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[Libguestfs] [PATCH nbdkit v3 2/3] Add new retry filter.



This filter can be used to transparently reopen/retry when a plugin
fails.  The connection is closed and reopened which for most plugins
causes them to attempt to reconnect to their source.

For example if doing a long or slow SSH copy:

  nbdkit -U - ssh host=remote /var/tmp/test.iso \
    --run 'qemu-img convert -p -f raw $nbd -O qcow2 test.qcow2'

if the SSH connection or network goes down in the middle then the
whole operation will fail.

By adding the retry filter:

  nbdkit -U - ssh --filter=retry host=remote /var/tmp/test.iso \
    --run 'qemu-img convert -p -f raw $nbd -O qcow2 test.qcow2'

this operation can recover from temporary failures in at least some
circumstances.  The NBD connection (a local Unix domain socket in the
example above) is not interrupted during retries, so NBD clients don't
need to be taught how to retry - everything is handled internally by
nbdkit.
---
 TODO                                  |   6 +
 configure.ac                          |   2 +
 docs/nbdkit-captive.pod               |   3 +
 filters/retry/Makefile.am             |  67 ++++++
 filters/retry/nbdkit-retry-filter.pod | 103 +++++++++
 filters/retry/retry.c                 | 301 ++++++++++++++++++++++++++
 plugins/curl/nbdkit-curl-plugin.pod   |   1 +
 plugins/ssh/nbdkit-ssh-plugin.pod     |   1 +
 plugins/vddk/nbdkit-vddk-plugin.pod   |   2 +
 9 files changed, 486 insertions(+)

diff --git a/TODO b/TODO
index d2cf0ae..02a095e 100644
--- a/TODO
+++ b/TODO
@@ -136,6 +136,12 @@ nbdkit-rate-filter:
 * split large requests to avoid long, lumpy sleeps when request size
   is much larger than rate limit
 
+nbdkit-retry-filter:
+
+* allow user to specify which errors cause a retry and which ones are
+  passed through; for example there's probably no point retrying on
+  ENOMEM
+
 Filters for security
 --------------------
 
diff --git a/configure.ac b/configure.ac
index 8261a7f..7ce3dbb 100644
--- a/configure.ac
+++ b/configure.ac
@@ -872,6 +872,7 @@ filters="\
         partition \
         rate \
         readahead \
+        retry \
         stats \
         truncate \
         xz \
@@ -952,6 +953,7 @@ AC_CONFIG_FILES([Makefile
                  filters/partition/Makefile
                  filters/rate/Makefile
                  filters/readahead/Makefile
+                 filters/retry/Makefile
                  filters/stats/Makefile
                  filters/truncate/Makefile
                  filters/xz/Makefile
diff --git a/docs/nbdkit-captive.pod b/docs/nbdkit-captive.pod
index 59df669..0adf0a0 100644
--- a/docs/nbdkit-captive.pod
+++ b/docs/nbdkit-captive.pod
@@ -92,6 +92,9 @@ help performance:
  nbdkit -U - --filter=readahead curl https://example.com/disk.img \
         --run 'qemu-img convert $nbd disk.img'
 
