O_NONBLOCK;
+	return fcntl(fd, F_SETFL, flags);
+}
+
+#elif defined(GIT_WINDOWS_NATIVE)
+
+#include "win32.h"
+
+int enable_pipe_nonblock(int fd)
+{
+	HANDLE h = (HANDLE)_get_osfhandle(fd);
+	DWORD mode;
+	DWORD type = GetFileType(h);
+	if (type == FILE_TYPE_UNKNOWN && GetLastError() != NO_ERROR) {
+		errno = EBADF;
+		return -1;
+	}
+	if (type != FILE_TYPE_PIPE)
+		BUG("unsupported file type: %lu", type);
+	if (!GetNamedPipeHandleState(h, &mode, NULL, NULL, NULL, NULL, 0)) {
+		errno = err_win_to_posix(GetLastError());
+		return -1;
+	}
+	mode |= PIPE_NOWAIT;
+	if (!SetNamedPipeHandleState(h, &mode, NULL, NULL)) {
+		errno = err_win_to_posix(GetLastError());
+		return -1;
+	}
+	return 0;
+}
+
+#else
+
+int enable_pipe_nonblock(int fd)
+{
+	errno = ENOSYS;
+	return -1;
+}
+
+#endif

compat/nonblock.h

@@ -0,0 +1,9 @@
+#ifndef COMPAT_NONBLOCK_H
+#define COMPAT_NONBLOCK_H
+
+/*
+ * Enable non-blocking I/O for the pipe specified by the passed-in descriptor.
+ */
+int enable_pipe_nonblock(int fd);
+
+#endif