From c0da910276e6318ce27c17bd776d183df94ffaf7 Mon Sep 17 00:00:00 2001
From: Axel Burri <axel@tty0.ch>
Date: Sun, 5 Jun 2022 17:36:01 +0200
Subject: [PATCH] btrbk: on resume, abort on unexpected if receive target is
 not present

A more sophisticated implementation would be to check this after
scheduling, only if the target really needs to be backuped.

We could as well automatically trigger a `btrfs snapshot -r` on target
in these cases, but this seems counter-intuitive.
---
 btrbk | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/btrbk b/btrbk
index 00a0ea3..22de3b7 100755
--- a/btrbk
+++ b/btrbk
@@ -7006,9 +7006,10 @@ MAIN:
             my $resume_total = 0;
             my $resume_success = 0;
 
+            my $unexpected_only = [];
             foreach my $snapshot (@snapshots)
             {
-              if(get_receive_targets($droot, $snapshot, exact => 1, warn => 1)){
+              if(get_receive_targets($droot, $snapshot, exact => 1, warn => 1, ret_unexpected_only => $unexpected_only)) {
                 DEBUG "Found correlated target of: $snapshot->{PRINT}";
                 next;
               }
@@ -7021,6 +7022,16 @@ MAIN:
                               });
             }
 
+            if(scalar @$unexpected_only && ((config_key($droot, "incremental") // "") eq "strict")) {
+              # If target exists at unexpected location ONLY, we can't send/receive it.
+              ABORTED($droot, "Receive targets of backup candidates exist at unexpected location only");
+              WARN "Skipping backup of \"$sroot->{PRINT}/${snapshot_basename}.*\": " . ABORTED_TEXT($droot),
+                "Please check your target configuration, or fix manually by running" . ($droot->{URL_PREFIX} ? " (on $droot->{URL_PREFIX}):" : ":"),
+                "`btrfs subvolume snapshot -r <found> <target>`",
+                map { "target: $droot->{PATH}/$_->{src_vol}{NAME}, found: " . _fs_path($_->{target_node}) } @$unexpected_only;
+              next;
+            }
+
             if(scalar @schedule)
             {
               DEBUG "Checking schedule for backup candidates";