Elide syscalls in fdes->port

* libguile/fports.h (scm_t_fport): Add options field.
  (SCM_FDES_RANDOM_P): Deprecate.
  (scm_i_fdes_to_port): Add options argument.
* libguile/fports.c (scm_i_fdes_to_port): Add options argument.  Only
  verify FD if SCM_FPORT_OPTION_VERIFY is there.
  (scm_fdes_to_port, scm_open_file_with_encoding): Adapt to
  scm_i_fdes_to_port changes.
  (fport_random_access_p): Don't try to seek if NOT_SEEKABLE option is
  set.
* libguile/deprecated.h:
* libguile/deprecated.c (SCM_FDES_RANDOM_P): Deprecate.
* NEWS: Add deprecation.
* libguile/filesys.c:
* libguile/ioext.c:
* libguile/posix.c:
* libguile/read.c:
* libguile/socket.c: Adapt callers.

1 files changed, 6 insertions, 10 deletions

diff --git a/libguile/read.c b/libguile/read.c
index 5c436e2b2..085cdb9f1 100644
--- a/libguile/read.c
+++ b/libguile/read.c
@@ -2105,21 +2105,17 @@ scm_i_scan_for_encoding (SCM port)
       memcpy (header, scm_port_buffer_take_pointer (buf, cur), bytes_read);
       header[bytes_read] = '\0';
     }
-  else
+  else if (pt->rw_random)
     {
-      /* Try to read some bytes and then seek back.  Not all ports
-         support seeking back; and indeed some file ports (like
-         /dev/urandom) will succeed on an lseek (fd, 0, SEEK_CUR)---the
-         check performed by SCM_FPORT_FDES---but fail to seek
-         backwards.  Hence this block comes second.  We prefer to use
-         the read buffer in-place.  */
-      if (SCM_FPORTP (port) && !SCM_FDES_RANDOM_P (SCM_FPORT_FDES (port)))
-        return NULL;
-
+      /* The port is seekable.  This is OK but grubbing in the read
+         buffer is better, so this case is just a fallback.  */
       bytes_read = scm_c_read (port, header, SCM_ENCODING_SEARCH_SIZE);
       header[bytes_read] = '\0';
       scm_seek (port, scm_from_int (0), scm_from_int (SEEK_SET));
     }
+  else
+    /* No input available and not seekable; scan fails.  */
+    return NULL;
 
   /* search past "coding[:=]" */
   pos = header;