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// Copyright 2014 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.actions.cache;
import com.google.common.base.Preconditions;
import com.google.common.io.BaseEncoding;
import com.google.devtools.build.lib.profiler.Profiler;
import com.google.devtools.build.lib.profiler.ProfilerTask;
import com.google.devtools.build.lib.util.BlazeClock;
import com.google.devtools.build.lib.util.LoggingUtil;
import com.google.devtools.build.lib.vfs.Path;
import java.io.IOException;
import java.util.Objects;
import java.util.logging.Level;
import javax.annotation.Nullable;
/**
* Utility class for getting md5 digests of files.
*/
public class DigestUtils {
// Object to synchronize on when serializing large file reads.
private static final Object MD5_LOCK = new Object();
/** Private constructor to prevent instantiation of utility class. */
private DigestUtils() {}
/**
* Returns true iff using MD5 digests is appropriate for an artifact.
*
* @param isFile whether or not Artifact is a file versus a directory, isFile() on its stat.
* @param size size of Artifact on filesystem in bytes, getSize() on its stat.
*/
public static boolean useFileDigest(boolean isFile, long size) {
// Use timestamps for directories. Use digests for everything else.
return isFile && size != 0;
}
/**
* Obtain file's MD5 metadata using synchronized method, ensuring that system
* is not overloaded in case when multiple threads are requesting MD5
* calculations and underlying file system cannot provide it via extended
* attribute.
*/
private static byte[] getDigestInExclusiveMode(Path path) throws IOException {
long startTime = BlazeClock.nanoTime();
synchronized (MD5_LOCK) {
Profiler.instance().logSimpleTask(startTime, ProfilerTask.WAIT, path.getPathString());
return getDigestInternal(path);
}
}
private static byte[] getDigestInternal(Path path) throws IOException {
long startTime = BlazeClock.nanoTime();
byte[] md5bin = path.getMD5Digest();
long millis = (BlazeClock.nanoTime() - startTime) / 1000000;
if (millis > 5000L) {
System.err.println("Slow read: a " + path.getFileSize() + "-byte read from " + path
+ " took " + millis + "ms.");
}
return md5bin;
}
private static boolean binaryDigestWellFormed(byte[] digest) {
Preconditions.checkNotNull(digest);
return digest.length == 16;
}
/**
* Returns the the fast md5 digest of the file, or null if not available.
*/
@Nullable
public static byte[] getFastDigest(Path path) throws IOException {
return path.getFastDigestFunctionType().equals("MD5") ? path.getFastDigest() : null;
}
/**
* Get the md5 digest of {@code path}, using a constant-time xattr call if the filesystem supports
* it, and calculating the digest manually otherwise.
*
* @param path Path of the file.
* @param fileSize size of the file. Used to determine if digest calculation should be done
* serially or in parallel. Files larger than a certain threshold will be read serially, in order
* to avoid excessive disk seeks.
*/
public static byte[] getDigestOrFail(Path path, long fileSize) throws IOException {
// TODO(bazel-team): the action cache currently only works with md5 digests but it ought to
// work with any opaque digest.
byte[] md5bin = null;
if (Objects.equals(path.getFastDigestFunctionType(), "MD5")) {
md5bin = getFastDigest(path);
}
if (md5bin != null && !binaryDigestWellFormed(md5bin)) {
// Fail-soft in cases where md5bin is non-null, but not a valid digest.
String msg = String.format("Malformed digest '%s' for file %s",
BaseEncoding.base16().lowerCase().encode(md5bin),
path);
LoggingUtil.logToRemote(Level.SEVERE, msg, new IllegalStateException(msg));
md5bin = null;
}
if (md5bin != null) {
return md5bin;
} else if (fileSize > 4096) {
// We'll have to read file content in order to calculate the digest. In that case
// it would be beneficial to serialize those calculations since there is a high
// probability that MD5 will be requested for multiple output files simultaneously.
// Exception is made for small (<=4K) files since they will not likely to introduce
// significant delays (at worst they will result in two extra disk seeks by
// interrupting other reads).
return getDigestInExclusiveMode(path);
} else {
return getDigestInternal(path);
}
}
}
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