errorcheck: print message to stderr for console UI (#5225)

CHANGELOG.md

@@ -8,6 +8,8 @@
   ([#5190](https://github.com/mitmproxy/mitmproxy/issues/5190), @redraw)
 * Fix a bug where the wrong SNI is sent to an upstream HTTPS proxy
   ([#5109](https://github.com/mitmproxy/mitmproxy/issues/5109), @mhils)
+* Make sure that mitmproxy displays error messages on startup.
+  ([#5225](https://github.com/mitmproxy/mitmproxy/issues/5225), @mhils)
 
 ## 19 March 2022: mitmproxy 8.0.0
 

mitmproxy/addons/errorcheck.py

@@ -1,15 +1,20 @@
 import asyncio
 import sys
+from typing import Optional
+
+from mitmproxy import log
 
 
 class ErrorCheck:
     """Monitor startup for error log entries, and terminate immediately if there are some."""
-    def __init__(self):
-        self.has_errored = False
 
-    def add_log(self, e):
-        if e.level == "error":
-            self.has_errored = True
+    def __init__(self, log_to_stderr: bool = False):
+        self.has_errored: Optional[str] = None
+        self.log_to_stderr = log_to_stderr
+
+    def add_log(self, e: log.LogEntry):
+        if not self.has_errored and e.level == "error":
+            self.has_errored = e.msg
 
     async def running(self):
         # don't run immediately, wait for all logging tasks to finish.
@@ -17,4 +22,6 @@ class ErrorCheck:
 
     async def _shutdown_if_errored(self):
         if self.has_errored:
+            if self.log_to_stderr:
+                print(f"Error on startup: {self.has_errored}", file=sys.stderr)
             sys.exit(1)

mitmproxy/tools/console/master.py

@@ -56,7 +56,7 @@ class ConsoleMaster(master.Master):
             readfile.ReadFile(),
             consoleaddons.ConsoleAddon(self),
             keymap.KeymapConfig(),
-            errorcheck.ErrorCheck(),
+            errorcheck.ErrorCheck(log_to_stderr=True),
         )
 
         self.window = None

test/mitmproxy/addons/test_errorcheck.py

@@ -6,11 +6,12 @@ from mitmproxy import log
 from mitmproxy.addons.errorcheck import ErrorCheck
 
 
-def test_errorcheck():
+@pytest.mark.parametrize("do_log", [True, False])
+def test_errorcheck(capsys, do_log):
     async def run():
         # suppress error that task exception was not retrieved.
         asyncio.get_running_loop().set_exception_handler(lambda *_: 0)
-        e = ErrorCheck()
+        e = ErrorCheck(do_log)
         e.add_log(log.LogEntry("fatal", "error"))
         await e.running()
         await asyncio.sleep(0)
@@ -18,6 +19,9 @@ def test_errorcheck():
     with pytest.raises(SystemExit):
         asyncio.run(run())
 
+    if do_log:
+        assert capsys.readouterr().err == "Error on startup: fatal\n"
+
 
 async def test_no_error():
     e = ErrorCheck()