diff --git a/mercurial/filelog.py b/mercurial/filelog.py
--- a/mercurial/filelog.py
+++ b/mercurial/filelog.py
@@ -49,7 +49,8 @@
         return self._revlog.node(rev)
 
     def lookup(self, node):
-        return self._revlog.lookup(node)
+        return storageutil.fileidlookup(self._revlog, node,
+                                        self._revlog.indexfile)
 
     def linkrev(self, rev):
         return self._revlog.linkrev(rev)
diff --git a/mercurial/repository.py b/mercurial/repository.py
--- a/mercurial/repository.py
+++ b/mercurial/repository.py
@@ -1099,9 +1099,6 @@
         that can be converted to an integer.
 
         Raises ``error.LookupError`` if a ndoe could not be resolved.
-
-        TODO this is only used by debug* commands and can probably be deleted
-        easily.
         """
 
     def parents(node):
diff --git a/mercurial/testing/storage.py b/mercurial/testing/storage.py
--- a/mercurial/testing/storage.py
+++ b/mercurial/testing/storage.py
@@ -85,21 +85,19 @@
         self.assertEqual(f.lookup(nullid), nullid)
         self.assertEqual(f.lookup(nullrev), nullid)
         self.assertEqual(f.lookup(hex(nullid)), nullid)
-
-        # String converted to integer doesn't work for nullrev.
-        with self.assertRaises(error.LookupError):
-            f.lookup(b'%d' % nullrev)
+        self.assertEqual(f.lookup(b'%d' % nullrev), nullid)
 
         with self.assertRaises(error.LookupError):
             f.lookup(b'badvalue')
 
-        self.assertEqual(f.lookup(hex(nullid)[0:12]), nullid)
+        with self.assertRaises(error.LookupError):
+            f.lookup(hex(nullid)[0:12])
 
         with self.assertRaises(error.LookupError):
             f.lookup(b'-2')
 
-        # TODO this is wonky.
-        self.assertEqual(f.lookup(b'0'), nullid)
+        with self.assertRaises(error.LookupError):
+            f.lookup(b'0')
 
         with self.assertRaises(error.LookupError):
             f.lookup(b'1')
@@ -197,7 +195,9 @@
         self.assertEqual(f.lookup(-1), nullid)
         self.assertEqual(f.lookup(b'0'), node)
         self.assertEqual(f.lookup(hex(node)), node)
-        self.assertEqual(f.lookup(hex(node)[0:12]), node)
+
+        with self.assertRaises(error.LookupError):
+            f.lookup(hex(node)[0:12])
 
         with self.assertRaises(IndexError):
             f.lookup(-2)
diff --git a/mercurial/utils/storageutil.py b/mercurial/utils/storageutil.py
--- a/mercurial/utils/storageutil.py
+++ b/mercurial/utils/storageutil.py
@@ -10,10 +10,13 @@
 import hashlib
 import re
 
+from ..i18n import _
 from ..node import (
+    bin,
     nullid,
 )
 from .. import (
+    error,
     pycompat,
 )
 
@@ -99,3 +102,53 @@
         stop = storelen
 
     return pycompat.xrange(start, stop, step)
+
+def fileidlookup(store, fileid, identifier):
+    """Resolve the file node for a value.
+
+    ``store`` is an object implementing the ``ifileindex`` interface.
+
+    ``fileid`` can be:
+
+    * A 20 byte binary node.
+    * An integer revision number
+    * A 40 byte hex node.
+    * A bytes that can be parsed as an integer representing a revision number.
+
+    ``identifier`` is used to populate ``error.LookupError`` with an identifier
+    for the store.
+
+    Raises ``error.LookupError`` on failure.
+    """
+    if isinstance(fileid, int):
+        return store.node(fileid)
+
+    if len(fileid) == 20:
+        try:
+            store.rev(fileid)
+            return fileid
+        except error.LookupError:
+            pass
+
+    if len(fileid) == 40:
+        try:
+            rawnode = bin(fileid)
+            store.rev(rawnode)
+            return rawnode
+        except TypeError:
+            pass
+
+    try:
+        rev = int(fileid)
+
+        if b'%d' % rev != fileid:
+            raise ValueError
+
+        try:
+            return store.node(rev)
+        except (IndexError, TypeError):
+            pass
+    except (ValueError, OverflowError):
+        pass
+
+    raise error.LookupError(fileid, identifier, _('no match found'))