[llvm] Support: Add vfs::OutputBackend and OutputFile to virtualize compiler outputs (PR #113363)

[Alexandre Ganea via llvm-commits]

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@@ -0,0 +1,598 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file implements the VirtualOutputBackend types, including:
+/// * NullOutputBackend: Outputs to NullOutputBackend are discarded.
+/// * FilteringOutputBackend: Filter paths from output.
+/// * MirroringOutputBackend: Mirror the output into two different backend.
+/// * OnDiskOutputBackend: Write output files to disk.
+///
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/VirtualOutputBackends.h"
+#include "llvm/ADT/ScopeExit.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/LockFileManager.h"
+#include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/Path.h"
+#include "llvm/Support/Process.h"
+#include "llvm/Support/Signals.h"
+#include "llvm/Support/VirtualOutputConfig.h"
+#include "llvm/Support/VirtualOutputError.h"
+
+using namespace llvm;
+using namespace llvm::vfs;
+
+void ProxyOutputBackend::anchor() {}
+void OnDiskOutputBackend::anchor() {}
+
+IntrusiveRefCntPtr<OutputBackend> vfs::makeNullOutputBackend() {
+  struct NullOutputBackend : public OutputBackend {
+    IntrusiveRefCntPtr<OutputBackend> cloneImpl() const override {
+      return const_cast<NullOutputBackend *>(this);
+    }
+    Expected<std::unique_ptr<OutputFileImpl>>
+    createFileImpl(StringRef Path, std::optional<OutputConfig>) override {
+      return std::make_unique<NullOutputFileImpl>();
+    }
+  };
+
+  return makeIntrusiveRefCnt<NullOutputBackend>();
+}
+
+IntrusiveRefCntPtr<OutputBackend> vfs::makeFilteringOutputBackend(
+    IntrusiveRefCntPtr<OutputBackend> UnderlyingBackend,
+    std::function<bool(StringRef, std::optional<OutputConfig>)> Filter) {
+  struct FilteringOutputBackend : public ProxyOutputBackend {
+    Expected<std::unique_ptr<OutputFileImpl>>
+    createFileImpl(StringRef Path,
+                   std::optional<OutputConfig> Config) override {
+      if (Filter(Path, Config))
+        return ProxyOutputBackend::createFileImpl(Path, Config);
+      return std::make_unique<NullOutputFileImpl>();
+    }
+
+    IntrusiveRefCntPtr<OutputBackend> cloneImpl() const override {
+      return makeIntrusiveRefCnt<FilteringOutputBackend>(
+          getUnderlyingBackend().clone(), Filter);
+    }
+
+    FilteringOutputBackend(
+        IntrusiveRefCntPtr<OutputBackend> UnderlyingBackend,
+        std::function<bool(StringRef, std::optional<OutputConfig>)> Filter)
+        : ProxyOutputBackend(std::move(UnderlyingBackend)),
+          Filter(std::move(Filter)) {
+      assert(this->Filter && "Expected a non-null function");
+    }
+    std::function<bool(StringRef, std::optional<OutputConfig>)> Filter;
+  };
+
+  return makeIntrusiveRefCnt<FilteringOutputBackend>(
+      std::move(UnderlyingBackend), std::move(Filter));
+}
+
+IntrusiveRefCntPtr<OutputBackend>
+vfs::makeMirroringOutputBackend(IntrusiveRefCntPtr<OutputBackend> Backend1,
+                                IntrusiveRefCntPtr<OutputBackend> Backend2) {
+  struct ProxyOutputBackend1 : public ProxyOutputBackend {
+    using ProxyOutputBackend::ProxyOutputBackend;
+  };
+  struct ProxyOutputBackend2 : public ProxyOutputBackend {
+    using ProxyOutputBackend::ProxyOutputBackend;
+  };
+  struct MirroringOutput final : public OutputFileImpl, raw_pwrite_stream {
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aganea wrote:

Can you please comment this class or bikeshed the naming a bit? It is seems it's more about demultiplexing, or writing concurently (from the API perspective) to several files. 

https://github.com/llvm/llvm-project/pull/113363