[PATCH] include/cygwin/limits.h: add XATTR_{NAME,SIZE,LIST}_MAX

[Brian Inglis <Brian.Inglis@Shaw.ca>]

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On Tue, 30 May 2023 13:25:38 +0200, Philippe Cerfon wrote:
> Hey there.
> 
> Linux exports XATTR_{NAME,SIZE,LIST}_MAX in it's linux/limits.h and
> e.g. the CPython interpreter uses them for it's XATTRs functions.
> 
> I made a corresponding PR at CPython
> https://github.com/python/cpython/pull/105075 to get the code built
> for Cygwin, but right now this would fail due to the missing
> XATTR_*_MAX symbols.
> 
> The attached patch below would add them to cygwin/limits.h.

Patches for Cygwin under winsup are submitted to cygwin-patches@cygwin.com 
(forwarded there).

> But beware, I'm absolutely no Windows/Cygwin expert ^^ - so whether
> the values I've chosen are actually correct, is more guesswork rather
> than definite knowledge.
> 
> As written in the commit message, I think:
> - XATTR_NAME_MAX corresponds to MAX_EA_NAME_LEN
> and
> - XATTR_SIZE_MAX to MAX_EA_VALUE_LEN
> 
> though I have no idea, whether these are just lower boundaries used by
> Cygwin, while e.g. Windows itself might set longer names or value
> lenghts, and thus - when Cygwin would try to read such - it might get
> into troubles (or rather e.g. CPython, as it's buffers wouldn't
> suffice to read the EA respectively XATTR.
> 
> Neither to i have an idea about XATTR_LIST_MAX. I'm not even 100% sure
> what it means (I guess the max number of XATTRs per file). Not to
> speak about whether there's such maximum for Windows EAs,
> And again - as above - what would happen if Windows itself would set
> more than that limit and within Cygwin one would try to read/list all.
> 
> Thanks,
> Philippe

-- 
Take care. Thanks, Brian Inglis              Calgary, Alberta, Canada

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From 824bcdf053bfb86570c7edda3c018626dc857a8b Mon Sep 17 00:00:00 2001
From: Philippe Cerfon <philcerf@gmail.com>
Date: Tue, 30 May 2023 13:16:18 +0200
Subject: [PATCH] export XATTR_{NAME,SIZE,LIST}_MAX

These are used for example by CPython. XATTR_NAME_MAX should correspond to
MAX_EA_NAME_LEN and XATTR_SIZE_MAX to MAX_EA_VALUE_LEN.

It's unclear whether Windows imposes a maximum number of EA's per file and which
value should be used for XATTR_LIST_MAX, so for now Linux' value.

Signed-off-by: Philippe Cerfon <philcerf@gmail.com>
---
 winsup/cygwin/include/cygwin/limits.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/winsup/cygwin/include/cygwin/limits.h b/winsup/cygwin/include/cygwin/limits.h
index aefc7c7bd..939ab4f38 100644
--- a/winsup/cygwin/include/cygwin/limits.h
+++ b/winsup/cygwin/include/cygwin/limits.h
@@ -56,4 +56,11 @@ details. */
 #define __PATH_MAX 4096
 #define __PIPE_BUF 4096
 
+/* Keep in sync with MAX_EA_NAME_LEN respectively MAX_EA_VALUE_LEN in
+ * winsup/cygwin/ntea.cc but don not use values that exceed their Linux
+ * counterparts as defined in linux/limits.h. */
+#define XATTR_NAME_MAX 255
+#define XATTR_SIZE_MAX 65536
+#define XATTR_LIST_MAX 65536
+
 #endif /* _CYGWIN_LIMITS_H__ */
-- 
2.40.1