Offer a writable database location instead of failing the open

`ida-tui /bin/ls` asked IDA to create /bin/ls.i64, which a normal user
cannot write, and what came back was the backend's account of it:

    idalib worker launcher 2425195 exited with status 1
    [ida-nexus] Failed to open database /usr/bin/ls

after a full analysis wait, naming neither the file it could not create
nor the reason. Nothing in the message points at permissions, and there
was no way forward from it.

idatui/idbpath.py answers the question before anything is opened: where
would the database go (mirroring ida_nexus's own derivation), can it go
there (os.access only — the pre-flight check must not create files), and
where instead. The proposal is stable per binary
(~/.local/share/idatui/db/<name>-<sha1(realpath)[:8]>.i64) so reopening
the same binary resumes the same names and comments, and hashed so two
busyboxes from different directories cannot collide. DATA_HOME rather
than CACHE_HOME on purpose: an .i64 holds the whole session, and cache
directories are something cleaners may empty.

The answer becomes NexusClient(output_database=...) — the option project
mode already uses to keep everything in its sidecar. `--idb PATH` (also
forwarded by `pane spawn`) says where without being asked. An RPC-driven
TUI is never asked: `pane spawn` polls for ready and would sit out its
whole timeout in front of a modal nobody can answer, so it relocates and
says so in the status line. A failed open re-offers relocation after a
real create probe, since os.access does not know about ACLs, a lying
network mount or a full disk.

tests/test_idbpath.py (pure, 32 checks) pins the path arithmetic, the two
properties of the proposal and that neither check touches the user's
directory; tests/test_idb_relocate_ui.py (textual, no worker, 24 checks)
drives the dialog, Esc, an edited path, the untouched common case, --idb,
the RPC path, and that output_database actually reaches the client.
Verified end to end against a real /bin/ls: 400 functions, and a rename
saved in one session comes back in the next.

1 files changed, 28 insertions, 0 deletions

diff --git a/idatui/launch.py b/idatui/launch.py
index 6e3b432..01778f3 100644
--- a/idatui/launch.py
+++ b/idatui/launch.py
@@ -103,6 +103,12 @@ def main(argv: list[str] | None = None) -> int:
         metavar="FILE",
         help="Tenet execution trace to explore alongside the binary",
     )
+    p.add_argument(
+        "--idb",
+        metavar="FILE",
+        help="keep the database here instead of beside the binary (an .i64 path; "
+        "the default for a binary in a directory you cannot write)",
+    )
     g = p.add_argument_group(
         "loading a headerless blob",
         "An ELF/PE/Mach-O says what it is. A raw firmware dump doesn't, and IDA "
@@ -145,6 +151,27 @@ def main(argv: list[str] | None = None) -> int:
     if args.ida_args:
         load["ida_args"] = args.ida_args
 
+    # Where the database goes, for a binary whose own directory cannot hold one
+    # (``/bin/ls`` -> ``/bin/ls.i64``). Validated HERE, before anything is staged
+    # or imported, so a contradictory invocation costs nothing.
+    idb_path = None
+    if args.idb:
+        from . import idbpath
+
+        if args.project:
+            # A project already keeps every database in its sidecar, so the two
+            # options mean contradictory things. Refusing beats ignoring one.
+            _log(
+                "--idb is for a single binary; a project keeps its databases in "
+                "its sidecar"
+            )
+            return 2
+        try:
+            idb_path = idbpath.ensure_parent(args.idb)
+        except OSError as e:
+            _log(f"cannot create the directory for --idb {args.idb}: {e}")
+            return 2
+
     project = None
     binary = None
     if args.project:
@@ -249,6 +276,7 @@ def main(argv: list[str] | None = None) -> int:
         trace_path=(
             os.path.abspath(os.path.expanduser(args.trace)) if args.trace else ""
         ),
+        idb_path=idb_path,
     ).run()
     return 0