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+// Copyright 2022 The Abseil Authors.
+//
+// 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
+//
+//      https://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.
+//
+// -----------------------------------------------------------------------------
+// File: log/log_sink.h
+// -----------------------------------------------------------------------------
+//
+// This header declares the interface class `absl::LogSink`.
+
+#ifndef ABSL_LOG_LOG_SINK_H_
+#define ABSL_LOG_LOG_SINK_H_
+
+#include "absl/base/config.h"
+#include "absl/log/log_entry.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// absl::LogSink
+//
+// `absl::LogSink` is an interface which can be extended to intercept and
+// process particular messages (with `LOG.ToSinkOnly()` or
+// `LOG.ToSinkAlso()`) or all messages (if registered with
+// `absl::AddLogSink`).  Implementations must be thread-safe, and should take
+// care not to take any locks that might be held by the `LOG` caller.
+class LogSink {
+ public:
+  virtual ~LogSink() = default;
+
+  // LogSink::Send()
+  //
+  // `Send` is called synchronously during the log statement.
+  //
+  // It is safe to use `LOG` within an implementation of `Send`.  `ToSinkOnly`
+  // and `ToSinkAlso` are safe in general but can be used to create an infinite
+  // loop if you try.
+  virtual void Send(const absl::LogEntry& entry) = 0;
+
+  // LogSink::Flush()
+  //
+  // Sinks that buffer messages should override this method to flush the buffer
+  // and return.
+  virtual void Flush() {}
+
+ private:
+  // https://lld.llvm.org/missingkeyfunction.html#missing-key-function
+  virtual void KeyFunction() const final;  // NOLINT(readability/inheritance)
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_LOG_LOG_SINK_H_