Merge pull request '(feature) Add backup safety and preflight validation' (#13) from issue-3-backup-safety-preflight-validation into master

Reviewed-on: #13

src/pobsync_backend/config_repository.py

@@ -77,6 +77,14 @@ def _host_yaml_data(host_config: HostConfig) -> dict[str, Any]:
     return validate_dict(data, HOST_SCHEMA, path="host")
 
 
+def global_config_object_data(global_config: GlobalConfig) -> dict[str, Any]:
+    return _global_yaml_data(global_config)
+
+
+def host_config_object_data(host_config: HostConfig) -> dict[str, Any]:
+    return _host_yaml_data(host_config)
+
+
 def global_config_data(name: str = "default") -> dict[str, Any]:
     try:
         global_config = GlobalConfig.objects.get(name=name)

src/pobsync_backend/preflight.py

@@ -0,0 +1,232 @@
+from __future__ import annotations
+
+import shlex
+import subprocess
+from dataclasses import dataclass
+from typing import Any
+
+from pobsync.config.merge import build_effective_config
+from pobsync.rsync import build_ssh_command
+
+from .config_repository import global_config_object_data, host_config_object_data
+from .config_source import DjangoConfigSource
+from .host_ops import collect_host_checks
+from .models import GlobalConfig, HostConfig
+from .self_check import SelfCheck
+
+
+DRY_RUN_BLOCKING_CHECKS = {
+    "Host global config",
+    "Host address",
+    "Host SSH key file",
+    "Host effective source root",
+    "Host effective SSH user",
+    "Host effective SSH port",
+    "Host effective SSH credential",
+    "Host effective rsync recursion",
+}
+
+
+@dataclass(frozen=True)
+class BackupGate:
+    state: str
+    message: str
+    checks: list[SelfCheck]
+    real_blockers: list[SelfCheck]
+    dry_run_blockers: list[SelfCheck]
+    warnings: list[SelfCheck]
+
+    @property
+    def can_queue_real(self) -> bool:
+        return not self.real_blockers
+
+    @property
+    def can_queue_dry_run(self) -> bool:
+        return not self.dry_run_blockers
+
+
+def collect_backup_gate(host: HostConfig, global_config: GlobalConfig | None = None) -> BackupGate:
+    checks = collect_host_checks(host, global_config)
+    remote_preflight_check = _remote_preflight_self_check(host)
+    if remote_preflight_check is not None:
+        checks.append(remote_preflight_check)
+    real_blockers = [check for check in checks if check.status == "failed"]
+    dry_run_blockers = [check for check in real_blockers if check.name in DRY_RUN_BLOCKING_CHECKS]
+    warnings = [check for check in checks if check.status == "warning"]
+
+    if real_blockers:
+        state = "blocked"
+        message = "Real backups are blocked until failed host checks are resolved."
+    elif warnings:
+        state = "warning"
+        message = "Backups can run, but review the warnings first."
+    else:
+        state = "ready"
+        message = "This host is ready for backup runs."
+
+    return BackupGate(
+        state=state,
+        message=message,
+        checks=checks,
+        real_blockers=real_blockers,
+        dry_run_blockers=dry_run_blockers,
+        warnings=warnings,
+    )
+
+
+def run_remote_preflight(host: HostConfig, *, timeout_seconds: int = 20) -> dict[str, Any]:
+    config = DjangoConfigSource().effective_config_for_host(host.host)
+    ssh_cfg = config.get("ssh", {}) or {}
+    rsync_cfg = config.get("rsync", {}) or {}
+    address = str(config.get("address") or host.address)
+    user = str(ssh_cfg.get("user") or "root")
+    source_root = str(config.get("source_root") or (config.get("defaults", {}) or {}).get("source_root") or "/")
+    rsync_binary = str(rsync_cfg.get("binary") or "rsync")
+    target = f"{user}@{address}"
+    ssh_cmd = build_ssh_command(ssh_cfg)
+
+    checks = [
+        _run_remote_check(
+            name="SSH reachability",
+            command=[*ssh_cmd, "-oBatchMode=yes", target, "true"],
+            timeout_seconds=timeout_seconds,
+        ),
+        _run_remote_check(
+            name="Remote rsync",
+            command=[
+                *ssh_cmd,
+                "-oBatchMode=yes",
+                target,
+                "sh",
+                "-lc",
+                f"command -v {shlex.quote(rsync_binary)} >/dev/null",
+            ],
+            timeout_seconds=timeout_seconds,
+        ),
+        _run_remote_check(
+            name="Remote source root",
+            command=[
+                *ssh_cmd,
+                "-oBatchMode=yes",
+                target,
+                "sh",
+                "-lc",
+                f"test -e {shlex.quote(source_root)} && test -r {shlex.quote(source_root)}",
+            ],
+            timeout_seconds=timeout_seconds,
+        ),
+    ]
+    result = {
+        "ok": all(check["ok"] for check in checks),
+        "checks": checks,
+        "target": target,
+        "source_root": source_root,
+        "rsync_binary": rsync_binary,
+        "timeout_seconds": timeout_seconds,
+    }
+    host.config = {**(host.config or {}), "last_preflight": result}
+    host.save(update_fields=["config", "updated_at"])
+    return result
+
+
+def effective_host_config_preview(host: HostConfig, global_config: GlobalConfig) -> dict[str, Any]:
+    config = build_effective_config(global_config_object_data(global_config), host_config_object_data(host))
+    credential = host.ssh_credential or global_config.default_ssh_credential
+    ssh = config.get("ssh", {}) or {}
+    rsync = config.get("rsync", {}) or {}
+    retention = config.get("retention", {}) or {}
+
+    return {
+        "source_root": config.get("source_root", ""),
+        "destination_subdir": (config.get("defaults", {}) or {}).get("destination_subdir", ""),
+        "includes": list(config.get("includes") or []),
+        "excludes": list(config.get("excludes_effective") or []),
+        "ssh": {
+            "user": ssh.get("user", ""),
+            "port": ssh.get("port", ""),
+            "options": list(ssh.get("options") or []),
+            "credential": str(credential) if credential else "",
+        },
+        "rsync": {
+            "binary": rsync.get("binary", ""),
+            "args": list(rsync.get("args_effective") or []),
+            "timeout_seconds": rsync.get("timeout_seconds", 0),
+            "bwlimit_kbps": rsync.get("bwlimit_kbps", 0),
+        },
+        "retention": {
+            "daily": retention.get("daily", 0),
+            "weekly": retention.get("weekly", 0),
+            "monthly": retention.get("monthly", 0),
+            "yearly": retention.get("yearly", 0),
+        },
+    }
+
+
+def _run_remote_check(*, name: str, command: list[str], timeout_seconds: int) -> dict[str, Any]:
+    try:
+        result = subprocess.run(
+            command,
+            check=False,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            text=True,
+            timeout=timeout_seconds,
+        )
+    except subprocess.TimeoutExpired as exc:
+        return {
+            "name": name,
+            "ok": False,
+            "exit_code": 124,
+            "message": f"{name} timed out after {timeout_seconds}s.",
+            "detail": _clip_output((exc.stderr or exc.stdout or "").strip()),
+        }
+    except OSError as exc:
+        return {
+            "name": name,
+            "ok": False,
+            "exit_code": None,
+            "message": f"{name} could not start.",
+            "detail": str(exc),
+        }
+
+    return {
+        "name": name,
+        "ok": result.returncode == 0,
+        "exit_code": result.returncode,
+        "message": f"{name} passed." if result.returncode == 0 else f"{name} failed.",
+        "detail": _clip_output((result.stderr or result.stdout or "").strip()),
+    }
+
+
+def _remote_preflight_self_check(host: HostConfig) -> SelfCheck | None:
+    preflight = (host.config or {}).get("last_preflight")
+    if not isinstance(preflight, dict):
+        return SelfCheck(
+            "Remote preflight",
+            "warning",
+            "No remote connection preflight has been run yet.",
+            "Run connection preflight before the first real backup.",
+        )
+    checks = preflight.get("checks")
+    if not isinstance(checks, list):
+        return SelfCheck("Remote preflight", "failed", "Stored remote preflight result is invalid.")
+    failed = [str(check.get("name", "unknown")) for check in checks if isinstance(check, dict) and not check.get("ok")]
+    if failed:
+        return SelfCheck(
+            "Remote preflight",
+            "failed",
+            "Remote connection preflight failed.",
+            ", ".join(failed),
+        )
+    return SelfCheck(
+        "Remote preflight",
+        "ok",
+        "Remote connection preflight passed.",
+        f"{preflight.get('target', '')} {preflight.get('source_root', '')}".strip(),
+    )
+
+
+def _clip_output(value: str, *, max_chars: int = 800) -> str:
+    if len(value) <= max_chars:
+        return value
+    return f"{value[:max_chars]}..."

