apport_python_hook: Fix crash if os.getcwd() fails

Example `getcwd.py`:

```python
import os
import tempfile

tempdir = tempfile.mkdtemp()
os.chdir(tempdir)
os.rmdir(tempdir)
os.getcwd()
```

Apport's Python hook will fail:

```
$ python3 getcwd.py
Traceback (most recent call last):
  File "getcwd.py", line 7, in <module>
    os.getcwd()
FileNotFoundError: [Errno 2] No such file or directory
Error in sys.excepthook:
Traceback (most recent call last):
  File "apport_python_hook.py", line 71, in apport_excepthook
    binary = os.path.realpath(os.path.join(os.getcwd(), sys.argv[0]))
FileNotFoundError: [Errno 2] No such file or directory

Original exception was:
Traceback (most recent call last):
  File "getcwd.py", line 7, in <module>
    os.getcwd()
FileNotFoundError: [Errno 2] No such file or directory
```

Since the program can change the current directory and mutate `sys.argv`
(and also change `/proc/self/cmdline` using the `setproctitle` Python
module), there is no alternative to record the binary name during starup
when registering the Python hook.

Bug: https://launchpad.net/bugs/1977954
Signed-off-by: Benjamin Drung <[email protected]>

apport_python_hook.py

@@ -32,7 +32,7 @@ def enabled():
         return True
 
 
-def apport_excepthook(exc_type, exc_obj, exc_tb):
+def apport_excepthook(binary, exc_type, exc_obj, exc_tb):
     """Catch an uncaught exception and make a traceback."""
 
     # create and save a problem report. Note that exceptions in this code
@@ -66,21 +66,12 @@ def apport_excepthook(exc_type, exc_obj, exc_tb):
         except (ImportError, IOError):
             return
 
-        # apport will look up the package from the executable path.
-        try:
-            binary = os.path.realpath(os.path.join(os.getcwd(), sys.argv[0]))
-        except (TypeError, AttributeError, IndexError):
-            # the module has mutated sys.argv, plan B
-            try:
-                binary = os.readlink("/proc/%i/exe" % os.getpid())
-            except OSError:
-                return
-
-        # for interactive python sessions, sys.argv[0] == ''; catch that and
-        # other irregularities
-        if not os.access(binary, os.X_OK) or not os.path.isfile(binary):
+        # for interactive Python sessions, sys.argv[0] == ""
+        if not binary:
             return
 
+        binary = os.path.realpath(binary)
+
         # filter out binaries in user accessible paths
         if not likely_packaged(binary):
             return
@@ -220,4 +211,23 @@ def dbus_service_unknown_analysis(exc_obj, report):
 def install():
     """Install the python apport hook."""
 
-    sys.excepthook = apport_excepthook
+    # Record before the program can mutate sys.argv and can call os.chdir().
+    binary = sys.argv[0]
+    if binary and not binary.startswith("/"):
+        # pylint: disable=import-outside-toplevel; for Python starup time
+        import os
+
+        try:
+            binary = f"{os.getcwd()}/{binary}"
+        except FileNotFoundError:
+            try:
+                binary = os.readlink("/proc/self/cwd")
+                if binary.endswith(" (deleted)"):
+                    binary = binary[:-10]
+            except IOError:
+                return
+
+    def partial_apport_excepthook(exc_type, exc_obj, exc_tb):
+        return apport_excepthook(binary, exc_type, exc_obj, exc_tb)
+
+    sys.excepthook = partial_apport_excepthook

tests/integration/test_python_crashes.py

@@ -45,7 +45,7 @@ def tearDown(self):
         for f in apport.fileutils.get_all_reports():
             os.unlink(f)
 
-    def _test_crash(self, extracode="", scriptname=None):
+    def _test_crash(self, extracode="", scriptname=None, relpath=False):
         """Create a test crash."""
 
         # put the script into /var/tmp, since that isn't ignored in the
@@ -73,12 +73,24 @@ def func(x):
         )
         os.close(fd)
         os.chmod(script, 0o755)
-        env = self.env.copy()
-        env["PYTHONPATH"] = f"{env.get('PYTHONPATH', '.')}:/my/bogus/path"
 
-        p = subprocess.Popen(
-            [script, "testarg1", "testarg2"], stderr=subprocess.PIPE, env=env
-        )
+        if relpath:
+            binary = os.path.join(".", os.path.basename(script))
+            orig_cwd = os.getcwd()
+            os.chdir(os.path.dirname(script))
+        else:
+            binary = script
+            orig_cwd = "."
+        env = self.env.copy()
+        env["PYTHONPATH"] = f"{env.get('PYTHONPATH', orig_cwd)}:/my/bogus/path"
+        try:
+            p = subprocess.Popen(
+                [binary, "testarg1", "testarg2"],
+                stderr=subprocess.PIPE,
+                env=env,
+            )
+        finally:
+            os.chdir(orig_cwd)
         err = p.communicate()[1].decode()
         self.assertEqual(
             p.returncode,
@@ -224,7 +236,7 @@ def test_symlink(self):
     def test_no_argv(self):
         """with zapped sys.argv."""
 
-        self._test_crash("import sys\nsys.argv = None")
+        script = self._test_crash("import sys\nsys.argv = None")
 
         # did we get a report?
         reports = apport.fileutils.get_new_reports()
@@ -260,9 +272,7 @@ def test_no_argv(self):
             "report has necessary fields",
         )
         self.assertIn("bin/python", pr["InterpreterPath"])
-        # we have no actual executable, so we should fall back to the
-        # interpreter
-        self.assertEqual(pr["ExecutablePath"], pr["InterpreterPath"])
+        self.assertEqual(pr["ExecutablePath"], script)
         if "ExecutableTimestamp" in pr:
             self.assertEqual(
                 pr["ExecutableTimestamp"],
@@ -503,6 +513,30 @@ def g():
             pr.crash_signature(), "%s:OSError:%s@11:g" % (exe, exe)
         )
 
+    def test_getcwd_error(self):
+        """FileNotFoundError on os.getcwd() call"""
+        for relpath in (True, False):
+            with self.subTest(relpath=relpath):
+                self._test_crash(
+                    extracode=textwrap.dedent(
+                        """\
+                        import os
+                        import tempfile
+
+                        tempdir = tempfile.mkdtemp()
+                        os.chdir(tempdir)
+                        os.rmdir(tempdir)
+                        os.getcwd()
+                        """
+                    ),
+                    relpath=relpath,
+                )
+                pr = self._load_report()
+                self.assertIn(":FileNotFoundError:", pr.crash_signature())
+                self.assertIn(
+                    "os.getcwd()\nFileNotFoundError", pr["Traceback"]
+                )
+
     def test_subclassed_os_error(self):
         """OSError with known subclass"""