Per-program Flet client app IDs so the dock shows the right icons

Programs/FletIdentity.py

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+"""Give a Flet program's window its own app ID on Linux, so the desktop can
+match it to the program's .desktop entry (icon, name) instead of showing
+every Flet program as "com.appveyor.flet" with a generic icon.
+
+The Flet client's ID is a string built into its small launcher binary
+(`flet`, ~24 KB); the client sets it as both its GTK application ID and its
+process name, so Flet's own FLET_APP_ID (which only changes argv[0]) can't
+override it, on Wayland or X11. Instead this makes a per-program copy of the
+client in ~/.cache/SaraSuite/FletClient/ with that string replaced; the big
+parts (lib/, data/, share/) are symlinks to the original, so a copy costs
+~24 KB. Pointing FLET_VIEW_PATH at it makes Flet use the copy.
+
+Used two ways:
+- SaraSuite imports it and calls ClientFor() before ft.run.
+- Installed programs' launchers run it with their own venv's python:
+      export FLET_VIEW_PATH="$(python FletIdentity.py srhtnin.SaraFem)"
+  It prints the copy's folder, or nothing if anything goes wrong, in which
+  case Flet just uses its normal client.
+"""
+
+import sys
+from pathlib import Path
+
+OriginalId = b"com.appveyor.flet"
+CopiesDir = Path.home() / ".cache" / "SaraSuite" / "FletClient"
+
+
+def ClientFor(AppId: str) -> str:
+    """Folder of a client copy whose app ID is AppId ("" if not possible).
+    AppId needs a dot (a GTK rule) and at most 17 characters, the length of
+    the original it replaces in place."""
+    try:
+        Encoded = AppId.encode("ascii")
+        if "." not in AppId or not 0 < len(Encoded) <= len(OriginalId):
+            return ""
+        from flet_desktop import ensure_client_cached  # the program's own Flet version
+
+        Original = Path(ensure_client_cached()) / "flet"
+        Binary = (Original / "flet").read_bytes()
+        Copy = CopiesDir / f"{Original.parent.name}-{AppId}" / "flet"
+        Patched = Binary.replace(
+            b"\x00" + OriginalId + b"\x00",
+            b"\x00" + Encoded.ljust(len(OriginalId), b"\x00") + b"\x00",
+        )
+        if Binary.count(b"\x00" + OriginalId + b"\x00") != 1:
+            return ""  # not the client layout this was written for
+
+        Target = Copy / "flet"
+        if not (Target.exists() and Target.read_bytes() == Patched):
+            Copy.mkdir(parents=True, exist_ok=True)
+            for Part in ("lib", "data", "share"):
+                Link = Copy / Part
+                if Link.is_symlink() or Link.exists():
+                    Link.unlink()
+                if (Original / Part).exists():
+                    Link.symlink_to(Original / Part)
+            Temporary = Target.with_suffix(".new")
+            Temporary.write_bytes(Patched)
+            Temporary.chmod(0o755)
+            Temporary.replace(Target)
+        return str(Copy)
+    except Exception:
+        return ""
+
+
+if __name__ == "__main__":
+    print(ClientFor(sys.argv[1]) if len(sys.argv) > 1 else "")