Introduce --report-log option

Fix #4488

src/_pytest/config/__init__.py

@@ -154,6 +154,7 @@ default_plugins = essential_plugins + (
     "assertion",
     "junitxml",
     "resultlog",
+    "report_log",
     "doctest",
     "cacheprovider",
     "freeze_support",

src/_pytest/report_log.py

@@ -0,0 +1,72 @@
+import json
+from pathlib import Path
+
+import pytest
+
+
+def pytest_addoption(parser):
+    group = parser.getgroup("terminal reporting", "report-log plugin options")
+    group.addoption(
+        "--report-log",
+        action="store",
+        metavar="path",
+        default=None,
+        help="Path to line-based json objects of test session events.",
+    )
+
+
+def pytest_configure(config):
+    report_log = config.option.report_log
+    if report_log and not hasattr(config, "slaveinput"):
+        config._report_log_plugin = ReportLogPlugin(config, Path(report_log))
+        config.pluginmanager.register(config._report_log_plugin)
+
+
+def pytest_unconfigure(config):
+    report_log_plugin = getattr(config, "_report_log_plugin", None)
+    if report_log_plugin:
+        report_log_plugin.close()
+        del config._report_log_plugin
+
+
+class ReportLogPlugin:
+    def __init__(self, config, log_path: Path):
+        self._config = config
+        self._log_path = log_path
+
+        log_path.parent.mkdir(parents=True, exist_ok=True)
+        self._file = log_path.open("w", buffering=1, encoding="UTF-8")
+
+    def close(self):
+        if self._file is not None:
+            self._file.close()
+            self._file = None
+
+    def _write_json_data(self, data):
+        self._file.write(json.dumps(data) + "\n")
+        self._file.flush()
+
+    def pytest_sessionstart(self):
+        data = {"pytest_version": pytest.__version__, "$report_type": "SessionStart"}
+        self._write_json_data(data)
+
+    def pytest_sessionfinish(self, exitstatus):
+        data = {"exitstatus": exitstatus, "$report_type": "SessionFinish"}
+        self._write_json_data(data)
+
+    def pytest_runtest_logreport(self, report):
+        data = self._config.hook.pytest_report_to_serializable(
+            config=self._config, report=report
+        )
+        self._write_json_data(data)
+
+    def pytest_collectreport(self, report):
+        data = self._config.hook.pytest_report_to_serializable(
+            config=self._config, report=report
+        )
+        self._write_json_data(data)
+
+    def pytest_terminal_summary(self, terminalreporter):
+        terminalreporter.write_sep(
+            "-", "generated report log file: {}".format(self._log_path)
+        )

src/_pytest/reports.py

@@ -329,18 +329,18 @@ class CollectErrorRepr(TerminalRepr):
 def pytest_report_to_serializable(report):
     if isinstance(report, (TestReport, CollectReport)):
         data = report._to_json()
-        data["_report_type"] = report.__class__.__name__
+        data["$report_type"] = report.__class__.__name__
         return data
 
 
 def pytest_report_from_serializable(data):
-    if "_report_type" in data:
-        if data["_report_type"] == "TestReport":
+    if "$report_type" in data:
+        if data["$report_type"] == "TestReport":
             return TestReport._from_json(data)
-        elif data["_report_type"] == "CollectReport":
+        elif data["$report_type"] == "CollectReport":
             return CollectReport._from_json(data)
         assert False, "Unknown report_type unserialize data: {}".format(
-            data["_report_type"]
+            data["$report_type"]
         )