Fix file-attributes race on GNU hosts

* doc/lispref/files.texi (File Attributes):
Document file-attributes atomicity.
* etc/NEWS: Document the fix.
* src/dired.c (file_attributes): New args DIRNAME and FILENAME,
for diagnostics.  All callers changed.  On platforms like
GNU/Linux that support O_PATH, fix a race condition in
file-attributes and similar functions, so that these functions do
not return nonsense if a directory entry is replaced while getting
its attributes.  On non-GNU platforms, do a better (though not
perfect) job of detecting the race, and return nil if detected.

1 files changed, 7 insertions, 0 deletions

diff --git a/doc/lispref/files.texi b/doc/lispref/files.texi
index 5a52765131c..36944e4713d 100644
--- a/doc/lispref/files.texi
+++ b/doc/lispref/files.texi
@@ -1249,6 +1249,13 @@ the default, but we plan to change that, so you should specify a
 non-@code{nil} value for @var{id-format} if you use the returned
 @acronym{UID} or @acronym{GID}.
 
+On GNU platforms when operating on a local file, this function is
+atomic: if the filesystem is simultaneously being changed by some
+other process, this function returns the file's attributes either
+before or after the change.  Otherwise this function is not atomic,
+and might return @code{nil} it detects the race condition, or might
+return a hodgepodge of the previous and current file attributes.
+
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