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PhabricatorRepositoryGraphCache.php
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Tue, Nov 19, 01:14

PhabricatorRepositoryGraphCache.php
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<?php
/**
* Given a commit and a path, efficiently determine the most recent ancestor
* commit where the path was touched.
*
* In Git and Mercurial, log operations with a path are relatively slow. For
* example:
*
* git log -n1 <commit> -- <path>
*
* ...routinely takes several hundred milliseconds, and equivalent requests
* often take longer in Mercurial.
*
* Unfortunately, this operation is fundamental to rendering a repository for
* the web, and essentially everything else that's slow can be reduced to this
* plus some trivial work afterward. Making this fast is desirable and powerful,
* and allows us to make other things fast by expressing them in terms of this
* query.
*
* Because the query is fundamentally a graph query, it isn't easy to express
* in a reasonable way in MySQL, and we can't do round trips to the server to
* walk the graph without incurring huge performance penalties.
*
* However, the total amount of data in the graph is relatively small. By
* caching it in chunks and keeping it in APC, we can reasonably load and walk
* the graph in PHP quickly.
*
* For more context, see T2683.
*
* Structure of the Cache
* ======================
*
* The cache divides commits into buckets (see @{method:getBucketSize}). To
* walk the graph, we pull a commit's bucket. The bucket is a map from commit
* IDs to a list of parents and changed paths, separated by `null`. For
* example, a bucket might look like this:
*
* array(
* 1 => array(0, null, 17, 18),
* 2 => array(1, null, 4),
* // ...
* )
*
* This means that commit ID 1 has parent commit 0 (a special value meaning
* no parents) and affected path IDs 17 and 18. Commit ID 2 has parent commit 1,
* and affected path 4.
*
* This data structure attempts to balance compactness, ease of construction,
* simplicity of cache semantics, and lookup performance. In the average case,
* it appears to do a reasonable job at this.
*
* @task query Querying the Graph Cache
* @task cache Cache Internals
*/
final class PhabricatorRepositoryGraphCache extends Phobject {
private $rebuiltKeys = array();
/* -( Querying the Graph Cache )------------------------------------------- */
/**
* Search the graph cache for the most modification to a path.
*
* @param int The commit ID to search ancestors of.
* @param int The path ID to search for changes to.
* @param float Maximum number of seconds to spend trying to satisfy this
* query using the graph cache. By default, `0.5` (500ms).
* @return mixed Commit ID, or `null` if no ancestors exist, or `false` if
* the graph cache was unable to determine the answer.
* @task query
*/
public function loadLastModifiedCommitID($commit_id, $path_id, $time = 0.5) {
$commit_id = (int)$commit_id;
$path_id = (int)$path_id;
$bucket_data = null;
$data_key = null;
$seen = array();
$t_start = microtime(true);
$iterations = 0;
while (true) {
$bucket_key = $this->getBucketKey($commit_id);
if (($data_key != $bucket_key) || $bucket_data === null) {
$bucket_data = $this->getBucketData($bucket_key);
$data_key = $bucket_key;
}
if (empty($bucket_data[$commit_id])) {
// Rebuild the cache bucket, since the commit might be a very recent
// one that we'll pick up by rebuilding.
$bucket_data = $this->getBucketData($bucket_key, $bucket_data);
if (empty($bucket_data[$commit_id])) {
// A rebuild didn't help. This can occur legitimately if the commit
// is new and hasn't parsed yet.
return false;
}
// Otherwise, the rebuild gave us the data, so we can keep going.
$did_fill = true;
} else {
$did_fill = false;
}
// Sanity check so we can survive and recover from bad data.
if (isset($seen[$commit_id])) {
phlog(pht('Unexpected infinite loop in %s!', __CLASS__));
return false;
} else {
$seen[$commit_id] = true;
}
// `$data` is a list: the commit's parent IDs, followed by `null`,
// followed by the modified paths in ascending order. We figure out the
// first parent first, then check if the path was touched. If the path
// was touched, this is the commit we're after. If not, walk backward
// in the tree.
$items = $bucket_data[$commit_id];
$size = count($items);
// Walk past the parent information.