src/pobsync_backend/templates/pobsync_backend/base.html

@@ -81,6 +81,7 @@
     .status.success { color: var(--success); border-color: #a7d8b9; background: #edf8f1; }
     .status.ok { color: var(--success); border-color: #a7d8b9; background: #edf8f1; }
     .status.failed { color: var(--failed); border-color: #e8b4b4; background: #fff0f0; }
+    .status.blocked { color: var(--failed); border-color: #e8b4b4; background: #fff0f0; }
     .status.running { color: var(--running); border-color: #e7cf8a; background: #fff8df; }
     .status.warning { color: var(--running); border-color: #e7cf8a; background: #fff8df; }
     .status.queued { color: var(--link); border-color: #b5cdea; background: #eef6ff; }

src/pobsync_backend/templates/pobsync_backend/host_detail.html

@@ -21,6 +21,10 @@
       {% csrf_token %}
       <button type="submit" class="secondary">Scan SSH host key</button>
     </form>
+    <form method="post" action="{% url 'run_host_preflight' host.host %}">
+      {% csrf_token %}
+      <button type="submit" class="secondary">Run connection preflight</button>
+    </form>
   </section>
 
   <section class="grid" aria-label="Host summary">
@@ -64,6 +68,46 @@
     </section>
   </div>
 
