feat(handler): on-the-fly package install for deployed mode

src/constants.py

@@ -50,3 +50,12 @@
 """Number of times the Flash-deployed endpoint will attempt to unpack the worker-flash tarball from mounted volume."""
 DEFAULT_TARBALL_UNPACK_INTERVAL = 30
 """Time in seconds the Flash-deployed endpoint will wait between tarball unpack attempts."""
+
+# Dependency Recovery Configuration
+MAX_IMPORT_RECOVERY_ATTEMPTS = 3
+"""Max on-the-fly package installs before giving up during handler loading.
+
+When a deployed handler fails to import due to a missing package, the worker
+attempts to install it and retry. This caps the retry loop to prevent unbounded
+installs (e.g. a package with many missing transitive deps).
+"""

src/handler.py

@@ -1,9 +1,11 @@
 import importlib.util
 import logging
 import os
+import sys
 from pathlib import Path
 from typing import Any, Dict, Optional
 
+from constants import MAX_IMPORT_RECOVERY_ATTEMPTS
 from logger import setup_logging
 from unpack_volume import maybe_unpack
 from version import format_version_banner
@@ -26,13 +28,112 @@ def _is_deployed_mode() -> bool:
     return bool(os.getenv("FLASH_RESOURCE_NAME"))
 
 
+class _HandlerRecoveryError(RuntimeError):
+    """Raised by _exec_handler_module when on-the-fly recovery fails.
+
+    Distinguished from generic RuntimeError so _load_generated_handler can
+    re-raise it without wrapping, while still wrapping user-code RuntimeErrors.
+    """
+
+
+def _extract_missing_package(error: ImportError) -> str | None:
+    """Extract the top-level package name from an ImportError.
+
+    Returns the root package name (e.g. 'numpy' from 'numpy.core') or None
+    if the module name cannot be determined.
+    """
+    module_name: str | None = getattr(error, "name", None)
+    if not module_name:
+        return None
+    return module_name.split(".")[0]
+
+
+def _try_install_missing_package(package_name: str) -> bool:
+    """Attempt to install a missing package on-the-fly via DependencyInstaller.
+
+    Returns True if installation succeeded, False otherwise.
+    """
+    from dependency_installer import DependencyInstaller
+
+    installer = DependencyInstaller()
+    result = installer.install_dependencies([package_name])
+    return bool(result.success)
+
+
+def _exec_handler_module(
+    spec: importlib.machinery.ModuleSpec,
+    handler_file: Path,
+) -> Any:
+    """Execute a handler module spec, installing missing packages on-the-fly.
+
+    When a deployed handler fails to import due to a missing package (e.g.
+    numpy excluded from the build artifact but needed at runtime), this
+    function installs the package and retries. This adds to cold start time
+    but prevents a fatal crash.
+
+    Returns:
+        The loaded module object.
+
+    Raises:
+        _HandlerRecoveryError: If the handler cannot be loaded after recovery attempts.
+    """
+    installed_packages: list[str] = []
+
+    for _attempt in range(MAX_IMPORT_RECOVERY_ATTEMPTS):
+        mod = importlib.util.module_from_spec(spec)
+        try:
+            spec.loader.exec_module(mod)  # type: ignore[union-attr]
+            return mod
+        except ImportError as e:
+            package_name = _extract_missing_package(e)
+            if not package_name or package_name in installed_packages:
+                # Can't determine package or already tried installing it
+                raise _HandlerRecoveryError(
+                    f"Generated handler {handler_file} failed to import: {e}. "
+                    f"This usually means a dependency was built for the wrong "
+                    f"Python version. Redeploy with 'flash deploy'."
+                ) from e
+
+            logger.warning(
+                "Package '%s' is not in the build artifact. Installing on-the-fly. "
+                "This adds to cold start time — consider adding it to your "
+                "dependencies list to include it in the build artifact.",
+                package_name,
+            )
+
+            if not _try_install_missing_package(package_name):
+                raise _HandlerRecoveryError(
+                    f"Failed to install missing package '{package_name}'. "
+                    f"Generated handler {handler_file} cannot be loaded. "
+                    f"Redeploy with 'flash deploy'."
+                ) from e
+
+            installed_packages.append(package_name)
+            # Clear the failed module from sys.modules so the retry gets a fresh import
+            for key in list(sys.modules):
+                if key == package_name or key.startswith(f"{package_name}."):
+                    del sys.modules[key]
+            logger.info("Installed '%s', retrying handler load", package_name)
+
+    raise _HandlerRecoveryError(
+        f"Generated handler {handler_file} failed to load after installing "
+        f"{len(installed_packages)} missing packages: {installed_packages}. "
+        f"Too many missing dependencies — redeploy with 'flash deploy'."
+    )
+
+
 def _load_generated_handler() -> Optional[Any]:
     """Load Flash-generated handler for deployed QB mode.
 
