zfs send -R -I "helpfully" doesn't error on being given a nonexistent snapshot

[rincebrain]

System information

Type	Version/Name
Distribution Name	Debian
Distribution Version	11
Kernel Version	5.10.0-8-amd64
Architecture	x86_64
OpenZFS Version	2.0.3 / 2a49ebb

Describe the problem you're observing

I was rather confused on 2.0.3 when I tried doing zfs send -R -I mydataset@2021-08-14 mydataset@2021-09-17 | ... | zfs recv somewhere and it spat up "cannot receive new filesystem stream: destination 'somewhere' exists".

On further debugging, mydataset@2021-08-14 had been destroyed, and zfs send -R -I was helpfully generating a send stream of every snapshot on the filesystem, but with a fromsnap of a non-present snapshot inside, which seems to result in a send stream you cannot receive to a new location (because it's got fromsnap) or the existing location without destroying everything.

This doesn't seem to be documented, so I could just go cut a man page update, but it seems surprising enough that I'd argue it's not really desired behavior.

Describe how to reproduce the problem

root@debrepro:~# for i in `seq 1 7`;do dd if=/dev/urandom of=/mypool/$i bs=1M count=64 && zfs snapshot mypool@snap$i;done;
64+0 records in
64+0 records out
67108864 bytes (67 MB, 64 MiB) copied, 2.22001 s, 30.2 MB/s
64+0 records in
64+0 records out
67108864 bytes (67 MB, 64 MiB) copied, 3.00796 s, 22.3 MB/s
64+0 records in
64+0 records out
67108864 bytes (67 MB, 64 MiB) copied, 2.85519 s, 23.5 MB/s
64+0 records in
64+0 records out
67108864 bytes (67 MB, 64 MiB) copied, 2.86249 s, 23.4 MB/s
64+0 records in
64+0 records out
67108864 bytes (67 MB, 64 MiB) copied, 2.59387 s, 25.9 MB/s
64+0 records in
64+0 records out
67108864 bytes (67 MB, 64 MiB) copied, 2.921 s, 23.0 MB/s
64+0 records in
64+0 records out
67108864 bytes (67 MB, 64 MiB) copied, 2.65469 s, 25.3 MB/s
root@debrepro:~# zfs list -t all -r mypool
NAME           USED  AVAIL     REFER  MOUNTPOINT
mypool         449M   383M      448M  /mypool
mypool@snap1    13K      -     64.1M  -
mypool@snap2    13K      -      128M  -
mypool@snap3    13K      -      192M  -
mypool@snap4    13K      -      256M  -
mypool@snap5    13K      -      320M  -
mypool@snap6    13K      -      384M  -
mypool@snap7     0B      -      448M  -
root@debrepro:~# zfs send -I mypool@nonsense mypool@snap5 > /tmp/whateven3
WARNING: could not send mypool@snap5:
incremental source (mypool@nonsense) does not exist
root@debrepro:~# zfs send -R -I mypool@nonsense mypool@snap5 > /tmp/whateven
root@debrepro:~# zfs send -R mypool@snap5 > /tmp/whateven2
root@debrepro:~# zstreamdump -vv < /tmp/whateven | grep -v checksum > /tmp/1
root@debrepro:~# zstreamdump -vv < /tmp/whateven2 | grep -v checksum > /tmp/2
root@debrepro:~# diff -urN /tmp/[12]
--- /tmp/1      2021-09-18 14:44:10.995540871 -0400
+++ /tmp/2      2021-09-18 14:44:14.843475467 -0400
@@ -8,10 +8,9 @@
        toguid = 0
        fromguid = 0
        toname = mypool@snap5
-       payloadlen = 812
+       payloadlen = 772

 nvlist version: 0
-       fromsnap = nonsense
        tosnap = snap5
        fss = (embedded nvlist)
        nvlist version: 0
@@ -220,7 +219,7 @@
 FREE object = 512 offset = 67108864 length = -1
 FREEOBJECTS firstobj = 513 numobjs = 31
 SUMMARY:
-       Total DRR_BEGIN records = 6 (812 bytes)
+       Total DRR_BEGIN records = 6 (772 bytes)
        Total DRR_END records = 7 (0 bytes)
        Total DRR_OBJECT records = 31 (1720 bytes)
        Total DRR_FREEOBJECTS records = 16 (0 bytes)
@@ -230,6 +229,6 @@
        Total DRR_FREE records = 52 (0 bytes)
        Total DRR_SPILL records = 0 (0 bytes)
        Total records = 2683
-       Total payload size = 335585252 (0x14009fe4)
+       Total payload size = 335585212 (0x14009fbc)
        Total header overhead = 837096 (0xcc5e8)
-       Total stream length = 336422348 (0x140d65cc)
+       Total stream length = 336422308 (0x140d65a4)
root@debrepro:~#

Include any warning/errors/backtraces from the system logs

Above.