$parent_id = null;
for ($ii = 0;; ++$ii) {
if ($items[$ii] === null) {
break;
}
if ($parent_id === null) {
$parent_id = $items[$ii];
}
}
// Look for a modification to the path.
for (; $ii < $size; ++$ii) {
$item = $items[$ii];
if ($item > $path_id) {
break;
}
if ($item === $path_id) {
return $commit_id;
}
}
if ($parent_id) {
$commit_id = $parent_id;
// Periodically check if we've spent too long looking for a result
// in the cache, and return so we can fall back to a VCS operation.
// This keeps us from having a degenerate worst case if, e.g., the
// cache is cold and we need to inspect a very large number of blocks
// to satisfy the query.
++$iterations;
// If we performed a cache fill in this cycle, always check the time
// limit, since cache fills may take a significant amount of time.
if ($did_fill || ($iterations % 64 === 0)) {
$t_end = microtime(true);
if (($t_end - $t_start) > $time) {
return false;
}
}
continue;
}
// If we have an explicit 0, that means this commit really has no parents.
// Usually, it is the first commit in the repository.
if ($parent_id === 0) {
return null;
}
// If we didn't find a parent, the parent data isn't available. We fail
// to find an answer in the cache and fall back to querying the VCS.
return false;
}
}
/* -( Cache Internals )---------------------------------------------------- */
/**
* Get the bucket key for a given commit ID.
*
* @param int Commit ID.
* @return int Bucket key.
* @task cache
*/
private function getBucketKey($commit_id) {
return (int)floor($commit_id / $this->getBucketSize());
}
/**
* Get the cache key for a given bucket key (from @{method:getBucketKey}).
*
* @param int Bucket key.
* @return string Cache key.
* @task cache
*/
private function getBucketCacheKey($bucket_key) {
static $prefix;
if ($prefix === null) {
$self = get_class($this);
$size = $this->getBucketSize();
$prefix = "{$self}:{$size}:2:";
}
return $prefix.$bucket_key;
}
/**
* Get the number of items per bucket.
*
* @return int Number of items to store per bucket.
* @task cache
*/
private function getBucketSize() {
return 4096;
}
/**
* Retrieve or build a graph cache bucket from the cache.
*
* Normally, this operates as a readthrough cache call. It can also be used
* to force a cache update by passing the existing data to `$rebuild_data`.
*
* @param int Bucket key, from @{method:getBucketKey}.
* @param mixed Current data, to force a cache rebuild of this bucket.
* @return array Data from the cache.
* @task cache
*/
private function getBucketData($bucket_key, $rebuild_data = null) {
$cache_key = $this->getBucketCacheKey($bucket_key);
// TODO: This cache stuff could be handled more gracefully, but the
// database cache currently requires values to be strings and needs
// some tweaking to support this as part of a stack. Our cache semantics
// here are also unusual (not purely readthrough) because this cache is
// appendable.
$cache_level1 = PhabricatorCaches::getRepositoryGraphL1Cache();
$cache_level2 = PhabricatorCaches::getRepositoryGraphL2Cache();
if ($rebuild_data === null) {
$bucket_data = $cache_level1->getKey($cache_key);
if ($bucket_data) {
return $bucket_data;
}
$bucket_data = $cache_level2->getKey($cache_key);
if ($bucket_data) {
$unserialized = @unserialize($bucket_data);
if ($unserialized) {
// Fill APC if we got a database hit but missed in APC.
$cache_level1->setKey($cache_key, $unserialized);
return $unserialized;
}
}
}
if (!is_array($rebuild_data)) {
$rebuild_data = array();
}
$bucket_data = $this->rebuildBucket($bucket_key, $rebuild_data);
// Don't bother writing the data if we didn't update anything.
if ($bucket_data !== $rebuild_data) {
$cache_level2->setKey($cache_key, serialize($bucket_data));
$cache_level1->setKey($cache_key, $bucket_data);
}
return $bucket_data;
}
/**
* Rebuild a cache bucket, amending existing data if available.
*
* @param int Bucket key, from @{method:getBucketKey}.
* @param array Existing bucket data.
* @return array Rebuilt bucket data.
