diff --git a/mercurial/scmutil.py b/mercurial/scmutil.py
--- a/mercurial/scmutil.py
+++ b/mercurial/scmutil.py
@@ -965,3 +965,41 @@ def gddeltaconfig(ui):
     """
     # experimental config: format.generaldelta
     return ui.configbool('format', 'generaldelta', False)
+
+class simplekeyvaluefile(object):
+    """A simple file with key=value lines
+
+    Keys must be alphanumerics and start with a letter, values must not
+    contain '\n' characters"""
+
+    def __init__(self, vfs, path, keys=None):
+        self.vfs = vfs
+        self.path = path
+
+    def read(self):
+        lines = self.vfs.readlines(self.path)
+        try:
+            d = dict(line[:-1].split('=', 1) for line in lines if line)
+        except ValueError as e:
+            raise error.CorruptedState(str(e))
+        return d
+
+    def write(self, data):
+        """Write key=>value mapping to a file
+        data is a dict. Keys must be alphanumerical and start with a letter.
+        Values must not contain newline characters."""
+        lines = []
+        for k, v in data.items():
+            if not k[0].isalpha():
+                e = "keys must start with a letter in a key-value file"
+                raise error.ProgrammingError(e)
+            if not k.isalnum():
+                e = "invalid key name in a simple key-value file"
+                raise error.ProgrammingError(e)
+            if '\n' in v:
+                e = "invalid value in a simple key-value file"
+                raise error.ProgrammingError(e)
+            lines.append("%s=%s\n" % (k, v))
+        with self.vfs(self.path, mode='wb', atomictemp=True) as fp:
+            fp.write(''.join(lines))
+
diff --git a/tests/test-simplekeyvaluefile.py b/tests/test-simplekeyvaluefile.py
new file mode 100644
--- /dev/null
+++ b/tests/test-simplekeyvaluefile.py
@@ -0,0 +1,72 @@
+from __future__ import absolute_import
+
+import unittest
+import silenttestrunner
+
+from mercurial import (
+    error,
+    scmutil,
+)
+
+class mockfile(object):
+    def __init__(self, name, fs):
+        self.name = name
+        self.fs = fs
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *args, **kwargs):
+        pass
+
+    def write(self, text):
+        self.fs.contents[self.name] = text
+
+    def read(self):
+        return self.fs.contents[self.name]
+
+class mockvfs(object):
+    def __init__(self):
+        self.contents = {}
+
+    def read(self, path):
+        return mockfile(path, self).read()
+
+    def readlines(self, path):
+        return mockfile(path, self).read().split('\n')
+
+    def __call__(self, path, mode, atomictemp):
+        return mockfile(path, self)
+
+class testsimplekeyvaluefile(unittest.TestCase):
+    def setUp(self):
+        self.vfs = mockvfs()
+
+    def testbasicwriting(self):
+        d = {'key1': 'value1', 'Key2': 'value2'}
+        scmutil.simplekeyvaluefile(self.vfs, 'kvfile').write(d)
+        self.assertEqual(sorted(self.vfs.read('kvfile').split('\n')),
+                         ['', 'Key2=value2', 'key1=value1'])
+
+    def testinvalidkeys(self):
+        d = {'0key1': 'value1', 'Key2': 'value2'}
+        with self.assertRaisesRegexp(error.ProgrammingError,
+                                     "keys must start with a letter.*"):
+            scmutil.simplekeyvaluefile(self.vfs, 'kvfile').write(d)
+        d = {'key1@': 'value1', 'Key2': 'value2'}
+        with self.assertRaisesRegexp(error.ProgrammingError, "invalid key.*"):
+            scmutil.simplekeyvaluefile(self.vfs, 'kvfile').write(d)
+
+    def testinvalidvalues(self):
+        d = {'key1': 'value1', 'Key2': 'value2\n'}
+        with self.assertRaisesRegexp(error.ProgrammingError, "invalid val.*"):
+            scmutil.simplekeyvaluefile(self.vfs, 'kvfile').write(d)
+
+    def testcorruptedfile(self):
+        self.vfs.contents['badfile'] = 'ababagalamaga\n'
+        with self.assertRaisesRegexp(error.CorruptedState,
+                                     "dictionary.*element.*"):
+            scmutil.simplekeyvaluefile(self.vfs, 'badfile').read()
+
+if __name__ == "__main__":
+    silenttestrunner.main(__name__)