monitor: limit the amount of data sent in JSONSeqMontior.result()

This commit limits the output in the json pipeline to a "reasonable"
length. We ran into issues (e.g. [0]) from a combination of a stage
that produce tons of output (dracut, ~256 kb, see issue#1976) and
the consumer ("images" osbuild/monitor.go) that used a golang scanner
with a max default buffer of 64kb before erroring. So limit it
here.

The stage result from via json is mostly for information and any error
will most likely at the end. Plus consumers can collect the individual
log lines on their own if desired via the "log()" messages that are
stream in "real-time" with the added benefit that e.g. timestamps
can be added to the logs etc.

[0] https://issues.redhat.com/browse/RHEL-77988

test/mod/test_monitor.py

@@ -19,7 +19,7 @@ import osbuild
 import osbuild.meta
 from osbuild.monitor import Context, JSONSeqMonitor, LogMonitor, Progress, log_entry
 from osbuild.objectstore import ObjectStore
-from osbuild.pipeline import Runner
+from osbuild.pipeline import BuildResult, Runner, Stage
 
 from .. import test
 
@@ -442,3 +442,21 @@ def test_json_progress_monitor_handles_racy_writes(tmp_path):
                 json.loads(line)
             except json.decoder.JSONDecodeError:
                 pytest.fail(f"the jsonseq stream is not valid json, got {line}")
+
+
+def test_json_progress_monitor_excessive_output_in_result(tmp_path):
+    index = osbuild.meta.Index(os.curdir)
+    info = index.get_module_info("Stage", "org.osbuild.noop")
+    stage = Stage(info, "source_opts", "build", "base", "options", "source_epoch")
+
+    output_path = tmp_path / "jsonseq.log"
+    output = "beginning-marker-vanishes\n" + "1" * 32_000 + "\nend-marker"
+    build_result = BuildResult(stage, 0, output, {})
+    with output_path.open("w") as fp:
+        mon = JSONSeqMonitor(fp.fileno(), 1)
+        mon.result(build_result)
+    with output_path.open() as fp:
+        line = fp.readline().strip("\x1e")
+        json_result = json.loads(line)
+    assert json_result["result"]["output"].startswith("[...1037 bytes hidden...]\n1111")
+    assert json_result["result"]["output"].endswith("1111\nend-marker")