New tmpdisk plugin.

This can be used for exporting temporary disks to thin clients, as a
kind of "remote tmpfs".

See also:
https://www.redhat.com/archives/libguestfs/2020-March/msg00134.html

.gitignore

@@ -134,6 +134,7 @@ plugins/*/*.3
 /tests/test-split
 /tests/test-streaming
 /tests/test-tcl
+/tests/test-tmpdisk
 /tests/test-xz
 /tests/test-xz-curl
 /test-driver

configure.ac

@@ -86,6 +86,7 @@ non_lang_plugins="\
         ssh \
         streaming \
         tar \
+        tmpdisk \
         vddk \
         zero \
         "
@@ -1016,6 +1017,7 @@ AC_CONFIG_FILES([Makefile
                  plugins/streaming/Makefile
                  plugins/tar/Makefile
                  plugins/tcl/Makefile
+                 plugins/tmpdisk/Makefile
                  plugins/vddk/Makefile
                  plugins/zero/Makefile
                  filters/Makefile

plugins/data/nbdkit-data-plugin.pod

@@ -269,6 +269,7 @@ L<nbdkit-null-plugin(1)>,
 L<nbdkit-partitioning-plugin(1)>,
 L<nbdkit-pattern-plugin(1)>,
 L<nbdkit-random-plugin(1)>,
+L<nbdkit-tmpdisk-plugin(1)>,
 L<nbdkit-zero-plugin(1)>,
 L<https://github.com/libguestfs/nbdkit/blob/master/plugins/data/disk2data.pl>,
 L<https://en.wikipedia.org/wiki/Base64>.

plugins/file/nbdkit-file-plugin.pod

@@ -111,6 +111,7 @@ L<nbdkit(1)>,
 L<nbdkit-plugin(3)>,
 L<nbdkit-split-plugin(1)>,
 L<nbdkit-partitioning-plugin(1)>,
+L<nbdkit-tmpdisk-plugin(1)>,
 L<nbdkit-noextents-filter(1)>.
 
 =head1 AUTHORS

plugins/linuxdisk/nbdkit-linuxdisk-plugin.pod

@@ -24,7 +24,9 @@ symbolic links, block special devices etc.
 
 To create a FAT-formatted virtual floppy disk, see
 L<nbdkit-floppy-plugin(1)>.  To create a CD/ISO, see
-L<nbdkit-iso-plugin(1)>.
+L<nbdkit-iso-plugin(1)>.  To create an empty filesystem for each
+client that connects (like a "remote tmpfs") use
+L<nbdkit-tmpdisk-plugin(1)>.
 
 =head1 EXAMPLES
 
@@ -184,7 +186,8 @@ L<nbdkit-file-plugin(1)>,
 L<nbdkit-floppy-plugin(1)>,
 L<nbdkit-iso-plugin(1)>,
 L<nbdkit-partition-filter(1)>,
-L<nbdkit-partitioning-plugin(1)>.
+L<nbdkit-partitioning-plugin(1)>,
+L<nbdkit-tmpdisk-plugin(1)>,
 
 =head1 AUTHORS
 

plugins/memory/nbdkit-memory-plugin.pod

@@ -103,7 +103,8 @@ L<nbdkit-plugin(3)>,
 L<nbdkit-loop(1)>,
 L<nbdkit-data-plugin(1)>,
 L<nbdkit-file-plugin(1)>,
-L<nbdkit-info-plugin(1)>.
+L<nbdkit-info-plugin(1)>,
+L<nbdkit-tmpdisk-plugin(1)>.
 
 =head1 AUTHORS
 

plugins/tmpdisk/Makefile.am

@@ -0,0 +1,66 @@
+# nbdkit
+# Copyright (C) 2017-2020 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-tmpdisk-plugin.pod
+
+plugin_LTLIBRARIES = nbdkit-tmpdisk-plugin.la
+
+nbdkit_tmpdisk_plugin_la_SOURCES = \
+	tmpdisk.c \
+	$(top_srcdir)/include/nbdkit-plugin.h \
+	$(NULL)
+
+nbdkit_tmpdisk_plugin_la_CPPFLAGS = \
+	-I$(top_srcdir)/include \
+	-I$(top_srcdir)/common/utils \
+	$(NULL)
+nbdkit_tmpdisk_plugin_la_CFLAGS = $(WARNINGS_CFLAGS)
+nbdkit_tmpdisk_plugin_la_LDFLAGS = \
+	-module -avoid-version -shared \
+	-Wl,--version-script=$(top_srcdir)/plugins/plugins.syms \
+	$(NULL)
+nbdkit_tmpdisk_plugin_la_LIBADD = \
+	$(top_builddir)/common/utils/libutils.la \
+	$(NULL)
+
+if HAVE_POD
+
+man_MANS = nbdkit-tmpdisk-plugin.1
+CLEANFILES += $(man_MANS)
+
+nbdkit-tmpdisk-plugin.1: nbdkit-tmpdisk-plugin.pod
+	$(PODWRAPPER) --section=1 --man $@ \
+	    --html $(top_builddir)/html/$@.html \
+	    $<
+
+endif HAVE_POD

