diff --git a/mercurial/utils/cborutil.py b/mercurial/utils/cborutil.py
new file mode 100644
--- /dev/null
+++ b/mercurial/utils/cborutil.py
@@ -0,0 +1,78 @@
+# cborutil.py - CBOR extensions
+#
+# Copyright 2018 Gregory Szorc <gregory.szorc@gmail.com>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+from __future__ import absolute_import
+
+import contextlib
+import struct
+
+def beginindefinitearray(encoder):
+    # major type = 4, information value 31 to specify indefinite length.
+    encoder.write(struct.pack(r'>B', 4 << 5 | 31))
+
+def beginindefinitemap(encoder):
+    # major type = 5, information value 31 to specify indefinite length.
+    encoder.write(struct.pack(r'>B', 5 << 5 | 31))
+
+def endindefinite(encoder):
+    encoder.write(b'\xff')
+
+@contextlib.contextmanager
+def streamarray(encoder):
+    """Write an array in a streaming manner.
+
+    Used as a context manager, the context manager resolves to a function
+    that should be called for each item to write to the array.
+
+    When the context manager exits, the indefinite length array is ended.
+    """
+    def writeitem(value):
+        encoder.encode(value)
+
+    beginindefinitearray(encoder)
+    yield writeitem
+    endindefinite(encoder)
+
+def streamarrayitems(encoder, items):
+    """Write out an iterable of items to a streaming array."""
+    with streamarray(encoder) as fn:
+        for value in items:
+            fn(value)
+
+@contextlib.contextmanager
+def streammap(encoder):
+    """Write a map in a streaming manner.
+
+    Used as a context manager, the context manager resolves to a function
+    that should be called with a key and value of each map item to write.
+
+    When the context manager exits, the indefinite length map is ended.
+
+    If is possible to nest streaming data structures. If the caller writes
+    out 2 values, the first value will be interpreted as a key and the second
+    a value. So a caller could do something like::
+
+        with streammap(encoder):
+            encoder.encode(b'mykey')
+            with streammap(encoder) as fn:
+                fn(b'innerkey', b'value')
+
+    This would decode to ``{b'mykey': {b'innerkey': b'value'}}``.
+    """
+    def writeitem(key, value):
+        encoder.encode(key)
+        encoder.encode(value)
+
+    beginindefinitemap(encoder)
+    yield writeitem
+    endindefinite(encoder)
+
+def streammapitems(encoder, items):
+    """Write out an iterable of (key, value) items to a streaming map."""
+    with streammap(encoder) as fn:
+        for key, value in items:
+            fn(key, value)
diff --git a/tests/test-cbor.py b/tests/test-cbor.py
new file mode 100644
--- /dev/null
+++ b/tests/test-cbor.py
@@ -0,0 +1,176 @@
+from __future__ import absolute_import
+
+import io
+import unittest
+
+from mercurial.thirdparty import (
+    cbor,
+)
+from mercurial.utils import (
+    cborutil,
+)
+
+class StreamArrayTests(unittest.TestCase):
+    def testempty(self):
+        b = io.BytesIO()
+        encoder = cbor.CBOREncoder(b)
+
+        with cborutil.streamarray(encoder):
+            pass
+
+        self.assertEqual(b.getvalue(), '\x9f\xff')
+        self.assertEqual(cbor.loads(b.getvalue()), [])
+
+    def testone(self):
+        b = io.BytesIO()
+        encoder = cbor.CBOREncoder(b)
+
+        with cborutil.streamarray(encoder) as fn:
+            fn(b'foo')
+
+        self.assertEqual(cbor.loads(b.getvalue()), [b'foo'])
+
+    def testmultiple(self):
+        b = io.BytesIO()
+        encoder = cbor.CBOREncoder(b)
+
+        with cborutil.streamarray(encoder) as fn:
+            fn(0)
+            fn(True)
+            fn(b'foo')
+            fn(None)
+
