fix: take a Debian cloud image from the newest build that has it

pve/vm-core.func

@@ -1832,6 +1832,32 @@ vm_prompt_start_vm() {
   echo -e "${GATEWAY}${BOLD}${DGN}Start VM when completed: ${BGN}${START_VM}${CL}"
 }
 
+# cloud.debian.org points latest/ at its newest build even when that build lacks
+# an image (20261111-2659 has no nocloud-amd64) or a mirror has not synced it.
+# Echoes the newest of the last five dated builds that has the file, else the URL.
+_vm_debian_build_url() {
+  local url="$1" base file stem ext dir cand
+  [[ "$url" =~ ^(https://cloud\.debian\.org/images/cloud/[a-z]+(/daily)?)/latest/([^/]+)$ ]] || { echo "$url"; return 0; }
+  base="${BASH_REMATCH[1]}" file="${BASH_REMATCH[3]}"
+  stem="${file%.*}" ext="${file##*.}"
+  curl -fsIL --max-time 15 -o /dev/null "$url" 2>/dev/null && { echo "$url"; return 0; }
+  # daily/latest/ names its files <image>-daily.<ext>; a caller may have left the suffix off.
+  local -a names=("$stem")
+  [[ "$base" == */daily && "$stem" != *-daily ]] && names=("$stem-daily" "$stem")
+  for cand in "${names[@]}"; do
+    curl -fsIL --max-time 15 -o /dev/null "$base/latest/$cand.$ext" 2>/dev/null && { echo "$base/latest/$cand.$ext"; return 0; }
+  done
+  for dir in $(curl -fsSL --max-time 15 "$base/" 2>/dev/null | grep -oE 'href="[0-9]{8}-[0-9]+/"' | grep -oE '[0-9]{8}-[0-9]+' | sort -ru | head -n 5); do
+    for cand in "${names[@]}"; do
+      if curl -fsIL --max-time 15 -o /dev/null "$base/$dir/$cand-$dir.$ext" 2>/dev/null; then
+        echo "$base/$dir/$cand-$dir.$ext"
+        return 0
+      fi
+    done
+  done
+  echo "$url"
+}
+
 # Downloads an image and proves it is the image. A bad mirror serves HTML with
 # status 200, so size decides rather than curl's exit code. Vendors also respin
 # images without updating the published checksum at the same byte count, so
@@ -1939,6 +1965,13 @@ vm_fetch_image() {
     esac
   done
 
+  local resolved
+  resolved="$(_vm_debian_build_url "$url")"
+  if [[ "$resolved" != "$url" ]]; then
+    msg_warn "cloud.debian.org has no $(basename "$url") in latest/ right now; using build $(basename "$(dirname "$resolved")")"
+    url="$resolved"
+  fi
+
   if [[ -n "$sha256" ]]; then
     sha_tool=sha256sum sha_want="${sha256,,}"
   elif [[ -n "$sha1" ]]; then

tests/vm-helpers.sh

@@ -158,6 +158,40 @@ done
 if vm_image_checksum "https://sourceforge.net/projects/x/files/a.iso/download" >/dev/null; then fail "unknown hosts have no checksum"; fi
 ok "vendor checksum files are derived from the download URL"
 
+# ── Debian latest/ without the image ────────────────────────────────────────
+# 20261111-2659 shipped no nocloud-amd64, and latest/ pointed at it: docker-vm
+# and debian-vm failed with 404 until the next build (VE #18702).
+DEB=https://cloud.debian.org/images/cloud/trixie
+fixture "$DEB/" <<'EOF'
+<a href="20261026-2643/">20261026-2643/</a> <a href="20261103-2651/">20261103-2651/</a>
+<a href="20261111-2659/">20261111-2659/</a> <a href="daily/">daily/</a> <a href="latest/">latest/</a>
+EOF
+fixture "$DEB/daily/" <<<'<a href="20261111-2659/">x</a> <a href="20261112-2660/">x</a>'
+HEAD_OK["$DEB/latest/debian-13-generic-amd64.qcow2"]=1
+HEAD_OK["$DEB/20261103-2651/debian-13-nocloud-amd64-20261103-2651.qcow2"]=1
+HEAD_OK["$DEB/20261026-2643/debian-13-nocloud-amd64-20261026-2643.qcow2"]=1
+HEAD_OK["$DEB/daily/20261112-2660/debian-13-generic-amd64-daily-20261112-2660.qcow2"]=1
+assert_eq "$(_vm_debian_build_url "$DEB/latest/debian-13-generic-amd64.qcow2")" "$DEB/latest/debian-13-generic-amd64.qcow2" "latest/ that has the image"
+assert_eq "$(_vm_debian_build_url "$DEB/latest/debian-13-nocloud-amd64.qcow2")" \
+  "$DEB/20261103-2651/debian-13-nocloud-amd64-20261103-2651.qcow2" "the newest build that has it"
+assert_eq "$(_vm_debian_build_url "$DEB/daily/latest/debian-13-generic-amd64-daily.qcow2")" \
+  "$DEB/daily/20261112-2660/debian-13-generic-amd64-daily-20261112-2660.qcow2" "daily builds"
+HEAD_OK["$DEB/daily/latest/debian-13-generic-amd64-daily.qcow2"]=1
+assert_eq "$(_vm_debian_build_url "$DEB/daily/latest/debian-13-generic-amd64.qcow2")"   "$DEB/daily/latest/debian-13-generic-amd64-daily.qcow2" "daily name without its suffix (waydroid-vm)"
+assert_eq "$(_vm_debian_build_url "$DEB/latest/debian-13-nocloud-riscv64.qcow2")" "$DEB/latest/debian-13-nocloud-riscv64.qcow2" "no build has it"
+assert_eq "$(_vm_debian_build_url https://dl.test/openwrt.img.gz)" https://dl.test/openwrt.img.gz "other hosts"
+ok "a Debian latest/ without the image falls back to the newest build that has it"
+
+reset_logs
+fixture "$DEB/20261103-2651/debian-13-nocloud-amd64-20261103-2651.qcow2" <"$IMAGE"
+fixture "$DEB/20261103-2651/SHA512SUMS" <<<"${IMAGE_SHA512}  debian-13-nocloud-amd64-20261103-2651.qcow2"
+vm_fetch_image "$DEB/latest/debian-13-nocloud-amd64.qcow2" "$TEST_DIR/cache/debian-13-nocloud-amd64.qcow2" ||
+  fail "a missing latest/ image must come from the build that has it"
+cmp -s "$IMAGE" "$TEST_DIR/cache/debian-13-nocloud-amd64.qcow2" || fail "the image kept its cache name"
+assert_contains "$LOG" "$DEB/20261103-2651/SHA512SUMS"
+assert_contains "$WARNINGS" "using build 20261103-2651"
+ok "vm_fetch_image downloads and verifies it from that build"
+
 # ── vm_fetch_image checksum options ─────────────────────────────────────────
 fixture "https://dl.test/openwrt.img.gz" <"$IMAGE"
 reset_logs