Conflicting writes.

src/allmydata/storage/http_server.py

@@ -31,7 +31,7 @@ from cbor2 import dumps, loads
 from .server import StorageServer
 from .http_common import swissnum_auth_header, Secrets
 from .common import si_a2b
-from .immutable import BucketWriter
+from .immutable import BucketWriter, ConflictingWriteError
 from ..util.hashutil import timing_safe_compare
 from ..util.base32 import rfc3548_alphabet
 
@@ -253,9 +253,11 @@ class HTTPServer(object):
             request.setResponseCode(http.NOT_FOUND)
             return b""
 
-        finished = bucket.write(offset, data)
-
-        # TODO if raises ConflictingWriteError, return HTTP CONFLICT code.
+        try:
+            finished = bucket.write(offset, data)
+        except ConflictingWriteError:
+            request.setResponseCode(http.CONFLICT)
+            return b""
 
         if finished:
             bucket.close()

src/allmydata/test/test_storage_http.py

@@ -626,9 +626,32 @@ class ImmutableHTTPAPITests(SyncTestCase):
         """
         If an uploaded chunk conflicts with an already uploaded chunk, a
         CONFLICT error is returned.
-
-        TBD in https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3860
         """
+        (upload_secret, _, storage_index, created) = self.create_upload({1}, 100)
+
+        # Write:
+        result_of(
+            self.im_client.write_share_chunk(
+                storage_index,
+                1,
+                upload_secret,
+                0,
+                b"0" * 10,
+            )
+        )
+
+        # Conflicting write:
+        with self.assertRaises(ClientException) as e:
+            result_of(
+                self.im_client.write_share_chunk(
+                    storage_index,
+                    1,
+                    upload_secret,
+                    0,
+                    b"0123456789",
+                )
+            )
+            self.assertEqual(e.exception.code, http.NOT_FOUND)
 
     def test_read_of_wrong_storage_index_fails(self):
         """