     Checks for a handler_<resource_name>.py file generated by the flash
     build pipeline. These handlers accept plain JSON input without
     FunctionRequest/cloudpickle serialization.
 
+    If the handler fails to import due to a missing package, attempts
+    on-the-fly installation before giving up. This handles cases where
+    a package was excluded from the build artifact (e.g. size-prohibitive
+    packages) but is needed at runtime.
+
     In deployed mode (FLASH_RESOURCE_NAME set), failures are fatal.
     FunctionRequest fallback is only valid for Live Serverless workers.
 
@@ -68,20 +169,16 @@ def _load_generated_handler() -> Optional[Any]:
             f"The file may be corrupted. Redeploy with 'flash deploy'."
         )
 
-    mod = importlib.util.module_from_spec(spec)
     try:
-        spec.loader.exec_module(mod)
-    except ImportError as e:
-        raise RuntimeError(
-            f"Generated handler {handler_file} failed to import: {e}. "
-            f"This usually means a dependency was built for the wrong Python version. "
-            f"Redeploy with 'flash deploy'."
-        ) from e
+        mod = _exec_handler_module(spec, handler_file)
     except SyntaxError as e:
         raise RuntimeError(
             f"Generated handler {handler_file} has a syntax error: {e}. "
             f"This indicates a bug in the flash build pipeline."
         ) from e
+    except _HandlerRecoveryError:
+        # Recovery-specific RuntimeErrors from _exec_handler_module — already formatted
+        raise
     except Exception as e:
         raise RuntimeError(
             f"Generated handler {handler_file} failed to load: {e} ({type(e).__name__}). "

tests/unit/test_handler.py

@@ -204,17 +204,58 @@ def test_raises_when_spec_creation_fails(self):
                     with pytest.raises(RuntimeError, match="Failed to create module spec"):
                         _load_generated_handler()
 
-    def test_raises_on_import_error(self, tmp_path):
-        """If generated handler has ImportError, raises RuntimeError."""
+    def test_raises_on_import_error_when_install_fails(self, tmp_path):
+        """If install of missing package fails, raises with install failure message."""
         handler_file = tmp_path / "handler_gpu_config.py"
         handler_file.write_text(
             "from nonexistent_package import missing_function\ndef handler(event): pass\n"
         )
 
         with patch.dict("os.environ", {"FLASH_RESOURCE_NAME": "gpu_config"}):
             with patch("handler.Path", return_value=handler_file):
-                with pytest.raises(RuntimeError, match="failed to import"):
-                    _load_generated_handler()
+                with patch("handler._try_install_missing_package", return_value=False):
+                    with pytest.raises(RuntimeError, match="Failed to install"):
+                        _load_generated_handler()
+
+    def test_recovery_installs_missing_package_and_retries(self, tmp_path):
+        """Successful on-the-fly install allows handler to load on retry."""
+        from handler import _exec_handler_module
+
+        handler_file = tmp_path / "handler_gpu_config.py"
+        handler_file.write_text("def handler(event): return {'recovered': True}\n")
+
+        import importlib.util
+
+        spec = importlib.util.spec_from_file_location("handler_gpu_config", handler_file)
+
+        call_count = 0
+        original_exec = spec.loader.exec_module
+
+        def exec_side_effect(module):
+            nonlocal call_count
+            call_count += 1
+            if call_count == 1:
+                raise ImportError("no module", name="fake_recovery_pkg")
+            original_exec(module)
+
+        with patch.object(spec.loader, "exec_module", side_effect=exec_side_effect):
+            with patch("handler._try_install_missing_package", return_value=True) as mock_install:
+                mod = _exec_handler_module(spec, handler_file)
+                assert hasattr(mod, "handler")
+                assert callable(mod.handler)
+                mock_install.assert_called_once_with("fake_recovery_pkg")
+
+    def test_recovery_stops_if_same_package_fails_twice(self, tmp_path):
+        """If the same package keeps failing after install, raises immediately."""
+        handler_file = tmp_path / "handler_gpu_config.py"
+        # Always raises ImportError for same package, even after "install"
+        handler_file.write_text("raise ImportError('still missing', name='stubborn_pkg')\n")
+
+        with patch.dict("os.environ", {"FLASH_RESOURCE_NAME": "gpu_config"}):
+            with patch("handler.Path", return_value=handler_file):
+                with patch("handler._try_install_missing_package", return_value=True):
+                    with pytest.raises(RuntimeError, match="failed to import"):
+                        _load_generated_handler()
 
     def test_raises_on_syntax_error(self, tmp_path):
         """SyntaxError in generated handler raises RuntimeError."""