+  {% if effective_config %}
+    <section class="panel">
+      <h2>Effective Config</h2>
+      <div class="two-col">
+        <div class="stack">
+          <div><strong>Source root:</strong> {{ effective_config.source_root }}</div>
+          <div><strong>Destination subdir:</strong> {{ effective_config.destination_subdir|default:"none" }}</div>
+          <div><strong>SSH:</strong> {{ effective_config.ssh.user }}@{{ host.address }}:{{ effective_config.ssh.port }}</div>
+          <div><strong>SSH key:</strong> {{ effective_config.ssh.credential|default:"none selected" }}</div>
+          <div><strong>SSH options:</strong> {{ effective_config.ssh.options|join:" " }}</div>
+          <div><strong>Rsync binary:</strong> {{ effective_config.rsync.binary }}</div>
+          <div><strong>Rsync args:</strong> {{ effective_config.rsync.args|join:" " }}</div>
+          <div><strong>Timeout:</strong> {{ effective_config.rsync.timeout_seconds }}s</div>
+          <div><strong>Bandwidth limit:</strong> {{ effective_config.rsync.bwlimit_kbps }} KB/s</div>
+          <div>
+            <strong>Retention:</strong>
+            d{{ effective_config.retention.daily }}
+            w{{ effective_config.retention.weekly }}
+            m{{ effective_config.retention.monthly }}
+            y{{ effective_config.retention.yearly }}
+          </div>
+        </div>
+        <div class="stack">
+          <div><strong>Includes:</strong> {{ effective_config.includes|length }}</div>
+          {% if effective_config.includes %}
+            <pre>{{ effective_config.includes|join:"&#10;" }}</pre>
+          {% else %}
+            <div class="muted">No include rules configured.</div>
+          {% endif %}
+          <div><strong>Excludes:</strong> {{ effective_config.excludes|length }}</div>
+          {% if effective_config.excludes %}
+            <pre>{{ effective_config.excludes|join:"&#10;" }}</pre>
+          {% else %}
+            <div class="muted">No exclude rules configured.</div>
+          {% endif %}
+        </div>
+      </div>
+    </section>
+  {% endif %}
+
   <section class="panel">
     <h2>Snapshot Discovery</h2>
     <div class="stack">
@@ -159,14 +203,47 @@
     </table>
   </section>
 
+  {% if last_preflight %}
+    <section class="panel">
+      <h2>Connection Preflight</h2>
+      <div class="stack spaced">
+        <div><strong>Status:</strong> <span class="status {% if last_preflight.ok %}ok{% else %}failed{% endif %}">{% if last_preflight.ok %}ok{% else %}failed{% endif %}</span></div>
+        <div><strong>Target:</strong> {{ last_preflight.target }}</div>
+        <div><strong>Source root:</strong> {{ last_preflight.source_root }}</div>
+        <div><strong>Remote rsync:</strong> {{ last_preflight.rsync_binary }}</div>
+      </div>
+      <table>
+        <thead>
+          <tr>
+            <th>Status</th>
+            <th>Check</th>
+            <th>Message</th>
+            <th>Detail</th>
+          </tr>
+        </thead>
+        <tbody>
+          {% for check in last_preflight.checks %}
+            <tr>
+              <td><span class="status {% if check.ok %}ok{% else %}failed{% endif %}">{% if check.ok %}ok{% else %}failed{% endif %}</span></td>
+              <td>{{ check.name }}</td>
+              <td>{{ check.message }}</td>
+              <td class="muted">{{ check.detail }}</td>
+            </tr>
+          {% endfor %}
+        </tbody>
+      </table>
+    </section>
+  {% endif %}
+
   <section class="panel">
     <h2>Backup Control</h2>
     <div class="operator-state">
       {% if active_run %}
         <span class="status {{ active_run.status }}">{{ active_run.status }}</span>
         <a href="{% url 'run_detail' active_run.id %}">Run {{ active_run.id }}</a>
-      {% elif can_queue_backup %}
-        <span class="status success">ready</span>
+      {% elif has_global_config and host.enabled %}
+        <span class="status {{ backup_gate.state }}">{{ backup_gate.state }}</span>
+        <span class="muted">{{ backup_gate.message }}</span>
       {% elif not host.enabled %}
         <span class="status failed">disabled</span>
       {% elif not has_global_config %}
@@ -180,20 +257,24 @@
         <input type="hidden" name="dry_run" value="on">
         <input type="hidden" name="verbose_output" value="on">
         <input type="hidden" name="prune_max_delete" value="10">
-        <button type="submit" class="secondary" {% if not can_queue_backup %}disabled{% endif %}>Queue dry-run</button>
+        <button type="submit" class="secondary" {% if not can_queue_dry_run %}disabled{% endif %}>Queue dry-run</button>
       </form>
       <form class="inline-form" method="post" action="{% url 'queue_manual_backup' host.host %}">
         {% csrf_token %}
         <input type="hidden" name="prune_max_delete" value="10">
-        <button type="submit" {% if not can_queue_backup %}disabled{% endif %}>Queue backup</button>
+        <button type="submit" {% if not can_queue_real_backup %}disabled{% endif %}>Queue backup</button>
       </form>
     </section>
 
-    {% if not can_queue_backup %}
+    {% if active_run %}
+      <p class="muted">Wait for the active run to finish, or cancel it from the run detail page.</p>
+    {% elif not can_queue_dry_run or not can_queue_real_backup %}
       {% if not has_global_config %}
         <p class="muted">Create the default global config before queueing backups.</p>
       {% elif not host.enabled %}
         <p class="muted">Enable this host before queueing backups.</p>
+      {% elif backup_gate.real_blockers %}
+        <p class="muted">Real backups are blocked by failed preflight checks. Dry-runs may still be available when storage-only checks fail.</p>
       {% endif %}
     {% endif %}
 
@@ -212,7 +293,7 @@
       {% endfor %}
 
       <div class="actions">
-        <button type="submit" {% if not can_queue_backup %}disabled{% endif %}>Queue with options</button>
+        <button type="submit" {% if not can_queue_dry_run and not can_queue_real_backup %}disabled{% endif %}>Queue with options</button>
       </div>
     </form>
   </section>

src/pobsync_backend/templates/pobsync_backend/run_detail.html

@@ -27,12 +27,51 @@
       <h2>Failure</h2>
       <div class="stack">
         <div><strong>Category:</strong> {{ failure.category|default:"unknown" }}</div>
-        <div><strong>Summary:</strong> {{ failure.summary|default:"" }}</div>
+        <div><strong>Summary:</strong> {{ failure_summary }}</div>
         <div><strong>Hint:</strong> {{ failure.hint|default:"" }}</div>
       </div>
     </section>
   {% endif %}
 
