Try to rewrite test_get_put.py::test_large_file into system-style test.

src/allmydata/test/test_system.py

@@ -15,11 +15,15 @@ from past.builtins import chr as byteschr, long
 from six import ensure_text
 
 import os, re, sys, time, json
+from subprocess import check_call
+from pathlib import Path
+from tempfile import mkdtemp
 
 from bs4 import BeautifulSoup
 
 from twisted.trial import unittest
 from twisted.internet import defer
+from twisted.internet.threads import deferToThread
 
 from allmydata import uri
 from allmydata.storage.mutable import MutableShareFile
@@ -1830,6 +1834,34 @@ class SystemTest(SystemTestMixin, RunBinTahoeMixin, unittest.TestCase):
         d.addCallback(_got_lit_filenode)
         return d
 
+    async def test_immutable_upload_download(self):
+        """
+        A reproducer for issue 3988: upload a large file and then download it.
+        """
+        DATA = b"abc123 this is not utf-8 decodable \xff\x00\x33 \x11" * 1_000_000
+        await self.set_up_nodes()
+
+        async def run(*args):
+            await deferToThread(check_call, ["tahoe", "--node-directory", self.getdir("client0")] + list(args))
+
+        for c in self.clients:
+            c.encoding_params['k'] = 2
+            c.encoding_params['happy'] = 3
+            c.encoding_params['n'] = 4
+
+        await run("create-alias", "getput")
+
+        tmp_path = Path(mkdtemp())
+        tempfile = tmp_path / "input"
+
+        with tempfile.open("wb") as f:
+            f.write(DATA)
+        await run("put", str(tempfile), "getput:largefile")
+
+        outfile = tmp_path / "out"
+        await run("get", "getput:largefile", str(outfile))
+        self.assertEqual(outfile.read_bytes(), tempfile.read_bytes())
+
 
 class Connections(SystemTestMixin, unittest.TestCase):
     FORCE_FOOLSCAP_FOR_STORAGE = True