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[8.8.0] Make naming scheme for repo contents cache entries more reliable #29073

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[8.8.0] Make naming scheme for repo contents cache entries more reliable #29073

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78 changes: 27 additions & 51 deletions src/main/java/com/google/devtools/build/lib/bazel/repository/cache/RepoContentsCache.java
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Original file line number Diff line number Diff line change
Expand Up @@ -31,10 +31,10 @@
import com.google.devtools.build.lib.vfs.Path;
import com.google.devtools.build.lib.vfs.Symlinks;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.time.Duration;
import java.time.Instant;
import java.util.Comparator;
import java.util.HashMap;
import java.util.HashSet;
import java.util.UUID;
import javax.annotation.Nullable;
Expand All @@ -47,12 +47,9 @@
* transitive bzl digest, repo attrs, starlark semantics, etc). Each distinct predeclared inputs
* hash is its own entry directory in the first layer.
*
* <p>Inside each entry directory are pairs of directories and files {@code <N, N.recorded_inputs>}
* where {@code N} is an integer. The file {@code N.recorded_inputs} contains the recorded inputs
* and their values of a cached repo, and the directory {@code N} contains the cached repo contents.
* There is also a file named {@code counter} that stores the next available {@code N} for this
* entry directory, and a file named {@code lock} to ensure exclusive access to the {@code counter}
* file.
* <p>Inside each entry directory are pairs of directories and files {@code <UUID,
* UUID.recorded_inputs>}. The file {@code UUID.recorded_inputs} contains the recorded inputs and
* their values of a cached repo, and the directory {@code UUID} contains the cached repo contents.
*
* <p>On a cache hit (that is, the predeclared inputs hash matches, and recorded inputs are
* up-to-date), the recorded inputs file has its mtime updated. Cached repos whose recorded inputs
Expand Down Expand Up @@ -124,17 +121,19 @@ public ImmutableList<CandidateRepo> getCandidateRepos(String predeclaredInputHas
Preconditions.checkState(path != null);
Path entryDir = path.getRelative(predeclaredInputHash);
try {
// Prefer more recently used cache entries over older ones. They're more likely to be
// up-to-date; plus, if a repo is force-fetched, we want to use the new repo instead of always
// being stuck with the old one. Since the inputs file is touched on use, we can just sort by
// mtime. This is slightly more complex than in runGc below as the files may be touched
// concurrently and we need to ensure that the equality relation is consistent.
var mtimes = new HashMap<Path, Long>();
return entryDir.getDirectoryEntries().stream()
.filter(path -> path.getBaseName().endsWith(RECORDED_INPUTS_SUFFIX))
// Prefer newer cache entries over older ones. They're more likely to be up-to-date; plus,
// if a repo is force-fetched, we want to use the new repo instead of always being stuck
// with the old one.
// To "prefer newer cache entries", we sort the entry file names by length DESC and then
// lexicographically DESC. This approximates sorting by converting to int and then DESC,
// but is defensive against non-numerically named entries.
.sorted(
Comparator.comparing((Path path) -> path.getBaseName().length())
.thenComparing(Path::getBaseName)
Comparator.comparingLong(
(Path path) ->
mtimes.computeIfAbsent(
path, RepoContentsCache::getLastModifiedTimeOrZero))
.reversed())
.map(CandidateRepo::fromRecordedInputsFile)
.collect(toImmutableList());
Expand Down Expand Up @@ -163,9 +162,9 @@ public Path moveToCache(
Preconditions.checkState(path != null);

Path entryDir = path.getRelative(predeclaredInputHash);
String counter = getNextCounterInDir(entryDir);
Path cacheRecordedInputsFile = entryDir.getChild(counter + RECORDED_INPUTS_SUFFIX);
Path cacheRepoDir = entryDir.getChild(counter);
String uniqueEntryName = UUID.randomUUID().toString();
Path cacheRecordedInputsFile = entryDir.getChild(uniqueEntryName + RECORDED_INPUTS_SUFFIX);
Path cacheRepoDir = entryDir.getChild(uniqueEntryName);

cacheRepoDir.deleteTree();
cacheRepoDir.getParentDirectory().createDirectoryAndParents();
Expand All @@ -187,27 +186,6 @@ public Path moveToCache(
return cacheRepoDir;
}

private static String getNextCounterInDir(Path entryDir)
throws IOException, InterruptedException {
Path counterFile = entryDir.getRelative("counter");
// This use of FileSystemLock.get is safe since the predeclared input hash is part of entryDir's
// path and in particular includes the canonical repository name. This ensures that the same
// lock file will not be acquired concurrently by multiple threads, which isn't supported.
try (var lock = FileSystemLock.get(entryDir.getRelative("lock"), LockMode.EXCLUSIVE)) {
int c = 0;
if (counterFile.exists()) {
try {
c = Integer.parseInt(FileSystemUtils.readContent(counterFile, StandardCharsets.UTF_8));
} catch (NumberFormatException e) {
// ignored
}
}
String counter = Integer.toString(c + 1);
FileSystemUtils.writeContent(counterFile, StandardCharsets.UTF_8, counter);
return counter;
}
}

public void acquireSharedLock() throws IOException, InterruptedException {
Preconditions.checkState(path != null);
Preconditions.checkState(sharedLock == null, "this process already has the shared lock");
Expand Down Expand Up @@ -273,18 +251,7 @@ private void runGc(Duration maxAge) throws InterruptedException, IOException {
var recordedInputsFiles =
path.getChild(dirent.getName()).getDirectoryEntries().stream()
.filter(file -> file.getBaseName().endsWith(RECORDED_INPUTS_SUFFIX))
.sorted(
comparingLong(
(Path path) -> {
try {
return path.getLastModifiedTime();
} catch (IOException e) {
// If we can't read the mtime from the entry, it's broken and treated
// as outdated.
return 0;
}
})
.reversed())
.sorted(comparingLong(RepoContentsCache::getLastModifiedTimeOrZero).reversed())
.collect(toImmutableList());
var seen = new HashSet<HashCode>();
for (Path recordedInputsFile : recordedInputsFiles) {
Expand All @@ -305,4 +272,13 @@ private void runGc(Duration maxAge) throws InterruptedException, IOException {
}
}
}

private static long getLastModifiedTimeOrZero(Path path) {
try {
return path.getLastModifiedTime();
} catch (IOException e) {
// If we can't read the mtime from the entry, it's broken and treated as outdated.
return 0L;
}
}
}