json: support BORG_JSON_COMPACT env var for single-line output

src/borg/helpers/parseformat.py

@@ -1418,9 +1418,25 @@ 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/parseformat_test.py

@@ -1,4 +1,5 @@
 import base64
+import json
 import os
 
 import re
@@ -718,3 +719,31 @@ def test_valid_chunkerparams(chunker_params, expected_return):
 def test_invalid_chunkerparams(invalid_chunker_params):
     with pytest.raises(ArgumentTypeError):
         ChunkerParams(invalid_chunker_params)
+
+
+@pytest.mark.parametrize(
+    "env_value, expect_newlines",
+    [
+        (None, True),  # default indent=4
+        ("none", False),  # compact single-line
+        ("0", True),  # newlines only, no spaces
+        ("4", True),  # explicit pretty-print
+        ("", True),  # empty string, newlines only
+        ("\t", True),  # tab indent string
+    ],
+)
+def test_json_dump_indent(monkeypatch, env_value, expect_newlines):
+    from ...helpers.parseformat import json_dump
+
+    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