Refuse to start a service command that is not on PATH

Falling back to the bare name let Windows CreateProcess search the current
directory. Look the command up on the child's PATH, as Node's spawn does,
and raise (surfaced as "Failed to start server") when it is missing.

Author: shrijayan

src/opencode/_service.py

@@ -540,7 +540,7 @@ class _Contender:
     def __init__(self, command: Sequence[str], env: Mapping[str, str | None]) -> None:
         merged = {**os.environ, **env}
         environment = {key: value for key, value in merged.items() if value is not None}
-        executable = _resolve(command[0])
+        executable = _resolve(command[0], environment.get("PATH", os.defpath))
         if sys.platform == "win32":  # pragma: no cover - Windows only
             self.process = subprocess.Popen(
                 [executable, *command[1:]],
@@ -604,18 +604,22 @@ def failure(self) -> ServiceError | None:
         return _startup_error(f"Server process terminated by {name}", self.stderr())
 
 
-def _resolve(command: str) -> str:
-    """Find ``command`` on ``PATH`` only.
+def _resolve(command: str, search_path: str) -> str:
+    """Find ``command`` on ``search_path``, the child's ``PATH``, as Node's spawn does.
 
     ``shutil.which`` on Windows also searches the current directory first, which Node's spawn in the TypeScript
     client does not, so a planted ``opencode.exe`` there could run. A command with a directory part is used as is.
+
+    Raises:
+        FileNotFoundError: ``command`` is not on ``PATH``.
     """
     if os.path.dirname(command):
         return command
-    for directory in os.environ.get("PATH", os.defpath).split(os.pathsep):
+    for directory in search_path.split(os.pathsep):
         if directory and (found := shutil.which(os.path.join(directory, command))) is not None:
             return found
-    return command
+    # Never hand Popen a bare name: Windows CreateProcess would search the current directory for it.
+    raise FileNotFoundError(f"{command!r} was not found on PATH")
 
 
 def _startup_error(message: str, stderr: str) -> ServiceError:

tests/test_service.py

@@ -282,8 +282,7 @@ def test_resolve_searches_path_only(tmp_path: Path, monkeypatch: pytest.MonkeyPa
         (folder / name).write_text("")
         (folder / name).chmod(0o755)
     monkeypatch.chdir(cwd)
-    monkeypatch.setenv("PATH", str(bin_dir))
-    assert Path(impl._resolve("tool")).parent == bin_dir
-    monkeypatch.setenv("PATH", "")
-    assert impl._resolve("tool") == "tool"
-    assert impl._resolve(str(cwd / name)) == str(cwd / name)
+    assert Path(impl._resolve("tool", str(bin_dir))).parent == bin_dir
+    with pytest.raises(FileNotFoundError, match="not found on PATH"):
+        impl._resolve("tool", "")  # the copy in the current directory is never used
+    assert impl._resolve(str(cwd / name), "") == str(cwd / name)