[PATCH v5 4/4] stat: support statx DONT_SYNC and FORCE_SYNC flags

[Jeff Layton]

Add a new --cache= command-line option that sets the appropriate hint
flags in the statx call. These are primarily used with network
filesystems to indicate what level of cache coherency the application
can tolerate. The new option is only implemented when built with
HAVE_STATX.

* NEWS: mention the enhancements
* src/stat.c: add new option to control synchronization with server
---
 NEWS       |  7 ++++++
 src/stat.c | 74 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 80 insertions(+), 1 deletion(-)

diff --git a/NEWS b/NEWS
index 12c864dcc9ad..bac18edfc052 100644
--- a/NEWS
+++ b/NEWS
@@ -2,6 +2,13 @@ GNU coreutils NEWS                                    -*- 
outline -*-
 
 * Noteworthy changes in release ?.? (????-??-??) [?]
 
+** New Features
+
+   stat now uses the statx() system call to do its bidding when it is
+   available. It will craft a statx mask with only the needed attributes.
+   When built with statx support, it also supports a new --cached=
+   option to control synchronization with the server on network file sytems.
+
 ** Bug fixes
 
   df now correctly parses the /proc/self/mountinfo file for unusual entries
diff --git a/src/stat.c b/src/stat.c
index 4b4dc52e5ca6..66446532ad4d 100644
--- a/src/stat.c
+++ b/src/stat.c
@@ -194,6 +194,23 @@ enum
   PRINTF_OPTION = CHAR_MAX + 1
 };
 
+enum cached_mode
+{
+  cached_default,
+  cached_never,
+  cached_always
+};
+
+static char const *const cached_args[] =
+{
+  "default", "never", "always", NULL
+};
+
+static enum cached_mode const cached_modes[] =
+{
+  cached_default, cached_never, cached_always
+};
+
 static struct option const long_options[] =
 {
   {"dereference", no_argument, NULL, 'L'},
@@ -201,6 +218,9 @@ static struct option const long_options[] =
   {"format", required_argument, NULL, 'c'},
   {"printf", required_argument, NULL, PRINTF_OPTION},
   {"terse", no_argument, NULL, 't'},
+#if HAVE_STATX && defined STATX_INO
+  {"cached", required_argument, NULL, 0},
+#endif
   {GETOPT_HELP_OPTION_DECL},
   {GETOPT_VERSION_OPTION_DECL},
   {NULL, 0, NULL, 0}
@@ -1200,6 +1220,15 @@ do_statfs (char const *filename, char const *format)
 }
 
 #if HAVE_STATX && defined STATX_INO
+/* Allowed command-line options */
+static const char *optstring = "c:fLt0:";
+
+/* Ask statx to avoid syncing? */
+static bool dont_sync;
+
+/* Ask statx to force sync? */
+static bool force_sync;
+
 struct printarg {
   struct statx *stx;
   struct stat *st;
@@ -1472,6 +1501,11 @@ do_stat (char const *filename, char const *format, char 
const *format2)
       flags = AT_SYMLINK_NOFOLLOW;
     }
 
+  if (dont_sync)
+    flags |= AT_STATX_DONT_SYNC;
+  else if (force_sync)
+    flags |= AT_STATX_FORCE_SYNC;
+
   fd = statx (fd, pathname, flags, format_to_mask (format), &stx);
   if (fd < 0)
     {
@@ -1492,6 +1526,9 @@ error (0, errno, _("cannot statx %s"), quoteaf 
(filename));
   return ! fail;
 }
 #else /* HAVE_STATX && defined STATX_INO */
+/* Allowed command-line options */
+static const char *optstring = "c:fLt";
+
 static struct timespec
 get_birthtime (int fd, char const *filename, struct stat const *st)
 {
@@ -1800,6 +1837,12 @@ Display file or file system status.\n\
   -L, --dereference     follow links\n\
   -f, --file-system     display file system status instead of file status\n\
 "), stdout);
+#if HAVE_STATX && defined STATX_INO
+      fputs (_("\
+      --cached=MODE     specify whether and how to use cached values;\n\
+                          (typically useful for network file systems)\n\
+"), stdout);
+#endif
       fputs (_("\
   -c  --format=FORMAT   use the specified FORMAT instead of the default;\n\
                           output a newline after each use of FORMAT\n\
@@ -1811,6 +1854,16 @@ Display file or file system status.\n\
       fputs (HELP_OPTION_DESCRIPTION, stdout);
       fputs (VERSION_OPTION_DESCRIPTION, stdout);
 
+#if HAVE_STATX && defined STATX_INO
+      fputs (_("\n\
+The valid MODE arguments to --cached are:\n\
+\n\
+  always:  always use cached values\n\
+  never:   never use cached values\n\
+  default: leave it up to the underlying file system\n\
+"), stdout);
+#endif
+
       fputs (_("\n\
 The valid format sequences for files (without --file-system):\n\
 \n\
@@ -1916,7 +1969,7 @@ main (int argc, char *argv[])
 
   atexit (close_stdout);
 
-  while ((c = getopt_long (argc, argv, "c:fLt", long_options, NULL)) != -1)
+  while ((c = getopt_long (argc, argv, optstring, long_options, NULL)) != -1)
     {
       switch (c)
         {
@@ -1944,6 +1997,25 @@ main (int argc, char *argv[])
           terse = true;
           break;
 
+#if HAVE_STATX && defined STATX_INO
+        case 0:
+          switch (XARGMATCH ("--cached", optarg, cached_args, cached_modes))
+            {
+              case cached_never:
+                force_sync = true;
+                dont_sync = false;
+                break;
+              case cached_always:
+                force_sync = false;
+                dont_sync = true;
+                break;
+              case cached_default:
+                force_sync = false;
+                dont_sync = false;
+            }
+          break;
+#endif
+
         case_GETOPT_HELP_CHAR;
 
         case_GETOPT_VERSION_CHAR (PROGRAM_NAME, AUTHORS);
-- 
2.21.0