Don't open directories as files

lib/system/sysio.nim

@@ -270,12 +270,30 @@ const
     # we always use binary here as for Nim the OS line ending
     # should not be translated.
 
+when defined(posix):
+  type
+    Mode {.importc: "mode_t", header: "<sys/types.h>".} = cint
+
+    Stat {.importc: "struct stat",
+             header: "<sys/stat.h>", final, pure.} = object ## struct stat
+      st_mode: Mode        ## Mode of file
+
+  proc S_ISDIR(m: Mode): bool {.importc, header: "<sys/stat.h>".}
+    ## Test for a directory.
+
+  proc fstat(a1: cint, a2: var Stat): cint {.importc, header: "<sys/stat.h>".}
 
 proc open(f: var File, filename: string,
           mode: FileMode = fmRead,
           bufSize: int = -1): bool =
   var p: pointer = fopen(filename, FormatOpen[mode])
   if p != nil:
+    when defined(posix):
+      var f2 = cast[File](p)
+      var res: Stat
+      if fstat(getFileHandle(f2), res) >= 0'i32 and S_ISDIR(res.st_mode):
+        close(f2)
+        return
     result = true
     f = cast[File](p)
     if bufSize > 0 and bufSize <= high(cint).int: