diff --git a/mercurial/formatter.py b/mercurial/formatter.py
--- a/mercurial/formatter.py
+++ b/mercurial/formatter.py
@@ -130,6 +130,7 @@ from . import (
     util,
 )
 from .utils import (
+    cborutil,
     dateutil,
     stringutil,
 )
@@ -341,6 +342,18 @@ class pickleformatter(baseformatter):
         baseformatter.end(self)
         self._out.write(pickle.dumps(self._data))
 
+class cborformatter(baseformatter):
+    '''serialize items as an indefinite-length CBOR array'''
+    def __init__(self, ui, out, topic, opts):
+        baseformatter.__init__(self, ui, topic, opts, _nullconverter)
+        self._out = out
+        self._out.write(cborutil.BEGIN_INDEFINITE_ARRAY)
+    def _showitem(self):
+        self._out.write(b''.join(cborutil.streamencode(self._item)))
+    def end(self):
+        baseformatter.end(self)
+        self._out.write(cborutil.BREAK)
+
 class jsonformatter(baseformatter):
     def __init__(self, ui, out, topic, opts):
         baseformatter.__init__(self, ui, topic, opts, _nullconverter)
@@ -617,7 +630,9 @@ class templateresources(templater.resour
 
 def formatter(ui, out, topic, opts):
     template = opts.get("template", "")
-    if template == "json":
+    if template == "cbor":
+        return cborformatter(ui, out, topic, opts)
+    elif template == "json":
         return jsonformatter(ui, out, topic, opts)
     elif template == "pickle":
         return pickleformatter(ui, out, topic, opts)
diff --git a/mercurial/help/scripting.txt b/mercurial/help/scripting.txt
--- a/mercurial/help/scripting.txt
+++ b/mercurial/help/scripting.txt
@@ -142,9 +142,11 @@ output containing authors, dates, descri
    using templates to make your life easier.
 
 The ``-T/--template`` argument allows specifying pre-defined styles.
-Mercurial ships with the machine-readable styles ``json`` and ``xml``,
-which provide JSON and XML output, respectively. These are useful for
-producing output that is machine readable as-is.
+Mercurial ships with the machine-readable styles ``cbor``, ``json``,
+and ``xml``, which provide CBOR, JSON, and XML output, respectively.
+These are useful for producing output that is machine readable as-is.
+
+(Mercurial 5.0 is required for CBOR style.)
 
 .. important::
 
diff --git a/mercurial/logcmdutil.py b/mercurial/logcmdutil.py
--- a/mercurial/logcmdutil.py
+++ b/mercurial/logcmdutil.py
@@ -542,7 +542,7 @@ def changesetdisplayer(ui, repo, opts, d
     regular display via changesetprinter() is done.
     """
     postargs = (differ, opts, buffered)
-    if opts.get('template') == 'json':
+    if opts.get('template') in {'cbor', 'json'}:
         fm = ui.formatter('log', opts)
         return changesetformatter(ui, repo, fm, *postargs)
 
diff --git a/tests/test-template-map.t b/tests/test-template-map.t
--- a/tests/test-template-map.t
+++ b/tests/test-template-map.t
@@ -669,6 +669,70 @@ Test xml styles:
   </log>
 
 
+test CBOR style:
+
+  $ cat <<'EOF' > "$TESTTMP/decodecborarray.py"
+  > from __future__ import absolute_import
+  > from mercurial import pycompat
+  > from mercurial.utils import (
+  >     cborutil,
+  >     stringutil,
+  > )
+  > data = pycompat.stdin.read()
+  > # our CBOR decoder doesn't support parsing indefinite-length arrays,
+  > # but the log output is indefinite stream by nature.
+  > assert data[:1] == cborutil.BEGIN_INDEFINITE_ARRAY
+  > assert data[-1:] == cborutil.BREAK
+  > items = cborutil.decodeall(data[1:-1])
+  > pycompat.stdout.write(stringutil.pprint(items, indent=1) + b'\n')
+  > EOF
+
+  $ hg log -k nosuch -Tcbor | "$PYTHON" "$TESTTMP/decodecborarray.py"
+  []
+
+  $ hg log -qr0:1 -Tcbor | "$PYTHON" "$TESTTMP/decodecborarray.py"
+  [
+   {
+    'node': '1e4e1b8f71e05681d422154f5421e385fec3454f',
+    'rev': 0
+   },
+   {
+    'node': 'b608e9d1a3f0273ccf70fb85fd6866b3482bf965',
+    'rev': 1
+   }
+  ]
+
+  $ hg log -vpr . -Tcbor --stat | "$PYTHON" "$TESTTMP/decodecborarray.py"
+  [
+   {
+    'bookmarks': [],
+    'branch': 'default',
+    'date': [
+     1577872860,
+     0
+    ],
+    'desc': 'third',
+    'diff': 'diff -r 29114dbae42b -r 95c24699272e fourth\n--- /dev/null\tThu Jan 01 00:00:00 1970 +0000\n+++ b/fourth\tWed Jan 01 10:01:00 2020 +0000\n@@ -0,0 +1,1 @@\n+second\ndiff -r 29114dbae42b -r 95c24699272e second\n--- a/second\tMon Jan 12 13:46:40 1970 +0000\n+++ /dev/null\tThu Jan 01 00:00:00 1970 +0000\n@@ -1,1 +0,0 @@\n-second\ndiff -r 29114dbae42b -r 95c24699272e third\n--- /dev/null\tThu Jan 01 00:00:00 1970 +0000\n+++ b/third\tWed Jan 01 10:01:00 2020 +0000\n@@ -0,0 +1,1 @@\n+third\n',
+    'diffstat': ' fourth |  1 +\n second |  1 -\n third  |  1 +\n 3 files changed, 2 insertions(+), 1 deletions(-)\n',
+    'files': [
+     'fourth',
+     'second',
+     'third'
+    ],
+    'node': '95c24699272ef57d062b8bccc32c878bf841784a',
+    'parents': [
+     '29114dbae42b9f078cf2714dbe3a86bba8ec7453'
+    ],
+    'phase': 'draft',
+    'rev': 8,
+    'tags': [
+     'tip'
+    ],
+    'user': 'test'
+   }
+  ]
+
+
 Test JSON style:
 
   $ hg log -k nosuch -Tjson