73 lines
2.6 KiB
C++
73 lines
2.6 KiB
C++
#pragma once
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///@file
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#include "local-fs-store.hh"
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namespace nix {
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/**
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* Mix-in class for implementing permanent roots as a pair of a direct
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* (strong) reference and indirect weak reference to the first
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* reference.
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*
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* See methods for details on the operations it represents.
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*
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* @note
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* To understand the purpose of this class, it might help to do some
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* "closed-world" rather than "open-world" reasoning, and consider the
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* problem it solved for us. This class was factored out from
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* `LocalFSStore` in order to support the following table, which
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* contains 4 concrete store types (non-abstract classes, exposed to the
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* user), and how they implemented the two GC root methods:
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*
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* @note
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* | | `addPermRoot()` | `addIndirectRoot()` |
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* |-------------------|-----------------|---------------------|
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* | `LocalStore` | local | local |
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* | `UDSRemoteStore` | local | remote |
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* | `SSHStore` | doesn't have | doesn't have |
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* | `MountedSSHStore` | remote | doesn't have |
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*
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* @note
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* Note how only the local implementations of `addPermRoot()` need
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* `addIndirectRoot()`; that is what this class enforces. Without it,
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* and with `addPermRoot()` and `addIndirectRoot()` both `virtual`, we
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* would accidentally be allowing for a combinatorial explosion of
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* possible implementations many of which make no sense. Having this and
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* that invariant enforced cuts down that space.
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*/
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struct IndirectRootStore : public virtual LocalFSStore
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{
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inline static std::string operationName = "Indirect GC roots registration";
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/**
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* Implementation of `LocalFSStore::addPermRoot` where the permanent
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* root is a pair of
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*
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* - The user-facing symlink which all implementations must create
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*
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* - An additional weak reference known as the "indirect root" that
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* points to that symlink.
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*
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* The garbage collector will automatically remove the indirect root
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* when it finds that the symlink has disappeared.
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*
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* The implementation of this method is concrete, but it delegates
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* to `addIndirectRoot()` which is abstract.
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*/
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Path addPermRoot(const StorePath & storePath, const Path & gcRoot) override final;
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/**
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* Add an indirect root, which is a weak reference to the
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* user-facing symlink created by `addPermRoot()`.
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*
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* @param path user-facing and user-controlled symlink to a store
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* path.
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*
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* The form this weak-reference takes is implementation-specific.
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*/
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virtual void addIndirectRoot(const Path & path) = 0;
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};
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}
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