json: support BORG_JSON_INDENT env var for JSON output formatting

src/borg/helpers/parseformat.py

@@ -1039,9 +1039,25 @@ def basic_json_data(manifest, *, cache=None, extra=None):
     return data
 
 
+def _json_indent_from_env():
+    """Parse BORG_JSON_INDENT env var for json.dumps indent parameter."""
+    value = os.environ.get("BORG_JSON_INDENT")
+    if value is None:
+        return 4
+    if value.lower() == "none":
+        return None
+    if value == "":
+        return ""
+    try:
+        return int(value)
+    except ValueError:
+        return value
+
+
 def json_dump(obj):
     """Dump using BorgJSONEncoder."""
-    return json.dumps(obj, sort_keys=True, indent=4, cls=BorgJsonEncoder)
+    indent = _json_indent_from_env()
+    return json.dumps(obj, sort_keys=True, indent=indent, cls=BorgJsonEncoder)
 
 
 def json_print(obj):

src/borg/testsuite/helpers.py

@@ -1239,3 +1239,31 @@ def test_ec_invalid():
 ))
 def test_max_ec(ec1, ec2, ec_max):
     assert max_ec(ec1, ec2) == ec_max
+
+@pytest.mark.parametrize(
+    'env_value, expect_newlines',
+    [
+        (None, True),
+        ('none', False),
+        ('0', True),
+        ('4', True),
+        ('', True),
+        ('\t', True),
+    ],
+)
+def test_json_dump_indent(monkeypatch, env_value, expect_newlines):
+    from ..helpers import json_dump
+    import json
+
+    obj = {'key': 'value', 'number': 42}
+    if env_value is not None:
+        monkeypatch.setenv('BORG_JSON_INDENT', env_value)
+    else:
+        monkeypatch.delenv('BORG_JSON_INDENT', raising=False)
+
+    result = json_dump(obj)
+    if expect_newlines:
+        assert '\n' in result
+    else:
+        assert '\n' not in result
+    assert json.loads(result) == obj