diff --git a/swh/core/config.py b/swh/core/config.py
--- a/swh/core/config.py
+++ b/swh/core/config.py
@@ -7,6 +7,8 @@
 import logging
 import os
 import yaml
+from itertools import chain
+from copy import deepcopy
 
 
 logger = logging.getLogger(__name__)
@@ -178,6 +180,77 @@
     return full_config
 
 
+def merge_configs(base, other):
+    """Merge two config dictionaries
+
+    This does merge config dicts recursively, with the rules, for every value
+    of the dicts (with 'val' not being a dict):
+
+    - None + type -> type
+    - type + None -> None
+    - dict + dict -> dict (merged)
+    - val + dict -> TypeError
+    - dict + val -> TypeError
+    - val + val -> val (other)
+
+    so merging
+
+    {
+      'key1': {
+        'skey1': value1,
+        'skey2': {'sskey1': value2},
+      },
+      'key2': value3,
+    }
+
+    with
+
+    {
+      'key1': {
+        'skey1': value4,
+        'skey2': {'sskey2': value5},
+      },
+      'key3': value6,
+    }
+
+    will give:
+
+    {
+      'key1': {
+        'skey1': value4,  # <-- note this
+        'skey2': {
+          'sskey1': value2,
+          'sskey2': value5,
+        },
+      },
+      'key2': value3,
+      'key3': value6,
+    }
+
+    Note that no type checking is done for anything but dicts.
+    """
+    if not isinstance(base, dict) or not isinstance(other, dict):
+        raise TypeError(
+            'Cannot merge a %s with a %s' % (type(base), type(other)))
+
+    output = {}
+    allkeys = set(chain(base.keys(), other.keys()))
+    for k in allkeys:
+        vb = base.get(k)
+        vo = other.get(k)
+
+        if isinstance(vo, dict):
+            output[k] = merge_configs(vb is not None and vb or {}, vo)
+        elif isinstance(vb, dict) and k in other and other[k] is not None:
+            output[k] = merge_configs(vb, vo is not None and vo or {})
+        elif k in other:
+            output[k] = deepcopy(vo)
+        else:
+            output[k] = deepcopy(vb)
+
+    return output
+
+
 def swh_config_paths(base_filename):
     """Return the Software Heritage specific configuration paths for the given
        filename."""
diff --git a/swh/core/tests/test_config.py b/swh/core/tests/test_config.py
--- a/swh/core/tests/test_config.py
+++ b/swh/core/tests/test_config.py
@@ -223,3 +223,90 @@
 
     assert os.path.exists(conf['path1']), "path1 should still exist!"
     assert os.path.exists(conf['path2']), "path2 should now exist."
+
+
+def test_merge_config():
+    cfg_a = {
+        'a': 42,
+        'b': [1, 2, 3],
+        'c': None,
+        'd': {'gheez': 27},
+        'e': {
+            'ea': 'Mr. Bungle',
+            'eb': None,
+            'ec': [11, 12, 13],
+            'ed': {'eda': 'Secret Chief 3',
+                   'edb': 'Faith No More'},
+            'ee': 451,
+        },
+        'f': 'Janis',
+    }
+    cfg_b = {
+        'a': 43,
+        'b': [41, 42, 43],
+        'c': 'Tom Waits',
+        'd': None,
+        'e': {
+            'ea': 'Igorrr',
+            'ec': [51, 52],
+            'ed': {'edb': 'Sleepytime Gorilla Museum',
+                   'edc': 'Nils Peter Molvaer'},
+        },
+        'g': 'Hüsker Dü',
+    }
+
+    # merge A, B
+    cfg_m = config.merge_configs(cfg_a, cfg_b)
+    assert cfg_m == {
+        'a': 43,  # b takes precedence
+        'b': [41, 42, 43],  # b takes precedence
+        'c': 'Tom Waits',  # b takes precedence
+        'd': None,  # b['d'] takes precedence (explicit None)
+        'e': {
+            'ea': 'Igorrr',  # a takes precedence
+            'eb': None,  # only in a
+            'ec': [51, 52],  # b takes precedence
+            'ed': {
+                'eda': 'Secret Chief 3',  # only in a
+                'edb': 'Sleepytime Gorilla Museum',  # b takes precedence
+                'edc': 'Nils Peter Molvaer'},  # only defined in b
+            'ee': 451,
+        },
+        'f': 'Janis',  # only defined in a
+        'g': 'Hüsker Dü',  # only defined in b
+    }
+
+    # merge B, A
+    cfg_m = config.merge_configs(cfg_b, cfg_a)
+    assert cfg_m == {
+        'a': 42,  # a takes precedence
+        'b': [1, 2, 3],  # a takes precedence
+        'c': None,  # a takes precedence
+        'd': {'gheez': 27},  # a takes precedence
+        'e': {
+            'ea': 'Mr. Bungle',  # a takes precedence
+            'eb': None,  # only defined in a
+            'ec': [11, 12, 13],  # a takes precedence
+            'ed': {
+                'eda': 'Secret Chief 3',  # only in a
+                'edb': 'Faith No More',  # a takes precedence
+                'edc': 'Nils Peter Molvaer'},  # only in b
+            'ee': 451,
+        },
+        'f': 'Janis',  # only in a
+        'g': 'Hüsker Dü',  # only in b
+    }
+
+
+def test_merge_config_type_error():
+    for v in (1, 'str', None):
+        with pytest.raises(TypeError):
+            config.merge_configs(v, {})
+        with pytest.raises(TypeError):
+            config.merge_configs({}, v)
+
+    for v in (1, 'str'):
+        with pytest.raises(TypeError):
+            config.merge_configs({'a': v}, {'a': {}})
+        with pytest.raises(TypeError):
+            config.merge_configs({'a': {}}, {'a': v})