+If the source suffers from temporary network failures
+L<nbdkit-retry-filter(1)> may help.
+
 To overwrite a file inside an uncompressed tar file (the file being
 overwritten must be the same size), use L<nbdkit-tar-plugin(1)> like
 this:
diff --git a/filters/retry/Makefile.am b/filters/retry/Makefile.am
new file mode 100644
index 0000000..86011bb
--- /dev/null
+++ b/filters/retry/Makefile.am
@@ -0,0 +1,67 @@
+# nbdkit
+# Copyright (C) 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 $(top_srcdir)/common-rules.mk
+
+EXTRA_DIST = nbdkit-retry-filter.pod
+
+filter_LTLIBRARIES = nbdkit-retry-filter.la
+
+nbdkit_retry_filter_la_SOURCES = \
+	retry.c \
+	$(top_srcdir)/include/nbdkit-filter.h \
+	$(NULL)
+
+nbdkit_retry_filter_la_CPPFLAGS = \
+	-I$(top_srcdir)/include \
+	-I$(top_srcdir)/common/include \
+	-I$(top_srcdir)/common/utils \
+	$(NULL)
+nbdkit_retry_filter_la_CFLAGS = $(WARNINGS_CFLAGS)
+nbdkit_retry_filter_la_LDFLAGS = \
+	-module -avoid-version -shared \
+	-Wl,--version-script=$(top_srcdir)/filters/filters.syms \
+	$(NULL)
+nbdkit_retry_filter_la_LIBADD = \
+	$(top_builddir)/common/utils/libutils.la \
+	$(NULL)
+
+if HAVE_POD
+
+man_MANS = nbdkit-retry-filter.1
+CLEANFILES += $(man_MANS)
+
+nbdkit-retry-filter.1: nbdkit-retry-filter.pod
+	$(PODWRAPPER) --section=1 --man $@ \
+	    --html $(top_builddir)/html/$  html \
+	    $<
+
+endif HAVE_POD
diff --git a/filters/retry/nbdkit-retry-filter.pod b/filters/retry/nbdkit-retry-filter.pod
new file mode 100644
index 0000000..aeed79c
--- /dev/null
+++ b/filters/retry/nbdkit-retry-filter.pod
@@ -0,0 +1,103 @@
+=head1 NAME
+
+nbdkit-retry-filter - reopen connection on error
+
+=head1 SYNOPSIS
+
+ nbdkit --filter=retry PLUGIN
+
+=head1 DESCRIPTION
+
+C<nbdkit-retry-filter> is a filter that transparently reopens the
+plugin connection when an error is encountered.  It can be used to
+make long-running copy operations reliable in the presence of
+temporary network failures, without requiring any changes to the
+plugin.
+
+A number of parameters are available to control:
+
+=over 4
+
+=item *
+
+How many times we retry.
+
+=item *
+
+The delay between retries, and whether we wait longer each time (known
+as "exponential back-off").
+
+=item *
+
+If we reopen the plugin in read-only mode after the first failure.
+
+=back
+
+The default (no parameters) is designed to offer a happy medium
+between recovering from short temporary failures but not doing
+anything too bad when permanent or unrecoverable failures happen: We
+retry 5 times with exponential back-off, waiting in total about 1
+minute before we give up.
+
+=head1 EXAMPLE
+
+In this example we copy and convert a large file using
+L<nbdkit-ssh-plugin(1)>, L<qemu-img(1)> and L<nbdkit-captive(1)>.
+
+ nbdkit -U - ssh --filter=retry host=remote /var/tmp/test.iso \
+   --run 'qemu-img convert -p -f raw $nbd -O qcow2 test.qcow2'
+
+Without I<--filter=retry> a temporary failure would cause the copy to
+fail (for example, the remote host’s firewall is restarted causing the
+SSH connection to be closed).  Adding this filter means that it may be
+possible to transparently recover.
+
+=head1 PARAMETERS
+
+=over 4
+
+=item B<retries=>N
+
+The number of times any single operation will be retried before we
+give up and fail the operation.  The default is 5.
+
+=item B<retry-delay=>N
+
+The number of seconds to wait before retrying.  The default is 2
+seconds.
+
+=item B<retry-exponential=yes>
+
+Use exponential back-off.  The retry delay is doubled between each
+retry.  This is the default.
+
+=item B<retry-exponential=no>
+
+Do not use exponential back-off.  The retry delay is the same between
+each retry.
+
+=item B<retry-readonly=yes>
+
+As soon as a failure occurs and we retry, switch the underlying plugin
+permanently to read-only mode.
+
+=item B<retry-readonly=no>
+
+Do not change the read-write/read-only mode of the plugin when
+retrying.  This is the default.
+
+=back
+
+=head1 SEE ALSO
+
+L<nbdkit(1)>,
+L<nbdkit-filter(3)>,
+L<nbdkit-readahead-filter(1)>.
+
+=head1 AUTHORS
+
+Richard W.M. Jones
+
+=head1 COPYRIGHT
+
+Copyright (C) 2019 Red Hat Inc.
diff --git a/filters/retry/retry.c b/filters/retry/retry.c
new file mode 100644
index 0000000..7ba3d96
--- /dev/null
