git-compat-util: make MAX_IO_SIZE define globally available

We define MAX_IO_SIZE within wrapper.c, but it's useful for any code
that wants to do a raw write() for whatever reason (say, because they
want different EAGAIN handling). Let's make it available everywhere.

The alternative would be adding xwrite_foo() variants to give callers
more options. But there's really no reason MAX_IO_SIZE needs to be
abstracted away, so this give callers the most flexibility.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/git-compat-util.h b/git-compat-util.h
index 58d7708296..79977caed6 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -995,6 +995,28 @@ static inline unsigned long cast_size_t_to_ulong(size_t a)
 	return (unsigned long)a;
 }
 
+/*
+ * Limit size of IO chunks, because huge chunks only cause pain.  OS X
+ * 64-bit is buggy, returning EINVAL if len >= INT_MAX; and even in
+ * the absence of bugs, large chunks can result in bad latencies when
+ * you decide to kill the process.
+ *
+ * We pick 8 MiB as our default, but if the platform defines SSIZE_MAX
+ * that is smaller than that, clip it to SSIZE_MAX, as a call to
+ * read(2) or write(2) larger than that is allowed to fail.  As the last
+ * resort, we allow a port to pass via CFLAGS e.g. "-DMAX_IO_SIZE=value"
+ * to override this, if the definition of SSIZE_MAX given by the platform
+ * is broken.
+ */
+#ifndef MAX_IO_SIZE
+# define MAX_IO_SIZE_DEFAULT (8*1024*1024)
+# if defined(SSIZE_MAX) && (SSIZE_MAX < MAX_IO_SIZE_DEFAULT)
+#  define MAX_IO_SIZE SSIZE_MAX
+# else
+#  define MAX_IO_SIZE MAX_IO_SIZE_DEFAULT
+# endif
+#endif
+
 #ifdef HAVE_ALLOCA_H
 # include <alloca.h>
 # define xalloca(size)      (alloca(size))