1 files changed, 105 insertions, 0 deletions
diff --git a/src/main/java/com/google/devtools/build/lib/util/LogHandlerQuerier.java b/src/main/java/com/google/devtools/build/lib/util/LogHandlerQuerier.java
new file mode 100644
index 0000000000..1af8b2aec5
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/util/LogHandlerQuerier.java
@@ -0,0 +1,105 @@
+// Copyright 2018 The Bazel Authors. All rights reserved.
+//
+// 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.
+
+package com.google.devtools.build.lib.util;
+
+import static com.google.common.base.Preconditions.checkState;
+
+import com.google.common.base.Supplier;
+import com.google.common.base.Suppliers;
+import com.google.errorprone.annotations.ForOverride;
+import java.nio.file.Path;
+import java.util.Optional;
+import java.util.logging.Handler;
+import java.util.logging.Logger;
+
+/**
+ * A retriever for logging handler properties, e.g. the log file path.
+ *
+ * <p>A querier is intended for situations where a logging handler is configured on the JVM command
+ * line, and where the code which needs to query the handler does not know the handler's class or
+ * cannot import it. The command line then should in addition specify an appropriate child class of
+ * {@link LogHandlerQuerier} via the {@code
+ * -Dcom.google.devtools.build.lib.util.LogHandlerQuerier.class} flag, and an instance of that
+ * appropriate child class can be obtained from {@code LogHandlerQuerier.getInstance()}.
+ */
+public abstract class LogHandlerQuerier {
+  private static final Supplier<LogHandlerQuerier> configuredInstanceSupplier =
+      Suppliers.memoize(LogHandlerQuerier::makeConfiguredInstance);
+
+  private static LogHandlerQuerier makeConfiguredInstance() {
+    String propertyName = LogHandlerQuerier.class.getName() + ".class";
+    String subclassName = System.getProperty(propertyName);
+    checkState(subclassName != null, "System property %s is not defined", propertyName);
+    try {
+      return (LogHandlerQuerier)
+          ClassLoader.getSystemClassLoader()
+              .loadClass(subclassName)
+              .getDeclaredConstructor()
+              .newInstance();
+    } catch (ReflectiveOperationException e) {
+      throw new IllegalStateException("System property " + propertyName + " value is invalid", e);
+    }
+  }
+
+  /**
+   * Returns the singleton instance of the LogHandlerQuerier child class which was configured as a
+   * system property on the JVM command line via the {@code
+   * -Dcom.google.devtools.build.lib.util.LogHandlerQuerier.class} flag.
+   *
+   * <p>This method is thread-safe.
+   *
+   * @throws IllegalStateException if the JVM property was not defined or if an instance of the
+   *     class named by the property could not be constructed
+   */
+  public static LogHandlerQuerier getConfiguredInstance() {
+    return configuredInstanceSupplier.get();
+  }
+
+  /**
+   * Returns a logger's handler's log file path, iterating through all handlers and the logger's
+   * ancestors' handlers as necessary.
+   *
+   * <p>The method will stop iterating at the first log handler that it can query, returning the log
+   * path if it is available for that log handler, or an empty {@link Optional} if the log file for
+   * that handler is currently unavailable.
+   *
+   * @param logger a logger whose handlers, and ancestors' handlers if necessary, will be queried
+   * @throws IllegalArgumentException if the {@link LogHandlerQuerier} cannot query any {@link
+   *     Handler} for this logger or its ancestors
+   */
+  public Optional<Path> getLoggerFilePath(Logger logger) {
+    for (; logger != null; logger = logger.getParent()) {
+      for (Handler handler : logger.getHandlers()) {
+        if (canQuery(handler)) {
+          return getLogHandlerFilePath(handler);
+        }
+      }
+    }
+    throw new IllegalArgumentException("Failed to find a queryable logging handler");
+  }
+
+  /** Checks if this {@link LogHandlerQuerier} can query the given handler. */
+  @ForOverride
+  protected abstract boolean canQuery(Handler handler);
+
+  /**
+   * Returns a logging handler's log file path.
+   *
+   * @param handler logging handler to query
+   * @return the log handler's log file path if the log file is currently available
+   */
+  @ForOverride
+  protected abstract Optional<Path> getLogHandlerFilePath(Handler handler);
+}