+        self.assertEqual(cbor.loads(b.getvalue()), [0, True, b'foo', None])
+
+    def testnested(self):
+        b = io.BytesIO()
+        encoder = cbor.CBOREncoder(b)
+
+        with cborutil.streamarray(encoder):
+            with cborutil.streamarray(encoder) as fn:
+                fn(b'foo')
+                fn(b'bar')
+
+        self.assertEqual(cbor.loads(b.getvalue()), [[b'foo', b'bar']])
+
+    def testitemslist(self):
+        b = io.BytesIO()
+        encoder = cbor.CBOREncoder(b)
+
+        orig = [b'foo', b'bar', None, True, 42]
+
+        cborutil.streamarrayitems(encoder, orig)
+        self.assertEqual(cbor.loads(b.getvalue()), orig)
+
+    def testitemsgen(self):
+        def makeitems():
+            yield b'foo'
+            yield b'bar'
+            yield None
+            yield 42
+
+        b = io.BytesIO()
+        encoder = cbor.CBOREncoder(b)
+
+        cborutil.streamarrayitems(encoder, makeitems())
+        self.assertEqual(cbor.loads(b.getvalue()), [b'foo', b'bar', None, 42])
+
+class StreamMapTests(unittest.TestCase):
+    def testempty(self):
+        b = io.BytesIO()
+        encoder = cbor.CBOREncoder(b)
+
+        with cborutil.streammap(encoder):
+            pass
+
+        self.assertEqual(b.getvalue(), '\xbf\xff')
+        self.assertEqual(cbor.loads(b.getvalue()), {})
+
+    def testone(self):
+        b = io.BytesIO()
+        encoder = cbor.CBOREncoder(b)
+
+        with cborutil.streammap(encoder) as fn:
+            fn(b'key1', b'value1')
+
+        self.assertEqual(cbor.loads(b.getvalue()), {b'key1': b'value1'})
+
+    def testmultiple(self):
+        b = io.BytesIO()
+        encoder = cbor.CBOREncoder(b)
+
+        with cborutil.streammap(encoder) as fn:
+            fn(0, 1)
+            fn(b'key1', b'value1')
+            fn(True, None)
+
+        self.assertEqual(cbor.loads(b.getvalue()), {
+            0: 1,
+            b'key1': b'value1',
+            True: None,
+        })
+
+    def testcomplex(self):
+        b = io.BytesIO()
+        encoder = cbor.CBOREncoder(b)
+
+        with cborutil.streammap(encoder) as fn:
+            fn(b'key1', b'value1')
+            fn(b'map', {b'inner1key': b'inner1value'})
+            fn(b'array', [0, 1, 2])
+
+        self.assertEqual(cbor.loads(b.getvalue()), {
+            b'key1': b'value1',
+            b'map': {b'inner1key': b'inner1value'},
+            b'array': [0, 1, 2],
+        })
+
+    def testnested(self):
+        b = io.BytesIO()
+        encoder = cbor.CBOREncoder(b)
+
+        with cborutil.streammap(encoder):
+            encoder.encode(b'streamkey')
+            with cborutil.streammap(encoder) as fn2:
+                fn2(b'inner1key', b'inner1value')
+                fn2(0, 1)
+
+        self.assertEqual(cbor.loads(b.getvalue()), {
+            b'streamkey': {
+                b'inner1key': b'inner1value',
+                0: 1,
+            },
+        })
+
+    def testitemsdict(self):
+        b = io.BytesIO()
+        encoder = cbor.CBOREncoder(b)
+
+        orig = [
+            (b'foo', b'bar'),
+            (42, 19),
+            (None, True),
+        ]
+
+        cborutil.streammapitems(encoder, orig)
+        self.assertEqual(cbor.loads(b.getvalue()), dict(orig))
+
+    def testitemsgen(self):
+        def makeitems():
+            yield b'foo', b'bar'
+            yield None, True
+            yield 42, 19
+
+        b = io.BytesIO()
+        encoder = cbor.CBOREncoder(b)
+
+        cborutil.streammapitems(encoder, makeitems())
+        self.assertEqual(cbor.loads(b.getvalue()), {
+            b'foo': b'bar',
+            None: True,
+            42: 19,
+        })
+
+if __name__ == '__main__':
+    import silenttestrunner
+    silenttestrunner.main(__name__)