OpenZFS 6738 - zfs send stream padding needs documentation

Authored by: Eli Rosenthal <eli.rosenthal@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/6738
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/c20404ff

diff --git a/module/zfs/dmu_send.c b/module/zfs/dmu_send.c
index 1c7b532694678216c24a44797724e28603184638..2f4087a67b0e9dd64fc196081a8d1a19447a5d3e 100644 (file)
--- a/module/zfs/dmu_send.c
+++ b/module/zfs/dmu_send.c
@@ -107,6 +107,19 @@ dump_bytes_cb(void *arg)
        dmu_sendarg_t *dsp = dbi->dbi_dsp;
        dsl_dataset_t *ds = dmu_objset_ds(dsp->dsa_os);
        ssize_t resid; /* have to get resid to get detailed errno */
+
+       /*
+        * The code does not rely on this (len being a multiple of 8).  We keep
+        * this assertion because of the corresponding assertion in
+        * receive_read().  Keeping this assertion ensures that we do not
+        * inadvertently break backwards compatibility (causing the assertion
+        * in receive_read() to trigger on old software).
+        *
+        * Removing the assertions could be rolled into a new feature that uses
+        * data that isn't 8-byte aligned; if the assertions were removed, a
+        * feature flag would have to be added.
+        */
+
        ASSERT0(dbi->dbi_len % 8);
 
        dsp->dsa_err = vn_rdwr(UIO_WRITE, dsp->dsa_vp,
@@ -1806,7 +1819,10 @@ receive_read(struct receive_arg *ra, int len, void *buf)
 {
        int done = 0;
 
-       /* some things will require 8-byte alignment, so everything must */
+       /*
+        * The code doesn't rely on this (lengths being multiples of 8).  See
+        * comment in dump_bytes.
+        */
        ASSERT0(len % 8);
 
        while (done < len) {