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 //===- Caching.h - LLVM Local File Cache ------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file defines the CachedFileStream and the localCache function, which
 // simplifies caching files on the local filesystem in a directory whose
 // contents are managed by a CachePruningPolicy.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_SUPPORT_CACHING_H
 #define LLVM_SUPPORT_CACHING_H
 
 #include "llvm/Support/Error.h"
 
 namespace llvm {
 
 class MemoryBuffer;
 
 /// This class wraps an output stream for a file. Most clients should just be
 /// able to return an instance of this base class from the stream callback, but
 /// if a client needs to perform some action after the stream is written to,
 /// that can be done by deriving from this class and overriding the destructor.
 class CachedFileStream {
 public:
   CachedFileStream(std::unique_ptr<raw_pwrite_stream> OS,
                    std::string OSPath = "")
       : OS(std::move(OS)), ObjectPathName(OSPath) {}
   std::unique_ptr<raw_pwrite_stream> OS;
   std::string ObjectPathName;
   virtual ~CachedFileStream() = default;
 };
 
 /// This type defines the callback to add a file that is generated on the fly.
 ///
 /// Stream callbacks must be thread safe.
 using AddStreamFn = std::function<Expected<std::unique_ptr<CachedFileStream>>(
     unsigned Task, const Twine &ModuleName)>;
 
 /// This is a callable that manages file caching operations. It accepts a task
 /// ID \p Task, a unique key \p Key, and a module name \p ModuleName, and
 /// returns AddStreamFn(). This function determines whether a cache hit or miss
 /// occurs and handles the appropriate actions.
 using FileCacheFunction = std::function<Expected<AddStreamFn>(
     unsigned Task, StringRef Key, const Twine &ModuleName)>;
 
 /// This type represents a file cache system that manages caching of files.
 /// It encapsulates a caching function and the directory path where the cache is
 /// stored. To request an item from the cache, pass a unique string as the Key.
 /// For hits, the cached file will be added to the link and this function will
 /// return AddStreamFn(). For misses, the cache will return a stream callback
 /// which must be called at most once to produce content for the stream. The
 /// file stream produced by the stream callback will add the file to the link
 /// after the stream is written to. ModuleName is the unique module identifier
 /// for the bitcode module the cache is being checked for.
 ///
 /// Clients generally look like this:
 ///
 /// if (AddStreamFn AddStream = Cache(Task, Key, ModuleName))
 ///   ProduceContent(AddStream);
 ///
 /// CacheDirectoryPath stores the directory path where cached files are kept.
 struct FileCache {
   FileCache(FileCacheFunction CacheFn, const std::string &DirectoryPath)
       : CacheFunction(std::move(CacheFn)), CacheDirectoryPath(DirectoryPath) {}
   FileCache() = default;
 
   Expected<AddStreamFn> operator()(unsigned Task, StringRef Key,
                                    const Twine &ModuleName) {
     assert(isValid() && "Invalid cache function");
     return CacheFunction(Task, Key, ModuleName);
   }
   const std::string &getCacheDirectoryPath() const {
     return CacheDirectoryPath;
   }
   bool isValid() const { return static_cast<bool>(CacheFunction); }
 
 private:
   FileCacheFunction CacheFunction = nullptr;
   std::string CacheDirectoryPath;
 };
 
 /// This type defines the callback to add a pre-existing file (e.g. in a cache).
 ///
 /// Buffer callbacks must be thread safe.
 using AddBufferFn = std::function<void(unsigned Task, const Twine &ModuleName,
                                        std::unique_ptr<MemoryBuffer> MB)>;
 
 /// Create a local file system cache which uses the given cache name, temporary
 /// file prefix, cache directory and file callback.  This function does not
 /// immediately create the cache directory if it does not yet exist; this is
 /// done lazily the first time a file is added.  The cache name appears in error
 /// messages for errors during caching. The temporary file prefix is used in the
 /// temporary file naming scheme used when writing files atomically.
 Expected<FileCache> localCache(
     const Twine &CacheNameRef, const Twine &TempFilePrefixRef,
     const Twine &CacheDirectoryPathRef,
     AddBufferFn AddBuffer = [](size_t Task, const Twine &ModuleName,
                                std::unique_ptr<MemoryBuffer> MB) {});
 } // namespace llvm
 
 #endif

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