+++ b/filters/retry/retry.c
@@ -0,0 +1,301 @@
+/* nbdkit
+ * Copyright (C) 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 <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <stdbool.h>
+#include <inttypes.h>
+#include <string.h>
+#include <sys/time.h>
+
+#include <nbdkit-filter.h>
+
+#include "cleanup.h"
+
+static int retries = 5;         /* 0 = filter is disabled */
+static int initial_delay = 2;
+static bool exponential_backoff = true;
+static bool force_readonly = false;
+
+static int
+retry_config (nbdkit_next_config *next, void *nxdata,
+              const char *key, const char *value)
+{
+  int r;
+
+  if (strcmp (key, "retries") == 0) {
+    if (sscanf (value, "%d", &retries) != 1 || retries < 0) {
+      nbdkit_error ("cannot parse retries: %s", value);
+      return -1;
+    }
+  }
+  else if (strcmp (key, "retry-delay") == 0) {
+    if (sscanf (value, "%d", &initial_delay) != 1 || initial_delay <= 0) {
+      nbdkit_error ("cannot parse retry-delay: %s", value);
+      return -1;
+    }
+  }
+  else if (strcmp (key, "retry-exponential") == 0) {
+    r = nbdkit_parse_bool (value);
+    if (r == -1)
+      return -1;
+    exponential_backoff = r;
+    return 0;
+  }
+  else if (strcmp (key, "retry-readonly") == 0) {
+    r = nbdkit_parse_bool (value);
+    if (r == -1)
+      return -1;
+    force_readonly = r;
+    return 0;
+  }
+
+  return next (nxdata, key, value);
+}
+
+#define retry_config_help \
+  "retries=<N>              Number of retries (default: 5).\n" \
+  "retry-delay=<N>          Second to wait before retry (default: 2).\n" \
+  "retry-exponential=yes|no Exponential back-off (default: yes).\n" \
+  "retry-readonly=yes|no    Force read-only on failure (default: no).\n"
+
+struct retry_handle {
+  int readonly;                 /* Save original readonly setting. */
+};
+
+static void *
+retry_open (nbdkit_next_open *next, void *nxdata, int readonly)
+{
+  struct retry_handle *h;
+
+  if (next (nxdata, readonly) == -1)
+    return NULL;
+
+  h = malloc (sizeof *h);
+  if (h == NULL) {
+    nbdkit_error ("malloc: %m");
+    return NULL;
+  }
+
+  h->readonly = readonly;
+
+  return h;
+}
+
+static void
+retry_close (void *handle)
+{
+  struct retry_handle *h = handle;
+
+  free (h);
+}
+
+/* This function encapsulates the common retry logic used across all
+ * data commands.  If it returns true then the data command will retry
+ * the operation.  ‘struct retry_data’ is stack data saved between
+ * retries within the same command, and is initialized to zero.
+ */
+struct retry_data {
+  int retry;                    /* Retry number (0 = first time). */
+  int delay;                    /* Seconds to wait before retrying. */
+};
+
+static bool
+do_retry (struct retry_handle *h,
+          struct retry_data *data,
+          struct nbdkit_next_ops *next_ops, void *nxdata,
+          int *err)
+{
+  /* If it's the first retry, initialize the other fields in *data. */
+  if (data->retry == 0)
+    data->delay = initial_delay;
+
+ again:
+  /* Log the original errno since it will be lost when we retry. */
+  nbdkit_debug ("retry %d: original errno = %d", data->retry+1, *err);
+
+  if (data->retry >= retries) {
+    nbdkit_debug ("could not recover after %d retries", retries);
+    return false;
+  }
+
+  nbdkit_debug ("waiting %d seconds before retrying", data->delay);
+  if (nbdkit_nanosleep (data->delay, 0) == -1) {
+    /* We could do this but it would overwrite the more important
+     * errno from the underlying data call.
+     */
+    /* *err = errno; */
+    return false;
+  }
+
+  /* Update *data in case we are called again. */
+  data->retry++;
+  if (exponential_backoff)
+    data->delay *= 2;
+
+  /* Reopen the connection. */
+  if (next_ops->reopen (nxdata, h->readonly || force_readonly) == -1) {
+    /* If the reopen fails we treat it the same way as a command
+     * failing.
+     */
+    goto again;
+  }
+
+  /* Retry the data command. */
+  return true;
+}
+
+static int
+retry_pread (struct nbdkit_next_ops *next_ops, void *nxdata,
+             void *handle, void *buf, uint32_t count, uint64_t offset,
+             uint32_t flags, int *err)
+{
+  struct retry_handle *h = handle;
+  struct retry_data data = {0};
+  int r;
+
+ again:
