Make memray detach fail no tracker is found

This seems more helpful to users than silently doing nothing would be,
and we could always open this up to not be an error if our users insist.

Signed-off-by: Matt Wozniski <mwozniski@bloomberg.net>

src/memray/commands/attach.py

@@ -48,6 +48,11 @@ def _get_current_heap_size() -> int:
     return rss_bytes
 
 
+class BareExceptionMessage(Exception):
+    def __repr__(self):
+        return self.args[0]
+
+
 class RepeatingTimer(threading.Thread):
     def __init__(self, interval, function):
         self._interval = interval
@@ -143,6 +148,8 @@ def deactivate_because_timer_elapsed():
 if {mode!r} == "ACTIVATE":
     activate_tracker()
 elif {mode!r} == "DEACTIVATE":
+    if not getattr(memray, "_last_tracker", None):
+        raise BareExceptionMessage("no previous `memray attach` call detected")
     deactivate_last_tracker()
 elif {mode!r} == "UNTIL_HEAP_SIZE":
     track_until_heap_size({heap_size})

tests/integration/test_attach.py

@@ -52,7 +52,7 @@ def baz():
 """
 
 
-def generate_command(method, output, *args):
+def generate_attach_command(method, output, *args):
     cmd = [
         sys.executable,
         "-m",
@@ -72,6 +72,23 @@ def generate_command(method, output, *args):
     return cmd
 
 
+def generate_detach_command(method, *args):
+    cmd = [
+        sys.executable,
+        "-m",
+        "memray",
+        "detach",
+        "--verbose",
+        "--method",
+        method,
+    ]
+
+    if args:
+        cmd.extend(args)
+
+    return cmd
+
+
 def run_process(cmd, wait_for_stderr=False):
     process_stderr = ""
     tracked_process = subprocess.Popen(
@@ -137,7 +154,7 @@ def test_basic_attach(tmp_path, method):
 
     # GIVEN
     output = tmp_path / "test.bin"
-    attach_cmd = generate_command(method, output)
+    attach_cmd = generate_attach_command(method, output)
 
     # WHEN
     run_process(attach_cmd)
@@ -155,7 +172,7 @@ def test_aggregated_attach(tmp_path, method):
 
     # GIVEN
     output = tmp_path / "test.bin"
-    attach_cmd = generate_command(method, output, "--aggregate")
+    attach_cmd = generate_attach_command(method, output, "--aggregate")
 
     # WHEN
     run_process(attach_cmd)
@@ -180,7 +197,7 @@ def test_attach_heap(tmp_path, method):
     # GIVEN
     limit = 50 * 1024 * 1024
     output = tmp_path / "test.bin"
-    attach_cmd = generate_command(method, output, "--heap-limit", str(limit))
+    attach_cmd = generate_attach_command(method, output, "--heap-limit", str(limit))
 
     # WHEN
     process_stderr = run_process(attach_cmd, wait_for_stderr=True)
@@ -197,10 +214,29 @@ def test_attach_time(tmp_path, method):
 
     # GIVEN
     output = tmp_path / "test.bin"
-    attach_cmd = generate_command(method, output, "--duration", "1")
+    attach_cmd = generate_attach_command(method, output, "--duration", "1")
 
     # WHEN
     process_stderr = run_process(attach_cmd, wait_for_stderr=True)
 
     # THEN
     assert "memray: Deactivating tracking: 1 seconds have elapsed" in process_stderr
+
+
+@pytest.mark.parametrize("method", ["lldb", "gdb"])
+def test_detach_without_attach(method):
+    if not debugger_available(method):
+        pytest.skip(f"a supported {method} debugger isn't installed")
+
+    # GIVEN
+    detach_cmd = generate_detach_command(method)
+
+    # WHEN
+    with pytest.raises(subprocess.CalledProcessError) as exc_info:
+        run_process(detach_cmd)
+
+    # THEN
+    assert (
+        "Failed to stop tracking in remote process:"
+        " no previous `memray attach` call detected"
+    ) in exc_info.value.stdout