+  {% if dry_run_summary %}
+    <section class="panel highlight {{ dry_run_summary.highlight_class }}">
+      <h2>Dry Run Summary</h2>
+      <section class="grid" aria-label="Dry run summary">
+        <div class="metric"><div class="label">Status</div><div class="value">{{ dry_run_summary.status }}</div></div>
+        <div class="metric"><div class="label">Files Seen</div><div class="value">{{ dry_run_summary.files_seen|default:"unknown" }}</div></div>
+        <div class="metric"><div class="label">Would Transfer</div><div class="value">{{ dry_run_summary.files_would_transfer|default:"unknown" }}</div></div>
+        <div class="metric"><div class="label">Transfer Estimate</div><div class="value">{{ dry_run_summary.transfer_estimate_bytes|filesizeformat }}</div></div>
+        <div class="metric"><div class="label">Total Size</div><div class="value">{{ dry_run_summary.total_file_size_bytes|filesizeformat }}</div></div>
+        <div class="metric"><div class="label">Link-Dest Saving</div><div class="value">{{ dry_run_summary.link_dest_estimated_savings_bytes|filesizeformat }}</div></div>
+      </section>
+      <div class="stack">
+        {% if dry_run_summary.duration_seconds is not None %}
+          <div><strong>Duration:</strong> {{ dry_run_summary.duration_seconds }}s</div>
+        {% endif %}
+        <div>
+          <strong>Log:</strong>
+          {% if dry_run_summary.log_available %}
+            <a href="{% url 'run_rsync_log' run.id %}">Open full rsync log</a>
+          {% elif rsync_log_path %}
+            <span class="muted">{{ rsync_log_path }} (missing)</span>
+          {% else %}
+            <span class="muted">not recorded yet</span>
+          {% endif %}
+        </div>
+        {% if dry_run_summary.warnings %}
+          <div><strong>Warnings:</strong></div>
+          <ul>
+            {% for warning in dry_run_summary.warnings %}
+              <li>{{ warning }}</li>
+            {% endfor %}
+          </ul>
+        {% else %}
+          <div><strong>Warnings:</strong> none recorded</div>
+        {% endif %}
+      </div>
+    </section>
+  {% endif %}
+
   <div class="two-col">
     <section class="panel">
       <h2>Timing</h2>

src/pobsync_backend/tests/test_views.py

@@ -557,18 +557,22 @@ class ViewTests(TestCase):
 
     def test_host_detail_renders_config_schedule_runs_and_snapshots(self) -> None:
         self.client.force_login(self.staff_user)
-        GlobalConfig.objects.create(name="default", backup_root="/backups")
-        host = HostConfig.objects.create(
-            host="web-01",
-            address="web-01.example.test",
-            source_root="/srv",
-            retention_daily=7,
-        )
-        ScheduleConfig.objects.create(host=host, cron_expr="15 2 * * *", prune=True, last_status="success")
-        snapshot = self._snapshot(host, "20260519-021500Z__ABCDEFGH")
-        BackupRun.objects.create(host=host, status=BackupRun.Status.SUCCESS, snapshot=snapshot)
+        with TemporaryDirectory() as tmp:
+            backup_root = Path(tmp)
+            GlobalConfig.objects.create(name="default", backup_root=str(backup_root), rsync_args=["--archive"])
+            host = HostConfig.objects.create(
+                host="web-01",
+                address="web-01.example.test",
+                source_root="/srv",
+                retention_daily=7,
+            )
+            for subdir in ("scheduled", "manual", ".incomplete"):
+                (backup_root / host.host / subdir).mkdir(parents=True)
+            ScheduleConfig.objects.create(host=host, cron_expr="15 2 * * *", prune=True, last_status="success")
+            snapshot = self._snapshot(host, "20260519-021500Z__ABCDEFGH")
+            BackupRun.objects.create(host=host, status=BackupRun.Status.SUCCESS, snapshot=snapshot)
 
-        response = self.client.get(reverse("host_detail", args=[host.host]))
+            response = self.client.get(reverse("host_detail", args=[host.host]))
 
         self.assertEqual(response.status_code, 200)
         self.assertContains(response, "web-01")
@@ -582,17 +586,122 @@ class ViewTests(TestCase):
         self.assertContains(response, "Discover snapshots")
         self.assertContains(response, "Edit schedule")
         self.assertContains(response, "Edit config")
+        self.assertContains(response, "Run connection preflight")
         self.assertContains(response, "Backup Control")
         self.assertContains(response, "Queue dry-run")
         self.assertContains(response, "Queue backup")
         self.assertContains(response, "Host Check")
         self.assertContains(response, reverse("prepare_host_directories", args=[host.host]))
-        self.assertContains(response, "ready")
+        self.assertContains(response, "warning")
         self.assertContains(response, "Snapshot Discovery")
         self.assertContains(response, reverse("queue_manual_backup", args=[host.host]))
         self.assertContains(response, reverse("run_detail", args=[BackupRun.objects.get().id]))
         self.assertContains(response, reverse("snapshot_detail", args=[snapshot.id]))
 