* @task cache
*/
private function rebuildBucket($bucket_key, array $current_data) {
// First, check if we've already rebuilt this bucket. In some cases (like
// browsing a repository at some commit) it's common to issue many lookups
// against one commit. If that commit has been discovered but not yet
// fully imported, we'll repeatedly attempt to rebuild the bucket. If the
// first rebuild did not work, subsequent rebuilds are very unlikely to
// have any effect. We can just skip the rebuild in these cases.
if (isset($this->rebuiltKeys[$bucket_key])) {
return $current_data;
} else {
$this->rebuiltKeys[$bucket_key] = true;
}
$bucket_min = ($bucket_key * $this->getBucketSize());
$bucket_max = ($bucket_min + $this->getBucketSize()) - 1;
// We need to reload all of the commits in the bucket because there is
// no guarantee that they'll get parsed in order, so we can fill large
// commit IDs before small ones. Later on, we'll ignore the commits we
// already know about.
$table_commit = new PhabricatorRepositoryCommit();
$table_repository = new PhabricatorRepository();
$conn_r = $table_commit->establishConnection('r');
// Find all the Git and Mercurial commits in the block which have completed
// change import. We can't fill the cache accurately for commits which have
// not completed change import, so just pretend we don't know about them.
// In these cases, we will ultimately fall back to VCS queries.
$commit_rows = queryfx_all(
$conn_r,
'SELECT c.id FROM %T c
JOIN %T r ON c.repositoryID = r.id AND r.versionControlSystem IN (%Ls)
WHERE c.id BETWEEN %d AND %d
AND (c.importStatus & %d) = %d',
$table_commit->getTableName(),
$table_repository->getTableName(),
array(
PhabricatorRepositoryType::REPOSITORY_TYPE_GIT,
PhabricatorRepositoryType::REPOSITORY_TYPE_MERCURIAL,
),
$bucket_min,
$bucket_max,
PhabricatorRepositoryCommit::IMPORTED_CHANGE,
PhabricatorRepositoryCommit::IMPORTED_CHANGE);
// If we don't have any data, just return the existing data.
if (!$commit_rows) {
return $current_data;
}
// Remove the commits we already have data for. We don't need to rebuild
// these. If there's nothing left, return the existing data.
$commit_ids = ipull($commit_rows, 'id', 'id');
$commit_ids = array_diff_key($commit_ids, $current_data);
if (!$commit_ids) {
return $current_data;
}
// Find all the path changes for the new commits.
$path_changes = queryfx_all(
$conn_r,
'SELECT commitID, pathID FROM %T
WHERE commitID IN (%Ld)
AND (isDirect = 1 OR changeType = %d)',
PhabricatorRepository::TABLE_PATHCHANGE,
$commit_ids,
DifferentialChangeType::TYPE_CHILD);
$path_changes = igroup($path_changes, 'commitID');
// Find all the parents for the new commits.
$parents = queryfx_all(
$conn_r,
'SELECT childCommitID, parentCommitID FROM %T
WHERE childCommitID IN (%Ld)
ORDER BY id ASC',
PhabricatorRepository::TABLE_PARENTS,
$commit_ids);
$parents = igroup($parents, 'childCommitID');
// Build the actual data for the cache.
foreach ($commit_ids as $commit_id) {
$parent_ids = array();
if (!empty($parents[$commit_id])) {
foreach ($parents[$commit_id] as $row) {
$parent_ids[] = (int)$row['parentCommitID'];
}
} else {
// We expect all rows to have parents (commits with no parents get
// an explicit "0" placeholder). If we're in an older repository, the
// parent information might not have been populated yet. Decline to fill
// the cache if we don't have the parent information, since the fill
// will be incorrect.
continue;
}
if (isset($path_changes[$commit_id])) {
$path_ids = $path_changes[$commit_id];
foreach ($path_ids as $key => $path_id) {
$path_ids[$key] = (int)$path_id['pathID'];
}
sort($path_ids);
} else {
$path_ids = array();
}
$value = $parent_ids;
$value[] = null;
foreach ($path_ids as $path_id) {
$value[] = $path_id;
}
$current_data[$commit_id] = $value;
}
return $current_data;
}
}

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