Implement seek on filters in read-only mode.

* include/mailutils/stream.h (mu_stream_skip_input_bytes): New proto.
* mailbox/stream.c (mu_stream_skip_input_bytes): New function.
* mailbox/fltstream.c (filter_seek): Re-implement on top of
mu_stream_skip_input_bytes.
(mu_filter_stream_create): Fix flag validity checking.

* examples/base64.c: Add new option (-s) for testing seek operations
on filters.

@@ -53,8 +53,9 @@ main (int argc, char * argv [])
   int flags = MU_STREAM_READ;
   char *input = NULL, *output = NULL;
   char *encoding = "base64";
+  mu_off_t shift = 0;
   
-  while ((c = getopt (argc, argv, "deE:hi:o:pvw")) != EOF)
+  while ((c = getopt (argc, argv, "deE:hi:o:ps:vw")) != EOF)
     switch (c)
       {
       case 'i':
@@ -81,6 +82,10 @@ main (int argc, char * argv [])
  	printable = 1; /* FIXME: Not implemented */
 	break;
 
+      case 's':
+	shift = strtoul (optarg, NULL, 0); 
+	break;
+	
       case 'v':
 	verbose = 1;
 	break;
@@ -98,7 +103,7 @@ main (int argc, char * argv [])
       }
 
   if (input)
-    MU_ASSERT (mu_file_stream_create (&in, input, MU_STREAM_READ));
+    MU_ASSERT (mu_file_stream_create (&in, input, MU_STREAM_READ|MU_STREAM_SEEK));
   else
     MU_ASSERT (mu_stdio_stream_create (&in, MU_STDIN_FD, 0));
   MU_ASSERT (mu_stream_open (in));
@@ -112,13 +117,18 @@ main (int argc, char * argv [])
 
   if (flags == MU_STREAM_READ)
     {
-      MU_ASSERT (mu_filter_create (&flt, in, encoding, mode, MU_STREAM_READ));
+      MU_ASSERT (mu_filter_create (&flt, in, encoding, mode,
+				   MU_STREAM_READ|MU_STREAM_SEEK));
+      if (shift)
+	MU_ASSERT (mu_stream_seek (flt, shift, MU_SEEK_SET, NULL));
       c_copy (out, flt);
     }
   else
     {
       MU_ASSERT (mu_filter_create (&flt, out, encoding, mode,
 				   MU_STREAM_WRITE));
+      if (shift)
+	MU_ASSERT (mu_stream_seek (in, shift, MU_SEEK_SET, NULL));
       c_copy (flt, in);
     }
       

@@ -71,6 +71,9 @@ void mu_stream_clearerr (mu_stream_t stream);
 int mu_stream_eof (mu_stream_t stream);
 int mu_stream_seek (mu_stream_t stream, mu_off_t offset, int whence,
 		    mu_off_t *pres);
+int mu_stream_skip_input_bytes (mu_stream_t stream, mu_off_t count,
+				mu_off_t *pres);
+
 int mu_stream_set_buffer (mu_stream_t stream, enum mu_buffer_type type,
 			  size_t size);
 int mu_stream_read (mu_stream_t stream, void *buf, size_t size, size_t *pread);

@@ -313,12 +313,17 @@ filter_wr_flush (mu_stream_t stream)
   return rc;
 }
 
-/* FIXME: Seeks in the *transport* stream */
 static int
 filter_seek (struct _mu_stream *stream, mu_off_t off, mu_off_t *ppos)
 {
   struct _mu_filter_stream *fs = (struct _mu_filter_stream *)stream;
-  return mu_stream_seek (fs->transport, off, MU_SEEK_SET, ppos);
+  int status;
+
+  status = mu_stream_seek (fs->transport, 0, MU_SEEK_SET, NULL);
+  if (status)
+    return status;
+  stream->offset = 0;
+  return mu_stream_skip_input_bytes (stream, off, ppos);
 }
 
 static int
@@ -392,7 +397,7 @@ mu_filter_stream_create (mu_stream_t *pflt,
 
   if ((flags & MU_STREAM_RDWR) == MU_STREAM_RDWR
       || !(flags & MU_STREAM_RDWR)
-      || (flags & MU_STREAM_SEEK))
+      || (flags & (MU_STREAM_WRITE|MU_STREAM_SEEK)) == (MU_STREAM_WRITE|MU_STREAM_SEEK))
     return EINVAL;
  
   fs = (struct _mu_filter_stream *) _mu_stream_create (sizeof (*fs), flags);

@@ -334,6 +334,71 @@ mu_stream_seek (mu_stream_t stream, mu_off_t offset, int whence,
   return 0;
 }
 
+/* Skip COUNT bytes from the current position in stream by reading from
+   it.  Return new offset in PRES.
+
+   Return 0 on success, EACCES if STREAM was not opened for reading.
+   Another non-zero exit codes are propagated from the underlying
+   input operations.
+   
+   This function is designed to help implement seek method in otherwise
+   unseekable streams (such as filters).  Do not use it if you absolutely
+   have to.  Using it on an unbuffered stream is a terrible waste of CPU. */
+int
+mu_stream_skip_input_bytes (mu_stream_t stream, mu_off_t count, mu_off_t *pres)
+{
+  mu_off_t pos;
+  int rc;
+
+  if (!(stream->flags & MU_STREAM_READ))
+    return _stream_seterror (stream, EACCES, 1);
+
+  if (stream->buftype == mu_buffer_none)
+    {
+      for (pos = 0; pos < count; pos++)
+	{
+	  char c;
+	  size_t nrd;
+	  rc = mu_stream_read (stream, &c, 1, &nrd);
+	  if (nrd == 0)
+	    rc = ESPIPE;
+	  if (rc)
+	    break;
+	}
+    }
+  else
+    {
+      if ((rc = _stream_flush_buffer (stream, 1)))
+	return rc;
+      for (pos = 0;;)
+	{
+	  if (stream->level == 0)
+	    {
+	      rc = _stream_fill_buffer (stream);
+	      if (rc)
+		break;
+	      if (stream->level == 0)
+		{
+		  rc = ESPIPE;
+		  break;
+		}
+	    }
+	  if (pos <= count && count < pos + stream->level)
+	    {
+	      rc = 0;
+	      stream->cur = stream->buffer + count - pos;
+	      pos = count;
+	      break;
+	    }
+	}
+    }
+  
+  stream->offset += pos;
+  if (pres)
+    *pres = stream->offset;
+  return rc;
+}
+
 int
 mu_stream_set_buffer (mu_stream_t stream, enum mu_buffer_type type,
 		      size_t size)