Storage: Reworked storage and garbage collection

The improved GC implementation uses refactored storage
classes instead of directly accessing "unknown" file paths.
The required storage class refactoring is quite substantial
and outlined in the following paragraphs.

The module `buildtool/file_system` was extended by:

- `ObjectCAS`: a plain CAS implementation for
  reading/writing blobs and computing digests for a given
  `ObjectType`. Depending on that type, files written to the
  file system may have different properties (e.g., the x-bit
  set) or the digest may be computed differently (e.g., tree
  digests in non-compatible mode).

A new module `buildtool/storage` was introduced containing:

- `LocalCAS`: provides a common interface for the "logical
  CAS", which internally combines three `ObjectCAS`s, one
  for each `ObjectType` (file, executable, tree).

- `LocalAC`: implements the action cache, which needs the
  `LocalCAS` for storing cache values.

- `TargetCache`: implements the high-level target cache,
  which also needs the `LocalCAS` for storing cache values.

- `LocalStorage`: combines the storage classes `LocalCAS`,
  `LocalAC`, and `TargetCache`. Those are initialized with
  settings from `StorageConfig`, such as the build root base
  path or number of generations for the garbage collector.
  `LocalStorage` is templated with a Boolean parameter
  `kDoGlobalUplink`, which indicates that, on every
  read/write access, the garbage collector should be used
  for uplinking across all generations (global).

- `GarbageCollector`: responsible for garbage collection and
  the global uplinking across all generations. To do so, it
  employs instances of `LocalStorage` with `kDoGlobalUplink`
  set to false, in order to avoid endless recursion. The
  actual (local) uplinking within two single generations is
  performed by the corresponding storage class (e.g.,
  `TargetCache` implements uplinking of target cache entries
  between two target cache generations etc.). Thereby, the
  actual knowledge how data should be uplinked is
  implemented by the instance that is responsible for
  creating the data in the first place.