+    def test_host_detail_renders_effective_config_preview(self) -> None:
+        self.client.force_login(self.staff_user)
+        credential = SshCredential.objects.create(name="default-key", key_path="/var/lib/pobsync/id_ed25519")
+        GlobalConfig.objects.create(
+            name="default",
+            backup_root="/backups",
+            default_ssh_credential=credential,
+            ssh_user="root",
+            ssh_port=2222,
+            ssh_options=["-oBatchMode=yes"],
+            rsync_args=["--archive", "--numeric-ids"],
+            rsync_extra_args=["--delete"],
+            rsync_timeout_seconds=300,
+            rsync_bwlimit_kbps=2048,
+            default_source_root="/",
+            excludes_default=["/proc/***", "/sys/***"],
+            retention_daily=14,
+            retention_weekly=4,
+            retention_monthly=2,
+            retention_yearly=1,
+        )
+        host = HostConfig.objects.create(
+            host="web-01",
+            address="web-01.example.test",
+            includes=["/srv/www/***"],
+            excludes_add=["/srv/www/cache/***"],
+            rsync_extra_args=["--one-file-system"],
+        )
+
+        response = self.client.get(reverse("host_detail", args=[host.host]))
+
+        self.assertEqual(response.status_code, 200)
+        self.assertContains(response, "Effective Config")
+        self.assertContains(response, "root@web-01.example.test:2222")
+        self.assertContains(response, "default-key")
+        self.assertContains(response, "-oBatchMode=yes")
+        self.assertContains(response, "--archive --numeric-ids --delete --one-file-system")
+        self.assertContains(response, "/srv/www/***")
+        self.assertContains(response, "/srv/www/cache/***")
+        self.assertContains(response, "d14")
+        self.assertContains(response, "w8")
+
+    def test_run_host_preflight_stores_remote_check_result(self) -> None:
+        self.client.force_login(self.staff_user)
+        GlobalConfig.objects.create(name="default", backup_root="/backups", rsync_args=["--archive"])
+        host = HostConfig.objects.create(host="web-01", address="web-01.example.test")
+
+        with patch(
+            "pobsync_backend.preflight.subprocess.run",
+            return_value=subprocess.CompletedProcess(args=["ssh"], returncode=0, stdout="", stderr=""),
+        ) as run:
+            response = self.client.post(reverse("run_host_preflight", args=[host.host]), follow=True)
+
+        self.assertRedirects(response, reverse("host_detail", args=[host.host]))
+        self.assertContains(response, "Connection preflight passed for web-01.")
+        self.assertContains(response, "Connection Preflight")
+        self.assertContains(response, "SSH reachability")
+        self.assertContains(response, "Remote rsync")
+        self.assertContains(response, "Remote source root")
+        self.assertEqual(run.call_count, 3)
+        host.refresh_from_db()
+        self.assertTrue(host.config["last_preflight"]["ok"])
+        self.assertEqual(host.config["last_preflight"]["target"], "root@web-01.example.test")
+
+    def test_queue_manual_backup_blocks_real_backup_after_failed_remote_preflight(self) -> None:
+        self.client.force_login(self.staff_user)
+        with TemporaryDirectory() as tmp:
+            backup_root = Path(tmp)
+            GlobalConfig.objects.create(name="default", backup_root=str(backup_root), rsync_args=["--archive"])
+            host = HostConfig.objects.create(
+                host="web-01",
+                address="web-01.example.test",
+                config={
+                    "last_preflight": {
+                        "ok": False,
+                        "target": "root@web-01.example.test",
+                        "source_root": "/",
+                        "rsync_binary": "rsync",
+                        "checks": [
+                            {
+                                "name": "Remote rsync",
+                                "ok": False,
+                                "exit_code": 127,
+                                "message": "Remote rsync failed.",
+                                "detail": "rsync missing",
+                            }
+                        ],
+                    }
+                },
+            )
+            for subdir in ("scheduled", "manual", ".incomplete"):
+                (backup_root / host.host / subdir).mkdir(parents=True)
+
+            response = self.client.post(
+                reverse("queue_manual_backup", args=[host.host]),
+                {"prune_max_delete": "10"},
+                follow=True,
+            )
+
+        self.assertRedirects(response, reverse("host_detail", args=[host.host]))
+        self.assertContains(response, "Cannot queue real backup until failed preflight checks are resolved")
+        self.assertContains(response, "Remote preflight")
+        self.assertFalse(BackupRun.objects.exists())
+
     def test_host_detail_renders_backup_trends(self) -> None:
         self.client.force_login(self.staff_user)
         GlobalConfig.objects.create(name="default", backup_root="/backups")
@@ -779,7 +888,7 @@ class ViewTests(TestCase):
 
     def test_queue_manual_backup_creates_queued_run_and_redirects_to_run_detail(self) -> None:
         self.client.force_login(self.staff_user)
-        GlobalConfig.objects.create(name="default", backup_root="/backups")
+        GlobalConfig.objects.create(name="default", backup_root="/backups", rsync_args=["--archive"])
         host = HostConfig.objects.create(host="web-01", address="web-01.example.test")
 
         response = self.client.post(
@@ -812,14 +921,18 @@ class ViewTests(TestCase):
 
     def test_queue_manual_backup_quick_action_can_queue_real_backup(self) -> None:
         self.client.force_login(self.staff_user)
-        GlobalConfig.objects.create(name="default", backup_root="/backups")
-        host = HostConfig.objects.create(host="web-01", address="web-01.example.test")
+        with TemporaryDirectory() as tmp:
+            backup_root = Path(tmp)
+            GlobalConfig.objects.create(name="default", backup_root=str(backup_root), rsync_args=["--archive"])
+            host = HostConfig.objects.create(host="web-01", address="web-01.example.test")
+            for subdir in ("scheduled", "manual", ".incomplete"):
+                (backup_root / host.host / subdir).mkdir(parents=True)
 
