Fix dry-run iSCSI ID map cascades; add pre-migration existence check

During dry run, "would create" iSCSI objects now populate id_map with
a source-ID placeholder so downstream objects (targets, target-extents)
can remap references without cascading failures.

Adds query_existing_iscsi() and clear_iscsi_config() to migrate.py, and
_prompt_clear_existing_iscsi() to the wizard: if the destination already
has iSCSI config, the user is shown a summary and offered Keep/Remove
before the dry run begins.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

truenas_migrate/cli.py

@@ -57,7 +57,7 @@ from .archive import parse_archive, list_archive_and_exit
 from .client import TrueNASClient, check_dataset_paths, create_missing_datasets, check_iscsi_zvols, create_missing_zvols
 from .colors import log, _bold, _bold_cyan, _bold_red, _bold_yellow, _cyan, _dim, _green, _yellow
 from .csv_source import parse_csv_sources
-from .migrate import migrate_smb_shares, migrate_nfs_shares, migrate_iscsi
+from .migrate import migrate_smb_shares, migrate_nfs_shares, migrate_iscsi, query_existing_iscsi, clear_iscsi_config
 from .summary import Summary
 
 
@@ -214,6 +214,53 @@ def _prompt_iscsi_portals(iscsi: dict) -> None:
     print()
 
 
+def _prompt_clear_existing_iscsi(host: str, port: int, api_key: str) -> None:
+    """
+    Check whether the destination already has iSCSI configuration.
+    If so, summarise what exists and offer to remove it before migration.
+    """
+    async def _check():
+        async with TrueNASClient(host=host, port=port, api_key=api_key, verify_ssl=False) as client:
+            return await query_existing_iscsi(client)
+
+    existing = asyncio.run(_check())
+    counts = {k: len(v) for k, v in existing.items()}
+    total = sum(counts.values())
+    if total == 0:
+        return
+
+    print(f"\n  {_bold_yellow('WARNING:')} Destination already has iSCSI configuration:")
+    labels = [
+        ("extents",       "extent(s)"),
+        ("initiators",    "initiator group(s)"),
+        ("portals",       "portal(s)"),
+        ("targets",       "target(s)"),
+        ("targetextents", "target-extent association(s)"),
+    ]
+    for key, label in labels:
+        n = counts[key]
+        if n:
+            print(f"    • {n} {label}")
+    print()
+    print(f"  {_dim('Keep existing:  new objects will be skipped if conflicts are detected.')}")
+    print(f"  {_dim('Remove existing: ALL iSCSI config will be deleted before migration.')}")
+    print()
+
+    raw = _prompt("  [K]eep existing  /  [R]emove all existing iSCSI config", default="K")
+    if raw.strip().lower().startswith("r"):
+        if _confirm(f"  Remove ALL {total} iSCSI object(s) from {host}?"):
+            async def _clear():
+                async with TrueNASClient(host=host, port=port, api_key=api_key, verify_ssl=False) as client:
+                    await clear_iscsi_config(client)
+            print()
+            asyncio.run(_clear())
+            print(f"  {_bold_cyan('✓')} iSCSI configuration cleared.\n")
+        else:
+            print(f"  {_yellow('–')} Removal cancelled — keeping existing config.\n")
+    else:
+        print(f"  {_dim('Keeping existing iSCSI configuration.')}\n")
+
+
 def _select_shares(shares: list[dict], share_type: str) -> list[dict]:
     """
     Display a numbered list of *shares* and return only those the user selects.
@@ -377,6 +424,10 @@ def interactive_mode() -> None:
         if "iscsi" in migrate and archive_data.get("iscsi", {}).get("portals"):
             _prompt_iscsi_portals(archive_data["iscsi"])
 
+        # ── iSCSI pre-migration check ────────────────────────────────────────
+        if "iscsi" in migrate:
+            _prompt_clear_existing_iscsi(host, port, api_key)
+
     # ── Select individual shares (common) ──────────────────────────────────────
     if "smb" in migrate and archive_data["smb_shares"]:
         archive_data["smb_shares"] = _select_shares(archive_data["smb_shares"], "SMB")