+  r = next_ops->pread (nxdata, buf, count, offset, flags, err);
+  if (r == -1 && do_retry (h, &data, next_ops, nxdata, err)) goto again;
+
+  return r;
+}
+
+/* Write. */
+static int
+retry_pwrite (struct nbdkit_next_ops *next_ops, void *nxdata,
+              void *handle,
+              const void *buf, uint32_t count, uint64_t offset,
+              uint32_t flags, int *err)
+{
+  struct retry_handle *h = handle;
+  struct retry_data data = {0};
+  int r;
+
+ again:
+  r = next_ops->pwrite (nxdata, buf, count, offset, flags, err);
+  if (r == -1 && do_retry (h, &data, next_ops, nxdata, err)) goto again;
+
+  return r;
+}
+
+/* Trim. */
+static int
+retry_trim (struct nbdkit_next_ops *next_ops, void *nxdata,
+            void *handle,
+            uint32_t count, uint64_t offset, uint32_t flags,
+            int *err)
+{
+  struct retry_handle *h = handle;
+  struct retry_data data = {0};
+  int r;
+
+ again:
+  r = next_ops->trim (nxdata, count, offset, flags, err);
+  if (r == -1 && do_retry (h, &data, next_ops, nxdata, err)) goto again;
+
+  return r;
+}
+
+/* Zero. */
+static int
+retry_zero (struct nbdkit_next_ops *next_ops, void *nxdata,
+            void *handle,
+            uint32_t count, uint64_t offset, uint32_t flags,
+            int *err)
+{
+  struct retry_handle *h = handle;
+  struct retry_data data = {0};
+  int r;
+
+ again:
+  r = next_ops->zero (nxdata, count, offset, flags, err);
+  if (r == -1 && do_retry (h, &data, next_ops, nxdata, err)) goto again;
+
+  return r;
+}
+
+/* Extents. */
+static int
+retry_extents (struct nbdkit_next_ops *next_ops, void *nxdata,
+               void *handle,
+               uint32_t count, uint64_t offset, uint32_t flags,
+               struct nbdkit_extents *extents, int *err)
+{
+  struct retry_handle *h = handle;
+  struct retry_data data = {0};
+  int r;
+
+ again:
+  r = next_ops->extents (nxdata, count, offset, flags, extents, err);
+  if (r == -1 && do_retry (h, &data, next_ops, nxdata, err)) goto again;
+
+  return r;
+}
+
+/* Cache. */
+static int
+retry_cache (struct nbdkit_next_ops *next_ops, void *nxdata,
+             void *handle,
+             uint32_t count, uint64_t offset, uint32_t flags,
+             int *err)
+{
+  struct retry_handle *h = handle;
+  struct retry_data data = {0};
+  int r;
+
+ again:
+  r = next_ops->cache (nxdata, count, offset, flags, err);
+  if (r == -1 && do_retry (h, &data, next_ops, nxdata, err)) goto again;
+
+  return r;
+}
+
+static struct nbdkit_filter filter = {
+  .name              = "retry",
+  .longname          = "nbdkit retry filter",
+  .config            = retry_config,
+  .config_help       = retry_config_help,
+  .open              = retry_open,
+  .close             = retry_close,
+  .pread             = retry_pread,
+  .pwrite            = retry_pwrite,
+  .trim              = retry_trim,
+  .zero              = retry_zero,
+  .extents           = retry_extents,
+  .cache             = retry_cache,
+};
+
+NBDKIT_REGISTER_FILTER(filter)
diff --git a/plugins/curl/nbdkit-curl-plugin.pod b/plugins/curl/nbdkit-curl-plugin.pod
index 1c96b4a..2d5c564 100644
--- a/plugins/curl/nbdkit-curl-plugin.pod
+++ b/plugins/curl/nbdkit-curl-plugin.pod
@@ -159,6 +159,7 @@ L<CURLOPT_COOKIE(3)>
 L<CURLOPT_VERBOSE(3)>,
 L<nbdkit(1)>,
 L<nbdkit-readahead-filter(1)>,
+L<nbdkit-retry-filter(1)>,
 L<nbdkit-ssh-plugin(1)>,
 L<nbdkit-plugin(3)>,
 L<http://curl.haxx.se>.
diff --git a/plugins/ssh/nbdkit-ssh-plugin.pod b/plugins/ssh/nbdkit-ssh-plugin.pod
index 2010837..316e852 100644
--- a/plugins/ssh/nbdkit-ssh-plugin.pod
+++ b/plugins/ssh/nbdkit-ssh-plugin.pod
@@ -288,6 +288,7 @@ libssh itself.
 L<nbdkit(1)>,
 L<nbdkit-curl-plugin(1)>,
 L<nbdkit-readahead-filter(1)>,
+L<nbdkit-retry-filter(1)>,
 L<nbdkit-plugin(3)>,
 L<ssh(1)>,
 L<ssh-agent(1)>,
diff --git a/plugins/vddk/nbdkit-vddk-plugin.pod b/plugins/vddk/nbdkit-vddk-plugin.pod
index 8820a50..9e451c6 100644
--- a/plugins/vddk/nbdkit-vddk-plugin.pod
+++ b/plugins/vddk/nbdkit-vddk-plugin.pod
@@ -431,6 +431,8 @@ writing.
 
 L<nbdkit(1)>,
 L<nbdkit-plugin(3)>,
+L<nbdkit-readahead-filter(1)>,
+L<nbdkit-retry-filter(1)>,
 L<virsh(1)>,
 L<https://www.vmware.com/support/developer/vddk/>
 
-- 
2.23.0


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