-        response = self.client.post(
-            reverse("queue_manual_backup", args=[host.host]),
-            {"prune_max_delete": "10"},
-            follow=True,
-        )
+            response = self.client.post(
+                reverse("queue_manual_backup", args=[host.host]),
+                {"prune_max_delete": "10"},
+                follow=True,
+            )
 
         run = BackupRun.objects.get(host=host)
         self.assertRedirects(response, reverse("run_detail", args=[run.id]))
@@ -834,6 +947,41 @@ class ViewTests(TestCase):
             },
         )
 
+    def test_queue_manual_backup_blocks_real_backup_when_host_directories_are_missing(self) -> None:
+        self.client.force_login(self.staff_user)
+        with TemporaryDirectory() as tmp:
+            backup_root = Path(tmp)
+            GlobalConfig.objects.create(name="default", backup_root=str(backup_root), rsync_args=["--archive"])
+            host = HostConfig.objects.create(host="web-01", address="web-01.example.test")
+
+            response = self.client.post(
+                reverse("queue_manual_backup", args=[host.host]),
+                {"prune_max_delete": "10"},
+                follow=True,
+            )
+
+        self.assertRedirects(response, reverse("host_detail", args=[host.host]))
+        self.assertContains(response, "Cannot queue real backup until failed preflight checks are resolved")
+        self.assertContains(response, "Host backup root")
+        self.assertFalse(BackupRun.objects.exists())
+
+    def test_queue_manual_backup_allows_dry_run_with_only_storage_preflight_failures(self) -> None:
+        self.client.force_login(self.staff_user)
+        with TemporaryDirectory() as tmp:
+            backup_root = Path(tmp)
+            GlobalConfig.objects.create(name="default", backup_root=str(backup_root), rsync_args=["--archive"])
+            host = HostConfig.objects.create(host="web-01", address="web-01.example.test")
+
+            response = self.client.post(
+                reverse("queue_manual_backup", args=[host.host]),
+                {"dry_run": "on", "verbose_output": "on", "prune_max_delete": "10"},
+                follow=True,
+            )
+
+        run = BackupRun.objects.get(host=host)
+        self.assertRedirects(response, reverse("run_detail", args=[run.id]))
+        self.assertEqual(run.result["requested"]["dry_run"], True)
+
     def test_queue_manual_backup_requires_default_global_config(self) -> None:
         self.client.force_login(self.staff_user)
         host = HostConfig.objects.create(host="web-01", address="web-01.example.test")
@@ -846,7 +994,7 @@ class ViewTests(TestCase):
 
     def test_queue_manual_backup_rejects_disabled_host(self) -> None:
         self.client.force_login(self.staff_user)
-        GlobalConfig.objects.create(name="default", backup_root="/backups")
+        GlobalConfig.objects.create(name="default", backup_root="/backups", rsync_args=["--archive"])
         host = HostConfig.objects.create(host="web-01", address="web-01.example.test", enabled=False)
 
         response = self.client.post(reverse("queue_manual_backup", args=[host.host]), {"dry_run": "on"}, follow=True)
@@ -918,12 +1066,69 @@ class ViewTests(TestCase):
         self.assertContains(response, "--archive")
         self.assertContains(response, "Rsync Log")
         self.assertContains(response, "sending incremental file list")
+        self.assertContains(response, "Dry Run Summary")
+        self.assertContains(response, "Files Seen")
+        self.assertContains(response, "Would Transfer")
+        self.assertContains(response, "Transfer Estimate")
+        self.assertContains(response, "Warnings:</strong> none recorded")
         self.assertContains(response, "Stats")
         self.assertContains(response, "Files seen:</strong> 10")
         self.assertContains(response, "Estimated link-dest saving")
         self.assertContains(response, "&quot;ok&quot;: true")
         self.assertContains(response, reverse("snapshot_detail", args=[snapshot.id]))
 
+    def test_run_detail_surfaces_dry_run_warnings_and_log_link(self) -> None:
+        self.client.force_login(self.staff_user)
+        host = HostConfig.objects.create(host="web-01", address="web-01.example.test")
+        with TemporaryDirectory() as tmp:
+            log_path = Path(tmp) / "dry-run" / "rsync.log"
+            log_path.parent.mkdir(parents=True)
+            log_path.write_text("WARNING: noisy shell output\npermission denied\n", encoding="utf-8")
+            run = BackupRun.objects.create(
+                host=host,
+                status=BackupRun.Status.FAILED,
+                rsync_exit_code=255,
+                result={
+                    "ok": False,
+                    "dry_run": True,
+                    "log": str(log_path),
+                    "failure": {
+                        "category": "transport",
+                        "message": "Rsync transport failed.",
+                        "hint": "Check SSH access.",
+                    },
+                    "stats": {
+                        "duration_seconds": 4,
+                        "rsync": {
+                            "files_total": 25,
+                            "files_transferred": 3,
+                            "total_file_size_bytes": 10_000,
+                            "total_transferred_file_size_bytes": 1_500,
+                        },
+                    },
+                    "rsync": {
+                        "exit_code": 255,
+                        "log_tail": ["WARNING: noisy shell output", "permission denied"],
+                    },
+                },
+            )
+
+            response = self.client.get(reverse("run_detail", args=[run.id]))
+
+        self.assertEqual(response.status_code, 200)
+        self.assertContains(response, "Dry Run Summary")
+        self.assertContains(response, "failed")
+        self.assertContains(response, "Files Seen")
+        self.assertContains(response, "25")
+        self.assertContains(response, "Would Transfer")
+        self.assertContains(response, "3")
+        self.assertContains(response, "1.5")
+        self.assertContains(response, "Open full rsync log")
+        self.assertContains(response, reverse("run_rsync_log", args=[run.id]))
+        self.assertContains(response, "Rsync transport failed.")
+        self.assertContains(response, "Check SSH access.")
+        self.assertContains(response, "WARNING: noisy shell output")
+
     def test_run_detail_links_existing_rsync_log(self) -> None:
         self.client.force_login(self.staff_user)
         host = HostConfig.objects.create(host="web-01", address="web-01.example.test")

