Re: write() _will_ fail on Mac OS X/XNU if nbytes > INT_MAX

Filipe Cabecinhas [off-list ref] writes:

Due to a bug in the Darwin kernel, write() calls have a maximum size of
INT_MAX bytes.

This patch introduces a new compat function: clipped_write
This function behaves the same as write() but will write, at most, INT_MAX
characters.
It may be necessary to include this function on Windows, too.

Signed-off-by: Filipe Cabecinhas <redacted>
---

Somehow your MUA seems to have lost _all_ tabs, not just in the new
lines in your patch but also in the existing context lines.

Here is what I resurrected and queued. I _think_ I did not make any
silly mistake while transcribing from your whitespace-mangled patch,
but please double check; I do not have any Darwin, so this hasn't
even been compile tested.

Also I have a small suggestion I'd like you to try on top of it,
which I'll be sending in a separate message.

Thanks.

-- >8 --
From: Filipe Cabecinhas <redacted>
Date: Fri, 10 May 2013 15:24:57 -0700
Subject: [PATCH] compate/clipped-write.c: large write(2) fails on Mac OS X/XNU

Due to a bug in the Darwin kernel, write(2) calls have a maximum size
of INT_MAX bytes.

Introduce a new compat function, clipped_write(), that only writes
at most INT_MAX bytes and returns the number of bytes written, as
a substitute for write(2), and allow platforms that need this to
enable it from the build mechanism with NEEDS_CLIPPED_WRITE.

Set it for Mac OS X by default.  It may be necessary to include this
function on Windows, too.

Signed-off-by: Filipe Cabecinhas <redacted>
Signed-off-by: Junio C Hamano <redacted>
---
 Makefile               |  5 +++++
 compat/clipped-write.c | 13 +++++++++++++
 config.mak.uname       |  1 +
 git-compat-util.h      |  5 +++++
 4 files changed, 24 insertions(+)
 create mode 100644 compat/clipped-write.c

diff --git a/Makefile b/Makefile
index 26d3332..7076b15 100644
--- a/Makefile
+++ b/Makefile

@@ -1463,6 +1463,11 @@ ifndef NO_MSGFMT_EXTENDED_OPTIONS
 	MSGFMT += --check --statistics
 endif
 
+ifdef NEEDS_CLIPPED_WRITE
+	BASIC_CFLAGS += -DNEEDS_CLIPPED_WRITE
+	COMPAT_OBJS += compat/clipped-write.o
+endif
+
 ifneq (,$(XDL_FAST_HASH))
 	BASIC_CFLAGS += -DXDL_FAST_HASH
 endif

diff --git a/compat/clipped-write.c b/compat/clipped-write.c
new file mode 100644
index 0000000..9183698
--- /dev/null
+++ b/compat/clipped-write.c

@@ -0,0 +1,13 @@
+#include <limits.h>
+#include <unistd.h>
+
+/*
+ * Version of write that will write at most INT_MAX bytes.
+ * Workaround a xnu bug on Mac OS X
+ */
+ssize_t clipped_write(int fildes, const void *buf, size_t nbyte)
+{
+	if (nbyte > INT_MAX)
+		nbyte = INT_MAX;
+	return write(fildes, buf, nbyte);
+}

diff --git a/config.mak.uname b/config.mak.uname
index e09af8f..e689a9a 100644
--- a/config.mak.uname
+++ b/config.mak.uname

@@ -95,6 +95,7 @@ ifeq ($(uname_S),Darwin)
 	NO_MEMMEM = YesPlease
 	USE_ST_TIMESPEC = YesPlease
 	HAVE_DEV_TTY = YesPlease
+	NEEDS_CLIPPED_WRITE = YesPlease
 	COMPAT_OBJS += compat/precompose_utf8.o
 	BASIC_CFLAGS += -DPRECOMPOSE_UNICODE
 endif

diff --git a/git-compat-util.h b/git-compat-util.h
index b636e0d..3144b8d 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h

@@ -181,6 +181,11 @@ typedef unsigned long uintptr_t;
 #define probe_utf8_pathname_composition(a,b)
 #endif
 
+#ifdef NEEDS_CLIPPED_WRITE
+ssize_t clipped_write(int fildes, const void *buf, size_t nbyte);
+#define write(x,y,z) clipped_write((x),(y),(z))
+#endif
+
 #ifdef MKDIR_WO_TRAILING_SLASH
 #define mkdir(a,b) compat_mkdir_wo_trailing_slash((a),(b))
 extern int compat_mkdir_wo_trailing_slash(const char*, mode_t);

-- 
1.8.3-rc1-268-g30389da