The implementation mostly delegates to the local file system except for the case where an - * action input is a remotely stored action output. Most notably {@link - * #getInputStream(PathFragment)} and {@link #createSymbolicLink(PathFragment, PathFragment)}. + *
It acts as a union filesystem over three different sources: * - *
This implementation only supports creating local action outputs. + *
Generally speaking, file operations consult the underlying sources in that order and operate + * on the first result found, although some (e.g. readdir) collate information from all sources. The + * contents of remotely stored files are transparently downloaded when an operation requires them. + * + *
Special care must be taken with operations that follow symlinks, as the symlink and its target + * path may reside on different sources, with an arbitrary number of indirections in between. This + * is required because some actions (notably SymlinkAction) may materialize an output as a symlink + * to an input. Most operations call resolveSymbolicLinks upfront (which is able to canonicalize + * paths taking every source into account) and only then delegate to the underlying sources. + * + *
The implementation assumes that an action never modifies its input files, but may otherwise + * modify any path in the output tree, including modifications that undo previous ones (e.g., + * writing to a path and then moving it elsewhere). No effort is made to detect irreconcilable + * differences between sources (e.g., distinct files of the same name in two different sources). */ public class RemoteActionFileSystem extends AbstractFileSystemWithCustomStat { private final PathFragment execRoot;