Fix JSON parser by not always mangling the input

Rather than by assuming the JSON file we are parsing has junk at the beginning
(which maybe only used to happen?), try parsing it as-is first, and then fall
back to trying again after skipping the first line

Fixes #1347

tests/test_add.py

@@ -91,3 +91,53 @@ def test_extract_input_uses_only_passed_extractors(tmp_path, process):
 
     assert (archived_item_path / "warc").exists()
     assert not (archived_item_path / "singlefile.html").exists()
+
+def test_json(tmp_path, process, disable_extractors_dict):
+    with open('../../mock_server/templates/example.json', 'r', encoding='utf-8') as f:
+        arg_process = subprocess.run(
+            ["archivebox", "add", "--index-only", "--parser=json"],
+            stdin=f,
+            capture_output=True,
+            env=disable_extractors_dict,
+        )
+
+    conn = sqlite3.connect("index.sqlite3")
+    c = conn.cursor()
+    urls = c.execute("SELECT url from core_snapshot").fetchall()
+    tags = c.execute("SELECT name from core_tag").fetchall()
+    conn.commit()
+    conn.close()
+
+    urls = list(map(lambda x: x[0], urls))
+    assert "http://127.0.0.1:8080/static/example.com.html" in urls
+    # if the following URL appears, we must have fallen back to another parser
+    assert not "http://www.example.com/should-not-exist" in urls
+
+    tags = list(map(lambda x: x[0], tags))
+    assert "Tag1" in tags
+    assert "Tag2" in tags
+
+def test_json_with_leading_garbage(tmp_path, process, disable_extractors_dict):
+    with open('../../mock_server/templates/example.json.bad', 'r', encoding='utf-8') as f:
+        arg_process = subprocess.run(
+            ["archivebox", "add", "--index-only", "--parser=json"],
+            stdin=f,
+            capture_output=True,
+            env=disable_extractors_dict,
+        )
+
+    conn = sqlite3.connect("index.sqlite3")
+    c = conn.cursor()
+    urls = c.execute("SELECT url from core_snapshot").fetchall()
+    tags = c.execute("SELECT name from core_tag").fetchall()
+    conn.commit()
+    conn.close()
+
+    urls = list(map(lambda x: x[0], urls))
+    assert "http://127.0.0.1:8080/static/example.com.html" in urls
+    # if the following URL appears, we must have fallen back to another parser
+    assert not "http://www.example.com/should-not-exist" in urls
+
+    tags = list(map(lambda x: x[0], tags))
+    assert "Tag1" in tags
+    assert "Tag2" in tags