features/lsp_transport_coverage.feature

@@ -23,6 +23,13 @@ Feature: LSP StdioTransport coverage
     When ltcov I try to start the transport
     Then ltcov the error should be an LspError with message "Failed to start"
 
+  Scenario: ltcov start raises LspError when process exits immediately after Popen succeeds
+    Given ltcov I create a StdioTransport for command "cat"
+    And ltcov Popen is mocked to return a process that already exited with code 1
+    When ltcov I try to start the transport
+    Then ltcov the error should be an LspError with message "exited immediately"
+    And ltcov the internal process should be None
+
   # ── stop() paths ────────────────────────────────────────────────
 
   Scenario: ltcov stop returns None when not started

features/steps/lsp_transport_coverage_steps.py

@@ -98,6 +98,19 @@ def step_ltcov_popen_oserror(context: Context, msg: str) -> None:
     context.add_cleanup(patcher.stop)
 
 
+@given(
+    "ltcov Popen is mocked to return a process that already exited with code {code:d}"
+)
+def step_ltcov_popen_exited(context: Context, code: int) -> None:
+    proc = _make_mock_process(poll_return=code, returncode=code)
+    patcher = patch(
+        "cleveragents.lsp.transport.subprocess.Popen",
+        return_value=proc,
+    )
+    patcher.start()
+    context.add_cleanup(patcher.stop)
+
+
 @given("ltcov the transport has a mock process that already exited with code {code:d}")
 def step_ltcov_exited_process(context: Context, code: int) -> None:
     proc = _make_mock_process(poll_return=code, returncode=code)

src/cleveragents/lsp/transport.py

@@ -18,6 +18,7 @@ and issue #826.
 
 from __future__ import annotations
 
+import contextlib
 import json
 import os
 import select
@@ -149,6 +150,45 @@ class StdioTransport:
             command=self._command,
         )
 
+        # Post-initialization check: if the child has already exited before
+        # we had a chance to interact with it (e.g. binary cannot be loaded,
+        # missing shared library, syntax error in script), clean up all pipe
+        # file descriptors to prevent resource leaks and zombie processes.
+        if self._process.poll() is not None:
+            pid = self._process.pid
+            code = self._process.returncode
+            logger.warning(
+                "lsp.transport.early_exit",
+                pid=pid,
+                exit_code=code,
+            )
+            # Wait to prevent zombie accumulation.
+            self._process.wait()
+            # Close stdin/stdout/stderr if they are still open; the OS may
+            # have already closed them after the child exited, so we suppress
+            # OSError on each close.
+            with contextlib.suppress(OSError):
+                if self._process.stdin is not None:
+                    self._process.stdin.close()
+            with contextlib.suppress(OSError):
+                if self._process.stdout is not None:
+                    self._process.stdout.close()
+            with contextlib.suppress(OSError):
+                if self._process.stderr is not None:
+                    self._process.stderr.close()
+            self._process = None
+            from cleveragents.lsp.errors import LspError
+
+            raise LspError(
+                f"LSP server process exited immediately (exit code {code})",
+                details={
+                    "command": cmd,
+                    "args": self._args,
+                    "exit_code": code,
+                    "pid": pid,
+                },
+            )
+
     def stop(self, timeout: float = _GRACEFUL_SHUTDOWN_TIMEOUT) -> int | None:
         """Terminate the subprocess gracefully, then force-kill if needed.