Experimental feature in GraalVM

Operating System Interfaces

Truffle-based GraalVM languages usually implement the system related functions using the Truffle abstraction layer, which is OS independent and provides extension points for the users when embedding GraalVM Python or other Truffle based languages into Java applications. See, for example, Truffle FileSystem service-provider.

The Python standard library also provides OS abstraction, but exposes lower level interfaces, for instance, the OS module directly exposes some POSIX functions. On non-POSIX platforms, this interface is emulated to a degree.

GraalVM Python runtime provides two alternative implementations of the system related functionality offered by the builtin Python modules such as os. Which implementation is used can be controlled by the option PosixModuleBackend: valid values are native and java.

Native backend #

The native backend calls directly the POSIX API in mostly the same way as CPython, the reference Python implementation, does.

This approach is the most compatible with CPython and provides bare access to the underlying OS interface without any emulation layer in between.

By default, this implementation bypasses the Truffle abstraction layer, therefore it is not sandboxed and does not support custom implementations of Truffle FileSystem service-provider and other Polyglot API providers related to system interfaces.

The native backend is chosen by default when GraalVM Python is started via the graalpython or any other Python related launcher inside GraalVM. The exception are Python related launchers with -managed suffix available only in GraalVM Enterprise (e.g., graalpython-managed), which by default use the java POSIX backend.

Limitations of the native backend #

Known limitations:

  • os.fork is not supported
  • _posixsubprocess.fork_exec does not support the preexec_fn parameter

Java backend #

The java backend uses the Truffle abstraction layer and therefore supports custom Polyglot API providers related to system interfaces and sandboxing. Since this abstraction is POSIX agnostic, it does not expose all the necessary functionality. Some functionality is emulated, and some functionality is not supported at all.

The java backend is the default when GraalVM Python is run via the Context API, i.e., embedded in Java applications, or when it is launched using Python related launchers with -managed suffix available only in GraalVM Enterprise.

Limitations of the emulated backend #

GraalVM Python can log info about known incompatibility of functions executed at runtime, which includes the OS interface related functions. To turn on this logging, use --log.python.compatibility=FINE or other desired logging level.

Known limitations of the emulated layer are:

  • Its state is disconnected from the actual OS state, which applies especially to:
    • file descriptors: Python level file descriptors are not usable in native code
    • current working directory: gets initialized to what was the actual current working directory at the startup, but then it is maintained separately, i.e., chdir in Python does not change the actual current working directory of the process.
    • umask: similarly to current working directory, but it is always initialized to 0022 regardless of the actual system value at startup.
  • Resolution of file access/modification times depends on the JDK. The best the emulated backend can guarantee is seconds resolution.
  • os.access and any other functionality based on faccessat POSIX function does not support:
    • effective IDs
    • follow_symlinks=False unless the mode is only F_OK

Relation to Python native extensions #

Apart from operating system interfaces exposed as builtin Python level modules, Python native extensions executed via the GraalVM LLVM runtime may also access OS interfaces at the C level. How such accesses are handled depends on the GraalVM LLVM runtime configuration.

At this point, the only combination where OS handles, such as file descriptors, can be shared between Python and the C code of Python extensions is with native PosixModuleBackend and native mode of GraalVM LLVM runtime. This combination is the default for the graalpython launcher.

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