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// Copyright 2014 Google Inc. 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.annotations.VisibleForTesting;
import com.google.common.base.Preconditions;
import com.google.devtools.build.lib.util.Fingerprint;
import com.google.devtools.build.lib.util.VarInt;
import java.io.IOException;
import java.io.OutputStream;
import java.nio.ByteBuffer;
import java.util.Arrays;
import java.util.Map;
/**
* A value class for capturing and comparing MD5-based digests.
*
* <p>Note that this class is responsible for digesting file metadata in an
* order-independent manner. Care must be taken to do this properly. The
* digest must be a function of the set of (path, metadata) tuples. While the
* order of these pairs must not matter, it would <b>not</b> be safe to make
* the digest be a function of the set of paths and the set of metadata.
*
* <p>Note that the (path, metadata) tuples must be unique, otherwise the
* XOR-based approach will fail.
*/
public class Digest {
static final int MD5_SIZE = 16;
private final byte[] digest;
/**
* Construct the digest from the given bytes.
* @param digest an MD5 digest. Must be sized properly.
*/
@VisibleForTesting
Digest(byte[] digest) {
Preconditions.checkState(digest.length == MD5_SIZE);
this.digest = digest.clone();
}
/**
* @param source the byte buffer source.
* @return the digest from the given buffer.
* @throws IOException if the byte buffer is incorrectly formatted.
*/
public static Digest read(ByteBuffer source) throws IOException {
int size = VarInt.getVarInt(source);
if (size != MD5_SIZE) {
throw new IOException("Unexpected digest length: " + size);
}
byte[] bytes = new byte[size];
source.get(bytes);
return new Digest(bytes);
}
/**
* Write the digest to the output stream.
*/
public void write(OutputStream sink) throws IOException {
VarInt.putVarInt(digest.length, sink);
sink.write(digest);
}
/**
* @param mdMap A collection of (execPath, Metadata) pairs.
* Values may be null.
* @return an <b>order-independent</b> digest from the given "set" of
* (path, metadata) pairs.
*/
public static Digest fromMetadata(Map<String, Metadata> mdMap) {
byte[] result = new byte[MD5_SIZE];
// Profiling showed that MD5 engine instantiation was a hotspot, so create one instance for
// this computation to amortize its cost.
Fingerprint fp = new Fingerprint();
for (Map.Entry<String, Metadata> entry : mdMap.entrySet()) {
xorWith(result, getDigest(fp, entry.getKey(), entry.getValue()));
fp.reset();
}
return new Digest(result);
}
/**
* @return this Digest as a Metadata with no mtime.
*/
public Metadata asMetadata() {
return new Metadata(digest);
}
@Override
public int hashCode() {
return Arrays.hashCode(digest);
}
@Override
public boolean equals(Object obj) {
return (obj instanceof Digest) && Arrays.equals(digest, ((Digest) obj).digest);
}
@Override
public String toString() {
return Fingerprint.hexDigest(digest);
}
private static byte[] getDigest(Fingerprint fp, String execPath, Metadata md) {
fp.addString(execPath);
if (md == null) {
// Move along, nothing to see here.
} else if (md.digest == null) {
// Use the timestamp if the digest is not present, but not both.
// Modifying a timestamp while keeping the contents of a file the
// same should not cause rebuilds.
fp.addLong(md.mtime);
} else {
fp.addBytes(md.digest);
}
return fp.digestAndReset();
}
/**
* Compute lhs ^= rhs bitwise operation of the arrays.
*/
private static void xorWith(byte[] lhs, byte[] rhs) {
for (int i = 0; i < lhs.length; i++) {
lhs[i] ^= rhs[i];
}
}
}
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