Add log streaming for macOS apps.

src/briefcase/platforms/macOS/app.py

@@ -84,6 +84,7 @@ def run_app(self, app: BaseConfig, **kwargs):
             self.subprocess.run(
                 [
                     'open',
+                    '-n',  # Force a new app to be launched
                     str(self.binary_path(app)),
                 ],
                 check=True,
@@ -94,6 +95,39 @@ def run_app(self, app: BaseConfig, **kwargs):
                 "Unable to start app {app.app_name}.".format(app=app)
             )
 
+        # Find the PID of the new instance using pgrep
+        try:
+            pid = self.subprocess.check_output(
+                ["pgrep", "-n", app.formal_name],
+                universal_newlines=True,
+            ).strip()
+        except subprocess.CalledProcessError:
+            print()
+            raise BriefcaseCommandError(
+                "Unable to find PID for app {app.app_name}.".format(app=app)
+            )
+
+        # Start streaming logs for the app.
+        try:
+            print()
+            print("[{app.app_name}] Following system log output (type CTRL-C to stop log)...".format(app=app))
+            print("=" * 75)
+            self.subprocess.run(
+                [
+                    "log",
+                    "stream",
+                    "--process", pid,
+                    "--style", "compact",
+                    "--type", "log",
+                ],
+                check=True,
+            )
+        except subprocess.CalledProcessError:
+            print()
+            raise BriefcaseCommandError(
+                "Unable to start log stream for app {app.app_name} (pid {pid}).".format(app=app, pid=pid)
+            )
+
 
 class macOSAppPackageCommand(macOSPackageMixin, macOSAppMixin, PackageCommand):
     description = "Package a macOS app for distribution."

tests/platforms/macOS/app/test_run.py

@@ -11,12 +11,26 @@ def test_run_app(first_app_config, tmp_path):
     "A macOS App can be started"
     command = macOSAppRunCommand(base_path=tmp_path)
     command.subprocess = mock.MagicMock()
+    command.subprocess.check_output.return_value = "3742"
 
     command.run_app(first_app_config)
 
-    command.subprocess.run.assert_called_with(
-        ['open', str(command.binary_path(first_app_config))],
-        check=True
+    # Calls were made to start the app and to start a log stream.
+    command.subprocess.run.assert_has_calls([
+        mock.call(
+            ['open', '-n', str(command.binary_path(first_app_config))],
+            check=True
+        ),
+        mock.call(
+            ['log', 'stream', '--process', '3742', '--style', 'compact', '--type', 'log'],
+            check=True,
+        )
+    ])
+
+    # A call to pgrep was also made
+    command.subprocess.check_output.assert_called_with(
+        ['pgrep', '-n', 'First App'],
+        universal_newlines=True
     )
 
 
@@ -25,7 +39,7 @@ def test_run_app_failed(first_app_config, tmp_path):
     command = macOSAppRunCommand(base_path=tmp_path)
     command.subprocess = mock.MagicMock()
     command.subprocess.run.side_effect = subprocess.CalledProcessError(
-        cmd=['open', str(command.binary_path(first_app_config))],
+        cmd=['open', '-n', str(command.binary_path(first_app_config))],
         returncode=1
     )
 
@@ -34,6 +48,72 @@ def test_run_app_failed(first_app_config, tmp_path):
 
     # The run command was still invoked, though
     command.subprocess.run.assert_called_with(
-        ['open', str(command.binary_path(first_app_config))],
+        ['open', '-n', str(command.binary_path(first_app_config))],
         check=True
     )
+
+    # but no attempt was made to find the PID
+    command.subprocess.check_output.assert_not_called()
+
+
+def test_run_app_no_pid(first_app_config, tmp_path):
+    "If there's a problem finding the PID of the app, an exception is raised"
+    command = macOSAppRunCommand(base_path=tmp_path)
+    command.subprocess = mock.MagicMock()
+    command.subprocess.check_output.side_effect = subprocess.CalledProcessError(
+        cmd=['pgrep', '-n', 'invalid app'],
+        returncode=1
+    )
+
+    with pytest.raises(BriefcaseCommandError):
+        command.run_app(first_app_config)
+
+    # The run command was still invoked, but the log stream wasn't
+    command.subprocess.run.assert_has_calls([
+        mock.call(
+            ['open', '-n', str(command.binary_path(first_app_config))],
+            check=True
+        ),
+    ])
+
+    # and an attempt was made to but no attempt was made to find the PID
+    command.subprocess.check_output.assert_called_with(
+        ['pgrep', '-n', 'First App'],
+        universal_newlines=True
+    )
+
+
+def test_run_app_log_failed(first_app_config, tmp_path):
+    "If the log can't be streamed, the app still starts"
+    command = macOSAppRunCommand(base_path=tmp_path)
+    command.subprocess = mock.MagicMock()
+    command.subprocess.check_output.return_value = "3742"
+    command.subprocess.run.side_effect = [
+        None,
+        subprocess.CalledProcessError(
+            cmd=['log', 'stream'],
+            returncode=1
+        )
+    ]
+
+    # The run command raises an error because the log stream couldn't start
+    with pytest.raises(BriefcaseCommandError):
+        command.run_app(first_app_config)
+
+    # Calls were made to start the app and to start a log stream.
+    command.subprocess.run.assert_has_calls([
+        mock.call(
+            ['open', '-n', str(command.binary_path(first_app_config))],
+            check=True
+        ),
+        mock.call(
+            ['log', 'stream', '--process', '3742', '--style', 'compact', '--type', 'log'],
+            check=True,
+        )
+    ])
+
+    # A call to pgrep was also made
+    command.subprocess.check_output.assert_called_with(
+        ['pgrep', '-n', 'First App'],
+        universal_newlines=True
+    )