1 files changed, 155 insertions, 0 deletions

diff --git a/src/buildtool/storage/garbage_collector.cpp b/src/buildtool/storage/garbage_collector.cpp
new file mode 100644
index 00000000..3e486672
--- /dev/null
+++ b/src/buildtool/storage/garbage_collector.cpp
@@ -0,0 +1,155 @@
+// Copyright 2022 Huawei Cloud Computing Technology Co., Ltd.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef BOOTSTRAP_BUILD_TOOL
+
+#include "src/buildtool/storage/garbage_collector.hpp"
+
+#include <vector>
+
+#include <nlohmann/json.hpp>
+
+#include "src/buildtool/common/artifact.hpp"
+#include "src/buildtool/common/bazel_types.hpp"
+#include "src/buildtool/compatibility/compatibility.hpp"
+#include "src/buildtool/compatibility/native_support.hpp"
+#include "src/buildtool/file_system/file_storage.hpp"
+#include "src/buildtool/file_system/file_system_manager.hpp"
+#include "src/buildtool/file_system/git_repo.hpp"
+#include "src/buildtool/file_system/object_type.hpp"
+#include "src/buildtool/logging/log_level.hpp"
+#include "src/buildtool/logging/logger.hpp"
+#include "src/buildtool/storage/config.hpp"
+#include "src/buildtool/storage/storage.hpp"
+#include "src/buildtool/storage/target_cache_entry.hpp"
+#include "src/utils/cpp/hex_string.hpp"
+
+auto GarbageCollector::GlobalUplinkBlob(bazel_re::Digest const& digest,
+                                        bool is_executable) noexcept -> bool {
+    // Try to find blob in all generations.
+    auto const& latest_cas = Storage::Generation(0).CAS();
+    for (std::size_t i = 0; i < StorageConfig::NumGenerations(); ++i) {
+        // Note that we uplink with _skip_sync_ as we want to prefer hard links
+        // from older generations over copies from the companion file/exec CAS.
+        if (Storage::Generation(i).CAS().LocalUplinkBlob(
+                latest_cas, digest, is_executable, /*skip_sync=*/true)) {
+            return true;
+        }
+    }
+    return false;
+}
+
+auto GarbageCollector::GlobalUplinkTree(bazel_re::Digest const& digest) noexcept
+    -> bool {
+    // Try to find tree in all generations.
+    auto const& latest_cas = Storage::Generation(0).CAS();
+    for (std::size_t i = 0; i < StorageConfig::NumGenerations(); ++i) {
+        if (Storage::Generation(i).CAS().LocalUplinkTree(latest_cas, digest)) {
+            return true;
+        }
+    }
+    return false;
+}
+
+auto GarbageCollector::GlobalUplinkActionCacheEntry(
+    bazel_re::Digest const& action_id) noexcept -> bool {
+    // Try to find action-cache entry in all generations.
+    auto const& latest_ac = Storage::Generation(0).ActionCache();
+    for (std::size_t i = 0; i < StorageConfig::NumGenerations(); ++i) {
+        if (Storage::Generation(i).ActionCache().LocalUplinkEntry(latest_ac,
+                                                                  action_id)) {
+            return true;
+        }
+    }
+    return false;
+}
+
+auto GarbageCollector::GlobalUplinkTargetCacheEntry(
+    TargetCacheKey const& key) noexcept -> bool {
+    // Try to find target-cache entry in all generations.
+    auto const& latest_tc = Storage::Generation(0).TargetCache();
+    for (std::size_t i = 0; i < StorageConfig::NumGenerations(); ++i) {
+        if (Storage::Generation(i).TargetCache().LocalUplinkEntry(latest_tc,
+                                                                  key)) {
+            return true;
+        }
+    }
+    return false;
+}
+
+auto GarbageCollector::SharedLock() noexcept -> std::optional<LockFile> {
+    return LockFile::Acquire(LockFilePath(), /*is_shared=*/true);
+}
+
+auto GarbageCollector::ExclusiveLock() noexcept -> std::optional<LockFile> {
+    return LockFile::Acquire(LockFilePath(), /*is_shared=*/false);
+}
+
+auto GarbageCollector::LockFilePath() noexcept -> std::filesystem::path {
+    return StorageConfig::CacheRoot() / "gc.lock";
+}
+
+auto GarbageCollector::TriggerGarbageCollection() noexcept -> bool {
+    auto pid = CreateProcessUniqueId();
+    if (not pid) {
+        return false;
+    }
+    auto remove_me = std::string{"remove-me-"} + *pid;
+    auto remove_me_dir = StorageConfig::CacheRoot() / remove_me;
+    if (FileSystemManager::IsDirectory(remove_me_dir)) {
+        if (not FileSystemManager::RemoveDirectory(remove_me_dir,
+                                                   /*recursively=*/true)) {
+            Logger::Log(LogLevel::Error,
+                        "Failed to remove directory {}",
+                        remove_me_dir.string());
+            return false;
+        }
+    }
+    {  // Create scope for critical renaming section protected by advisory lock.
+        auto lock = ExclusiveLock();
+        if (not lock) {
+            Logger::Log(LogLevel::Error,
+                        "Failed to exclusively lock the local build root");
+            return false;
+        }
+        for (std::size_t i = StorageConfig::NumGenerations(); i > 0; --i) {
+            auto cache_root = StorageConfig::GenerationCacheRoot(i - 1);
+            if (FileSystemManager::IsDirectory(cache_root)) {
+                auto new_cache_root =
+                    (i == StorageConfig::NumGenerations())
+                        ? remove_me_dir
+                        : StorageConfig::GenerationCacheRoot(i);
+                if (not FileSystemManager::Rename(cache_root, new_cache_root)) {
+                    Logger::Log(LogLevel::Error,
+                                "Failed to rename {} to {}.",
+                                cache_root.string(),
+                                new_cache_root.string());
+                    return false;
+                }
+            }
+        }
+    }
+    if (FileSystemManager::IsDirectory(remove_me_dir)) {
+        if (not FileSystemManager::RemoveDirectory(remove_me_dir,
+                                                   /*recursively=*/true)) {
+            Logger::Log(LogLevel::Warning,
+                        "Failed to remove directory {}",
+                        remove_me_dir.string());
+            return false;
+        }
+    }
+    return true;
+}
+
+#endif  // BOOTSTRAP_BUILD_TOOL