lib/zodb: Add zstor_2zurl - way to convert a ZODB storage into URL to access it

Wendelin.core 2 will need to spawn WCFS filesystem server that accesses
the same ZODB database as the program that spawns it. The database
argument passed to WCFS is passed in the form of URL[1,2].  Even though
zodburi provides way to convert an URL into ZODB storage instance, there
is currently no way for reverse operation - to convert ZODB storage
instance into URL to access it(*). So we have to build it by our own.

Provide zstor_2zurl stub that currently works for FileStorage only.
ZEO and NEO support is TODO.

In the future we might want to move this functionality into
zodbtools/py.

[1] https://lab.nexedi.com/nexedi/zodbtools/blob/a2e4dd23/zodbtools/help.py#L27-53
[2] https://lab.nexedi.com/kirr/neo/blob/3d909114/go/zodb/zodbtools/help.go#L25-51

(*) contrary to ZODB/go where this functionality is provided out of the box:
    https://godoc.org/lab.nexedi.com/kirr/neo/go/zodb#IStorage

lib/tests/test_zodb.py

@@ -17,8 +17,9 @@
 #
 # See COPYING file for full licensing terms.
 # See https://www.nexedi.com/licensing for rationale and options.
-from wendelin.lib.zodb import LivePersistent, deactivate_btree, dbclose, zconn_at, zmajor
+from wendelin.lib.zodb import LivePersistent, deactivate_btree, dbclose, zconn_at, zstor_2zurl, zmajor
 from wendelin.lib.testing import getTestDB
+from wendelin.lib import testing
 from persistent import Persistent, UPTODATE, GHOST, CHANGED
 from ZODB import DB, POSException
 from BTrees.IOBTree import IOBTree
@@ -351,6 +352,21 @@ def test_zodb_onresync():
     conn.close()
 
 
+# test that zurl does not change from one open to another storage open.
+def test_zurlstable():
+    if not isinstance(testdb, testing.TestDB_FileStorage):
+        pytest.xfail(reason="zstor_2zurl is TODO for ZEO and NEO")
+    zurl0 = None
+    for i in range(10):
+        zstor = testdb.getZODBStorage()
+        zurl  = zstor_2zurl(zstor)
+        zstor.close()
+        if i == 0:
+            zurl0 = zurl
+        else:
+            assert zurl == zurl0
+
+
 # ---- misc ----
 
 # zsync syncs ZODB storage.

lib/zodb.py

@@ -299,3 +299,20 @@ elif zmajor == 4:
 # ZODB3: TODO
 else:
     pass    # raises in onResyncCallback
+
+
+
+# zstor_2zurl converts a ZODB storage to URL to access it.
+def zstor_2zurl(zstor):
+    # There is, sadly, no unified way to do it, as even if storages are created via
+    # zodburi, after creation its uri is lost. And storages could be created not
+    # only through URI but e.g. via ZConfig and manually. We want to support all
+    # those cases...
+    #
+    # For this reason extract URL with important for wcfs use-case parameters in
+    # ad-hoc way.
+    if isinstance(zstor, FileStorage):
+        return "file://%s" % (zstor._file_name,)
+
+    # TODO ZEO + NEO support
+    raise NotImplementedError("don't know how to extract zurl from %r" % zstor)