src/pobsync_backend/views.py

@@ -29,8 +29,9 @@ from .forms import (
     ScheduleConfigForm,
     SshCredentialForm,
 )
-from .host_ops import collect_host_checks, ensure_host_directories
+from .host_ops import ensure_host_directories
 from .models import BackupRun, GlobalConfig, HostConfig, ScheduleConfig, SnapshotRecord, SshCredential
+from .preflight import collect_backup_gate, effective_host_config_preview, run_remote_preflight
 from .retention import run_sql_retention_apply, run_sql_retention_plan
 from .self_check import collect_self_checks, summarize_self_checks
 from .scheduler import next_due_after
@@ -260,14 +261,15 @@ def create_host_config(request):
 @staff_member_required
 def host_detail(request, host: str):
     host_config = get_object_or_404(HostConfig, host=host)
+    global_config = GlobalConfig.objects.filter(name="default").first()
     schedule = _schedule_for_host(host_config)
     queued_runs = host_config.runs.filter(status=BackupRun.Status.QUEUED)
     running_runs = host_config.runs.filter(status=BackupRun.Status.RUNNING)
     active_run = host_config.runs.filter(
         status__in=[BackupRun.Status.QUEUED, BackupRun.Status.RUNNING]
     ).order_by("created_at", "id").first()
-    has_global_config = GlobalConfig.objects.filter(name="default").exists()
-    host_checks = collect_host_checks(host_config)
+    has_global_config = global_config is not None
+    backup_gate = collect_backup_gate(host_config, global_config)
     stats_summary = collect_host_stats(host=host_config, limit=10)
     context = {
         "host": host_config,
@@ -275,11 +277,15 @@ def host_detail(request, host: str):
         "next_run_at": _next_run_for_schedule(schedule, host_config),
         "scheduler_timezone": timezone.get_current_timezone_name(),
         "discovery": inspect_snapshot_discovery(host=host_config),
-        "host_checks": host_checks,
-        "host_check_summary": summarize_self_checks(host_checks),
+        "host_checks": backup_gate.checks,
+        "host_check_summary": summarize_self_checks(backup_gate.checks),
+        "backup_gate": backup_gate,
+        "last_preflight": (host_config.config or {}).get("last_preflight") if isinstance(host_config.config, dict) else {},
+        "effective_config": effective_host_config_preview(host_config, global_config) if global_config else {},
         "stats_summary": stats_summary,
         "manual_backup_form": ManualBackupForm(initial=_default_manual_backup_initial(host_config)),
-        "can_queue_backup": host_config.enabled and has_global_config,
+        "can_queue_dry_run": host_config.enabled and has_global_config and backup_gate.can_queue_dry_run and active_run is None,
+        "can_queue_real_backup": host_config.enabled and has_global_config and backup_gate.can_queue_real and active_run is None,
         "has_global_config": has_global_config,
         "active_run": active_run,
         "latest_runs": host_config.runs.select_related("snapshot").order_by("-created_at")[:10],
@@ -331,6 +337,34 @@ def scan_host_known_key(request, host: str):
     return redirect("host_detail", host=host_config.host)
 
 
+@staff_member_required
+@require_POST
+def run_host_preflight(request, host: str):
+    host_config = get_object_or_404(HostConfig, host=host)
+    if not host_config.enabled:
+        messages.error(request, f"Cannot run preflight for disabled host {host_config.host}.")
+        return redirect("host_detail", host=host_config.host)
+    if not GlobalConfig.objects.filter(name="default").exists():
+        messages.error(request, "Create the default global config before running preflight.")
+        return redirect("host_detail", host=host_config.host)
+
+    try:
+        result = run_remote_preflight(host_config)
+    except Exception as exc:
+        messages.error(request, f"Connection preflight failed for {host_config.host}: {exc}")
+    else:
+        if result.get("ok"):
+            messages.success(request, f"Connection preflight passed for {host_config.host}.")
+        else:
+            failed = [
+                str(check.get("name"))
+                for check in result.get("checks", [])
+                if isinstance(check, dict) and not check.get("ok")
+            ]
+            messages.error(request, f"Connection preflight failed for {host_config.host}: {', '.join(failed)}.")
+    return redirect("host_detail", host=host_config.host)
+
+
 @staff_member_required
 @require_POST
 def queue_manual_backup(request, host: str):
@@ -338,7 +372,8 @@ def queue_manual_backup(request, host: str):
     if not host_config.enabled:
         messages.error(request, f"Cannot queue backup for disabled host {host_config.host}.")
         return redirect("host_detail", host=host_config.host)
-    if not GlobalConfig.objects.filter(name="default").exists():
+    global_config = GlobalConfig.objects.filter(name="default").first()
+    if global_config is None:
         messages.error(request, "Create the default global config before queueing backups.")
         return redirect("host_detail", host=host_config.host)
 