plugins/tmpdisk/nbdkit-tmpdisk-plugin.pod

@@ -0,0 +1,157 @@
+=head1 NAME
+
+nbdkit-tmpdisk-plugin - create a fresh temporary filesystem for each client
+
+=head1 SYNOPSIS
+
+ nbdkit tmpdisk [size=]SIZE
+                [type=ext4|xfs|vfat|...] [label=LABEL]
+                [command=COMMAND]
+
+=head1 DESCRIPTION
+
+This L<nbdkit(1)> plugin is used for creating temporary filesystems
+for thin clients.  Each time a client connects it will see a fresh,
+empty filesystem for its exclusive use.  B<The filesystem is deleted>
+when the client disconnects.
+
+When a new client connects, a blank, sparse file of the required size
+is created in C<$TMPDIR> (or F</var/tmp>).  L<mkfs(8)> is then run on
+the file to create the empty filesystem, and this filesystem is served
+to the client.  Unlike L<nbdkit-linuxdisk-plugin(1)> each client of
+this plugin sees a different disk.
+
+The size of the disk is chosen using the C<size> parameter.  The
+filesystem type is C<ext4> but this can be changed using the C<type>
+parameter (controlling the I<-t> option of mkfs).
+
+Instead of running mkfs you can run an arbitrary C<command> to create
+the disk.
+
+=head2 Example
+
+One use for this is to create a kind of "remote L<tmpfs(5)>" for thin
+clients.  On the server you would run (see L<nbdkit-service(1)>):
+
+ nbdkit tmpdisk 16G
+
+and each thin client would have a file F</etc/rc.d/rc.local>
+containing:
+
+ nm-online
+ modprobe nbd
+ nbd-client server /dev/nbd0
+ mount /dev/nbd0 /var/scratch
+
+Clients would see a fresh, empty C</var/scratch> directory after boot.
+
+=head2 Security considerations
+
+Because each client gets a new disk, the amount of disk space
+required on the server can be as much as
+S<<< I<number of clients> × I<size parameter> >>>.  It is therefore
+best to limit the number of clients using L<nbdkit-limit-filter(1)> or
+take steps to limit where clients can connect from using
+L<nbdkit-ip-filter(1)>, firewalls, or TLS client certificates.
+
+=head1 PARAMETERS
+
+=over 4
+
+=item B<command='>COMMANDB<'>
+
+Instead of running L<mkfs(8)> to create the initial filesystem, run
+C<COMMAND> (which usually must be quoted to protect it from the
+shell).  The following shell variables may be used in C<COMMAND>:
+
+=over 4
+
+=item C<$disk>
+
+The absolute path of the file that the command must initialize.  This
+file is created by nbdkit before the command runs.
+
+=item C<$label>
+
+The filesystem label (from the C<label> parameter).
+
+=item C<$size>
+
+The virtual size in bytes, which is the same as the file size.
+
+=item C<$type>
+
+The filesystem type (from the C<type> parameter), defaulting to
+C<ext4>.  (Commands can ignore this if it is not relevant).
+
+=back
+
+=item B<label=>LABEL
+
+Select the filesystem label.  The default is not set.
+
+=item [B<size=>]SIZE
+
+Specify the virtual size of the disk image.
+
+This parameter is required.
+
+C<size=> is a magic config key and may be omitted in most cases.
+See L<nbdkit(1)/Magic parameters>.
+
+=item B<type=>FS
+
+Select the filesystem type.  The default is C<ext4>.  Most
+non-networked, non-cluster filesystem types supported by the
+L<mkfs(8)> command can be used here.
+
+=back
+
+=head1 ENVIRONMENT VARIABLES
+
+=over 4
+
+=item C<TMPDIR>
+
+The temporary disks for this plugin are created in this directory, one
+per connected client.  If not set this defaults to F</var/tmp>.
+
+=back
+
+=head1 FILES
+
+=over 4
+
+=item F<$plugindir/nbdkit-tmpdisk-plugin.so>
+
+The plugin.
+
+Use C<nbdkit --dump-config> to find the location of C<$plugindir>.
+
+=back
+
+=head1 VERSION
+
+C<nbdkit-tmpdisk-plugin> first appeared in nbdkit 1.20.
+
+=head1 SEE ALSO
+
+L<nbdkit(1)>,
+L<nbdkit-plugin(3)>,
+L<nbdkit-data-plugin(1)>,
+L<nbdkit-file-plugin(1)>,
+L<nbdkit-ip-filter(1)>,
+L<nbdkit-limit-filter(1)>,
+L<nbdkit-linuxdisk-plugin(1)>,
+L<nbdkit-memory-plugin(1)>,
+L<nbdkit-loop(1)>,
+L<nbdkit-tls(1)>,
+L<mkfs(8)>.
+
+=head1 AUTHORS
+
+Richard W.M. Jones
+
+=head1 COPYRIGHT
+
+Copyright (C) 2018-2020 Red Hat Inc.