Improve copy-file diagnostics on MS-Windows.

 src/fileio.c (Fcopy_file) [WINDOWSNT]: Improve diagnostics when CopyFile
 fails by looking at what GetLastError returns.

2 files changed, 10 insertions, 1 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index f40f936d13a..e95df2b8953 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -3,6 +3,7 @@
         * fileio.c (Fset_file_selinux_context, Fset_file_acl): Return t if
         file's SELinux context or ACLs successfully set, nil otherwise.
         (Bug#13298)
+        (Fcopy_file) [WINDOWSNT]: Improve diagnostics when CopyFile fails.
 
         * w32proc.c (reader_thread): Avoid passing NULL handles to
         SetEvent and WaitForSingleObject.
diff --git a/src/fileio.c b/src/fileio.c
index 241775abec1..0e63e3f18d9 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -2029,7 +2029,15 @@ entries (depending on how Emacs was built).  */)
   if (!CopyFile (SDATA (encoded_file),
                  SDATA (encoded_newname),
                  FALSE))
-    report_file_error ("Copying file", Fcons (file, Fcons (newname, Qnil)));
+    {
+      /* CopyFile doesn't set errno when it fails.  By far the most
+         "popular" reason is that the target is read-only.  */
+      if (GetLastError () == 5)
+        errno = EACCES;
+      else
+        errno = EPERM;
+      report_file_error ("Copying file", Fcons (file, Fcons (newname, Qnil)));
+    }
   /* CopyFile retains the timestamp by default.  */
   else if (NILP (keep_time))
     {