Add nofilter filter.

This is a filter that does nothing, approximately the filter
equivalent of the null plugin.  It is used for two purposes:

(1) Testing, where we need a filter that does nothing except pass
through requests.

(2) Checking that internally nbdkit can handle a filter that has no
callbacks.

configure.ac

@@ -902,6 +902,7 @@ filters="\
         log \
         nocache \
         noextents \
+        nofilter \
         noparallel \
         nozero \
         offset \
@@ -987,6 +988,7 @@ AC_CONFIG_FILES([Makefile
                  filters/log/Makefile
                  filters/nocache/Makefile
                  filters/noextents/Makefile
+                 filters/nofilter/Makefile
                  filters/noparallel/Makefile
                  filters/nozero/Makefile
                  filters/offset/Makefile

filters/nofilter/Makefile.am

@@ -0,0 +1,63 @@
+# nbdkit
+# Copyright (C) 2018 Red Hat Inc.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+# * Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+#
+# * Redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution.
+#
+# * Neither the name of Red Hat nor the names of its contributors may be
+# used to endorse or promote products derived from this software without
+# specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+# THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+# PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+# USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+# ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+# OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+# SUCH DAMAGE.
+
+include $(top_srcdir)/common-rules.mk
+
+EXTRA_DIST = nbdkit-nofilter-filter.pod
+
+filter_LTLIBRARIES = nbdkit-nofilter-filter.la
+
+nbdkit_nofilter_filter_la_SOURCES = \
+	nofilter.c \
+	$(top_srcdir)/include/nbdkit-filter.h \
+	$(NULL)
+
+nbdkit_nofilter_filter_la_CPPFLAGS = \
+	-I$(top_srcdir)/include \
+	-I$(top_srcdir)/common/include \
+	$(NULL)
+nbdkit_nofilter_filter_la_CFLAGS = $(WARNINGS_CFLAGS)
+nbdkit_nofilter_filter_la_LDFLAGS = \
+	-module -avoid-version -shared \
+	-Wl,--version-script=$(top_srcdir)/filters/filters.syms \
+	$(NULL)
+
+if HAVE_POD
+
+man_MANS = nbdkit-nofilter-filter.1
+CLEANFILES += $(man_MANS)
+
+nbdkit-nofilter-filter.1: nbdkit-nofilter-filter.pod
+	$(PODWRAPPER) --section=1 --man $@ \
+	    --html $(top_builddir)/html/$@.html \
+	    $<
+
+endif HAVE_POD

filters/nofilter/nbdkit-nofilter-filter.pod

@@ -0,0 +1,41 @@
+=head1 NAME
+
+nbdkit-nofilter-filter - nbdkit passthrough filter
+
+=head1 SYNOPSIS
+
+ nbdkit --filter=nofilter plugin [plugin-args...]
+
+=head1 DESCRIPTION
+
+C<nbdkit-nofilter-filter> passes through requests unchanged.
+It is mostly used for testing nbdkit.
+
+=head1 FILES
+
+=over 4
+
+=item F<$filterdir/nbdkit-nofilter-filter.so>
+
+The filter.
+
+Use C<nbdkit --dump-config> to find the location of C<$filterdir>.
+
+=back
+
+=head1 VERSION
+
+C<nbdkit-nofilter-filter> first appeared in nbdkit 1.18.
+
+=head1 SEE ALSO
+
+L<nbdkit(1)>,
+L<nbdkit-filter(3)>.
+
+=head1 AUTHORS
+
+Richard W.M. Jones
+
+=head1 COPYRIGHT
+
+Copyright (C) 2019 Red Hat Inc.

filters/nofilter/nofilter.c

@@ -0,0 +1,42 @@
+/* nbdkit
+ * Copyright (C) 2018-2019 Red Hat Inc.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ *
+ * * Neither the name of Red Hat nor the names of its contributors may be
+ * used to endorse or promote products derived from this software without
+ * specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+ * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+ * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <config.h>
+
+#include <nbdkit-filter.h>
+
+static struct nbdkit_filter filter = {
+  .name              = "nofilter",
+  .longname          = "nbdkit nofilter filter",
+};
+
+NBDKIT_REGISTER_FILTER(filter)

tests/Makefile.am

@@ -138,6 +138,7 @@ EXTRA_DIST = \
 	test-nbd-extents.sh \
 	test-nbd-tls.sh \
 	test-nbd-tls-psk.sh \
+	test-nofilter.sh \
 	test-nozero.sh \
 	test-null-extents.sh \
 	test_ocaml_plugin.ml \
@@ -1014,6 +1015,9 @@ TESTS += test-ip-filter.sh
 # log filter test.
 TESTS += test-log.sh
 
+# nofilter test.
+TESTS += test-nofilter.sh
+
 if HAVE_NBD_PLUGIN
 # nozero filter test.
 TESTS += test-nozero.sh

tests/test-nofilter.sh

@@ -0,0 +1,42 @@
+#!/usr/bin/env bash
+# nbdkit
+# Copyright (C) 2018-2019 Red Hat Inc.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+# * Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+#
+# * Redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution.
+#
+# * Neither the name of Red Hat nor the names of its contributors may be
+# used to endorse or promote products derived from this software without
+# specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+# THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+# PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
+# USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+# ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+# OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+# SUCH DAMAGE.
+
+source ./functions.sh
+set -e
+set -x
+
+files="nofilter.img"
+rm -f $files
+cleanup_fn rm -f $files
+
+nbdkit -U - -v --filter=nofilter memory 10M \
+       --run 'qemu-img convert $nbd nofilter.img'