Re: read() MAX_IO_SIZE bytes, more than SSIZE_MAX?

[Junio C Hamano <gitster@pobox.com>]

Joachim Schmitz <jojo@schmitz-digital.de> writes:

> Joachim Schmitz <jojo <at> schmitz-digital.de> writes:
>
>> 
>> Junio C Hamano <gitster <at> pobox.com> writes:
>> 
>> <snip> 
>> > OK, then let's do this.
>> > 
>
>
> Except for the type "taht"

Also #ifndef part X-<

Here is what I queued for the day.

-- >8 --
Subject: xread/xwrite: clip MAX_IO_SIZE to SSIZE_MAX

Since 0b6806b9 (xread, xwrite: limit size of IO to 8MB, 2013-08-20),
we chomp our calls to read(2) and write(2) into chunks of
MAX_IO_SIZE bytes (8 MiB), because a large IO results in a bad
latency when the program needs to be killed.  This also brought our
IO below SSIZE_MAX, which is a limit POSIX allows read(2) and
write(2) to fail when the IO size exceeds it, for OS X, where a
problem was originally reported.

However, there are other systems that define SSIZE_MAX smaller than
our default, and feeding 8 MiB to underlying read(2)/write(2) would
fail.  Make sure we clip our calls to the lower limit as well.

Reported-by: Joachim Schmitz <jojo@schmitz-digital.de>
Helped-by: Torsten Bögershausen <tboegi@web.de>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 wrapper.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/wrapper.c b/wrapper.c
index f92b147..c77c2eb 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -135,8 +135,21 @@ void *xcalloc(size_t nmemb, size_t size)
  * 64-bit is buggy, returning EINVAL if len >= INT_MAX; and even in
  * the absense 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 platform is broken.
  */
-#define MAX_IO_SIZE (8*1024*1024)
+#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
 
 /*
  * xread() is the same a read(), but it automatically restarts read()
-- 
2.3.0-186-g9f73ee1