rm -f: ignore EROFS when it's really ENOENT

rm -f must not print a diagnostic for a nonexistent file.  However,
most linux-based kernel unlinkat functions set errno to EROFS when
the named file (regardless of whether it exists) would lie on a
read-only file system.  remove.c now performs an extra fstatat call
in that case, to determine whether the file exists.
* src/remove.c (excise): Map EROFS to ENOENT, if a file is nonexistent.
Reported by Steven Drake in <http://savannah.gnu.org/bugs/?27923>.
* NEWS (Changes in behavior): Mention it.

NEWS

@@ -49,6 +49,12 @@ GNU coreutils NEWS                                    -*- outline -*-
 
   echo and printf now interpret \e as the Escape character (0x1B).
 
+  rm -f /read-only-fs/nonexistent now succeeds and prints no diagnostic
+  on systems with an unlinkat syscall that sets errno to EROFS in that case.
+  Before, it would fail with a "Read-only file system" diagnostic.
+  Also, "rm /read-only-fs/nonexistent" now reports "file not found" rather
+  than the less precise "Read-only file system" error.
+
 ** New features
 
   env and printenv now accept the option --null (-0), as a means to

THANKS

@@ -541,6 +541,7 @@ Stephen Smoogen                     smooge@mindspring.com
 Steve McConnel                      steve@acadcomp.sil.org
 Steve McIntyre                      steve@einval.com
 Steve Ward                          planet36@gmail.com
+Steven Drake                        sbd@users.sourceforge.net
 Steven G. Johnson                   stevenj@alum.mit.edu
 Steven Mocking                      ufo@quicknet.nl
 Steven Parkes                       smparkes@smparkes.net

src/remove.c

@@ -437,6 +437,18 @@ excise (FTS *fts, FTSENT *ent, struct rm_options const *x, bool is_dir)
       return RM_OK;
     }
 
+  /* The unlinkat from kernels like linux-2.6.32 reports EROFS even for
+     nonexistent files.  When the file is indeed missing, map that to ENOENT,
+     so that rm -f ignores it, as required.  Even without -f, this is useful
+     because it makes rm print the more precise diagnostic.  */
+  if (errno == EROFS)
+    {
+      struct stat st;
+      if ( ! (lstatat (fts->fts_cwd_fd, ent->fts_accpath, &st)
+                       && errno == ENOENT))
+        errno = EROFS;
+    }
+
   if (ignorable_missing (x, errno))
     return RM_OK;