10:24 p.m.
Add a perl language binding for the .zero callback, used for
implementing NBD_CMD_WRITE_ZEROES. The caller doesn't have to
return anything, but should use Nbdkit::set_error(POSIX::EOPNOTSUPP)
to get an automatic fallback to pwrite.
Enhance the example to show the use of the fallback mechanism,
and to serve as a test of Nbdkit::set_error().
Signed-off-by: Eric Blake <eblake(a)redhat.com>
---
plugins/perl/example.pl | 16 +++++++++++++
plugins/perl/nbdkit-perl-plugin.pod | 26 +++++++++++++++++++++
plugins/perl/perl.c | 45 ++++++++++++++++++++++++++++++++++++-
3 files changed, 86 insertions(+), 1 deletion(-)
diff --git a/plugins/perl/example.pl b/plugins/perl/example.pl
index fcdac33..7ea24ff 100644
--- a/plugins/perl/example.pl
+++ b/plugins/perl/example.pl
@@ -1,4 +1,5 @@
use strict;
+use POSIX ();
# Example Perl plugin.
#
@@ -83,3 +84,18 @@ sub pwrite
substr ($disk, $offset, $count) = $buf;
}
+
+sub zero
+{
+ my $h = shift;
+ my $count = shift;
+ my $offset = shift;
+ my $may_trim = shift;
+
+ if ($may_trim) {
+ substr ($disk, $offset, $count) = "\0" x $count;
+ } else {
+ Nbdkit::set_error(POSIX::EOPNOTSUPP);
+ die "fall back to pwrite";
+ }
+}
diff --git a/plugins/perl/nbdkit-perl-plugin.pod b/plugins/perl/nbdkit-perl-plugin.pod
index 82766d0..920bdef 100644
--- a/plugins/perl/nbdkit-perl-plugin.pod
+++ b/plugins/perl/nbdkit-perl-plugin.pod
@@ -290,6 +290,32 @@ store.
If there is an error, the function should call C<die>, optionally using
C<Nbdkit::set_error> first.
+=item C<zero>
+
+(Optional)
+
+ sub zero
+ {
+ my $handle = shift;
+ my $count = shift;
+ my $offset = shift;
+ my $may_trim = shift;
+ # No return value
+ }
+
+The body of your C<zero> function should ensure that C<$count> bytes
+of the disk, starting at C<$offset>, will read back as zero. If
+C<$may_trim> is true, the operation may be optimized as a trim as long
+as subsequent reads see zeroes.
+
+NBD only supports whole writes, so your function should try to write
+the whole region (perhaps requiring a loop). If the write fails or is
+partial, your function should C<die>, optionally using
+C<Nbdkit::set_error> first. In particular, if you would like to
+automatically fall back to C<pwrite> (perhaps because there is nothing
+to optimize if C<$may_trim> is false), use
+C<Nbdkit::set_error(POSIX::EOPNOTSUPP)>.
+
=back
=head2 MISSING CALLBACKS
diff --git a/plugins/perl/perl.c b/plugins/perl/perl.c
index 9fb189c..52694d9 100644
--- a/plugins/perl/perl.c
+++ b/plugins/perl/perl.c
@@ -141,12 +141,15 @@ check_perl_failure (void)
return 0;
}
+static int last_error;
+
XS(set_error)
{
dXSARGS;
/* Is it worth adding error checking for bad arguments? */
if (items >= 1)
- nbdkit_set_error (SvIV (ST (0)));
+ last_error = SvIV (ST (0));
+ nbdkit_set_error (last_error);
XSRETURN_EMPTY;
}
@@ -514,6 +517,45 @@ perl_can_trim (void *handle)
}
static int
+perl_zero (void *handle, uint32_t count, uint64_t offset, int may_trim)
+{
+ dSP;
+
+ if (callback_defined ("zero")) {
+ last_error = 0;
+ ENTER;
+ SAVETMPS;
+ PUSHMARK (SP);
+ XPUSHs (handle);
+ XPUSHs (sv_2mortal (newSViv (count)));
+ XPUSHs (sv_2mortal (newSViv (offset)));
+ XPUSHs (sv_2mortal (newSViv (may_trim)));
+ PUTBACK;
+ call_pv ("zero", G_EVAL|G_SCALAR);
+ SPAGAIN;
+ PUTBACK;
+ FREETMPS;
+ LEAVE;
+
+ if (last_error == EOPNOTSUPP) {
+ /* When user requests this particular error, we want to
+ gracefully fall back, and to accomodate both a normal return
+ and an exception. */
+ nbdkit_debug ("zero requested falling back to pwrite");
+ return -1;
+ }
+ if (check_perl_failure () == -1)
+ return -1;
+
+ return 0;
+ }
+
+ nbdkit_debug ("zero falling back to pwrite");
+ nbdkit_set_error (EOPNOTSUPP);
+ return -1;
+}
+
+static int
perl_is_rotational (void *handle)
{
dSP;
@@ -644,6 +686,7 @@ static struct nbdkit_plugin plugin = {
.pwrite = perl_pwrite,
.flush = perl_flush,
.trim = perl_trim,
+ .zero = perl_zero,
.errno_is_reliable = perl_errno_is_reliable,
};
--
2.9.3