@@ -347,6 +382,17 @@ def queue_manual_backup(request, host: str):
         messages.error(request, "Manual backup options are invalid.")
         return redirect("host_detail", host=host_config.host)
 
+    backup_gate = collect_backup_gate(host_config, global_config)
+    if form.cleaned_data["dry_run"]:
+        if not backup_gate.can_queue_dry_run:
+            blockers = ", ".join(check.name for check in backup_gate.dry_run_blockers)
+            messages.error(request, f"Cannot queue dry-run until failed preflight checks are resolved: {blockers}.")
+            return redirect("host_detail", host=host_config.host)
+    elif not backup_gate.can_queue_real:
+        blockers = ", ".join(check.name for check in backup_gate.real_blockers)
+        messages.error(request, f"Cannot queue real backup until failed preflight checks are resolved: {blockers}.")
+        return redirect("host_detail", host=host_config.host)
+
     run = queue_backup_run(
         host=host_config,
         dry_run=form.cleaned_data["dry_run"],
@@ -369,19 +415,29 @@ def run_detail(request, run_id: int):
     prune_result = result.get("prune") if isinstance(result.get("prune"), dict) else {}
     rsync_log_path = _run_rsync_log_path(run)
     rsync_log_tail = _run_rsync_log_tail(rsync_result, rsync_log_path)
+    requested = result.get("requested") if isinstance(result.get("requested"), dict) else {}
     context = {
         "run": run,
         "can_cancel": run.status in {BackupRun.Status.QUEUED, BackupRun.Status.RUNNING},
-        "requested": result.get("requested") if isinstance(result.get("requested"), dict) else {},
+        "requested": requested,
         "stats": run_stats if isinstance(run_stats, dict) else {},
         "rsync": rsync_result,
         "rsync_command": _run_rsync_command(rsync_result),
         "failure": failure,
+        "failure_summary": failure.get("message") or failure.get("summary") or "",
         "prune_result": prune_result,
         "has_prune_result": bool(prune_result),
         "rsync_log_path": str(rsync_log_path) if rsync_log_path is not None else "",
         "rsync_log_exists": bool(rsync_log_path and rsync_log_path.exists()),
         "rsync_log_tail": rsync_log_tail,
+        "dry_run_summary": _dry_run_summary(
+            result=result,
+            requested=requested,
+            stats=run_stats if isinstance(run_stats, dict) else {},
+            failure=failure,
+            rsync_log_tail=rsync_log_tail,
+            rsync_log_exists=bool(rsync_log_path and rsync_log_path.exists()),
+        ),
         "result_json": _pretty_json(run.result),
     }
     return render(request, "pobsync_backend/run_detail.html", context)
@@ -693,6 +749,48 @@ def _run_rsync_log_tail(rsync_result: dict, log_path: Path | None, *, max_lines:
         return []
 
 
+def _dry_run_summary(
+    *,
+    result: dict,
+    requested: dict,
+    stats: dict,
+    failure: dict,
+    rsync_log_tail: list[str],
+    rsync_log_exists: bool,
+) -> dict[str, object]:
+    if not (result.get("dry_run") or requested.get("dry_run")):
+        return {}
+
+    rsync_stats = stats.get("rsync") if isinstance(stats.get("rsync"), dict) else {}
+    warnings = []
+    if failure:
+        message = failure.get("message") or failure.get("summary")
+        hint = failure.get("hint")
+        if message:
+            warnings.append(str(message))
+        if hint:
+            warnings.append(str(hint))
+    for line in rsync_log_tail:
+        lowered = line.lower()
+        if "warning" in lowered or "permission denied" in lowered or "failed" in lowered:
+            warnings.append(line)
+
+    return {
+        "ok": result.get("ok"),
+        "status": "passed" if result.get("ok") else ("failed" if result.get("ok") is False else "running"),
+        "highlight_class": "success" if result.get("ok") else ("failed" if result.get("ok") is False else "warning"),
+        "files_seen": rsync_stats.get("files_total"),
+        "files_would_transfer": rsync_stats.get("files_transferred"),
+        "total_file_size_bytes": rsync_stats.get("total_file_size_bytes"),
+        "transfer_estimate_bytes": rsync_stats.get("total_transferred_file_size_bytes")
+        or rsync_stats.get("literal_data_bytes"),
+        "link_dest_estimated_savings_bytes": rsync_stats.get("link_dest_estimated_savings_bytes"),
+        "duration_seconds": stats.get("duration_seconds"),
+        "log_available": rsync_log_exists,
+        "warnings": list(dict.fromkeys(warnings)),
+    }
+
+
 def _log_context(request) -> dict[str, object]:
     units = ("pobsync-web.service", "pobsync-worker.service", "pobsync-scheduler.service")
     priorities = {

src/pobsync_server/urls.py

@@ -21,6 +21,7 @@ urlpatterns = [
     path("hosts/<str:host>/config/", views.edit_host_config, name="edit_host_config"),
     path("hosts/<str:host>/prepare-directories/", views.prepare_host_directories, name="prepare_host_directories"),
     path("hosts/<str:host>/scan-known-key/", views.scan_host_known_key, name="scan_host_known_key"),
+    path("hosts/<str:host>/preflight/", views.run_host_preflight, name="run_host_preflight"),
     path("hosts/<str:host>/queue-backup/", views.queue_manual_backup, name="queue_manual_backup"),
     path("hosts/<str:host>/discover-snapshots/", views.discover_host_snapshots, name="discover_host_snapshots"),
     path("hosts/<str:host>/retention-apply/", views.apply_host_retention, name="apply_host_retention"),