lix/src/libstore/gc.cc
Eelco Dolstra b33da599c5 * In the garbage collector, delete invalid paths before deleting
unreachable paths.  This matters when using --max-freed etc.:
  unreachable paths could become reachable again, so it's nicer to
  keep them if there is "real" garbage to be deleted.  Also, don't use
  readDirectory() but read the Nix store and delete invalid paths in
  parallel.  This reduces GC latency on very large Nix stores.
2011-12-22 15:55:53 +00:00

669 lines
22 KiB
C++

#include "globals.hh"
#include "misc.hh"
#include "local-store.hh"
#include <boost/shared_ptr.hpp>
#include <functional>
#include <queue>
#include <algorithm>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
namespace nix {
static string gcLockName = "gc.lock";
static string tempRootsDir = "temproots";
static string gcRootsDir = "gcroots";
static const int defaultGcLevel = 1000;
/* Acquire the global GC lock. This is used to prevent new Nix
processes from starting after the temporary root files have been
read. To be precise: when they try to create a new temporary root
file, they will block until the garbage collector has finished /
yielded the GC lock. */
int LocalStore::openGCLock(LockType lockType)
{
Path fnGCLock = (format("%1%/%2%")
% nixStateDir % gcLockName).str();
debug(format("acquiring global GC lock `%1%'") % fnGCLock);
AutoCloseFD fdGCLock = open(fnGCLock.c_str(), O_RDWR | O_CREAT, 0600);
if (fdGCLock == -1)
throw SysError(format("opening global GC lock `%1%'") % fnGCLock);
if (!lockFile(fdGCLock, lockType, false)) {
printMsg(lvlError, format("waiting for the big garbage collector lock..."));
lockFile(fdGCLock, lockType, true);
}
/* !!! Restrict read permission on the GC root. Otherwise any
process that can open the file for reading can DoS the
collector. */
return fdGCLock.borrow();
}
void createSymlink(const Path & link, const Path & target, bool careful)
{
/* Create directories up to `gcRoot'. */
createDirs(dirOf(link));
/* !!! shouldn't removing and creating the symlink be atomic? */
/* Remove the old symlink. */
if (pathExists(link)) {
if (careful && (!isLink(link) || !isInStore(readLink(link))))
throw Error(format("cannot create symlink `%1%'; already exists") % link);
unlink(link.c_str());
}
/* And create the new one. */
if (symlink(target.c_str(), link.c_str()) == -1)
throw SysError(format("symlinking `%1%' to `%2%'")
% link % target);
}
void LocalStore::syncWithGC()
{
AutoCloseFD fdGCLock = openGCLock(ltRead);
}
void LocalStore::addIndirectRoot(const Path & path)
{
string hash = printHash32(hashString(htSHA1, path));
Path realRoot = canonPath((format("%1%/%2%/auto/%3%")
% nixStateDir % gcRootsDir % hash).str());
createSymlink(realRoot, path, false);
}
Path addPermRoot(StoreAPI & store, const Path & _storePath,
const Path & _gcRoot, bool indirect, bool allowOutsideRootsDir)
{
Path storePath(canonPath(_storePath));
Path gcRoot(canonPath(_gcRoot));
assertStorePath(storePath);
if (isInStore(gcRoot))
throw Error(format(
"creating a garbage collector root (%1%) in the Nix store is forbidden "
"(are you running nix-build inside the store?)") % gcRoot);
if (indirect) {
createSymlink(gcRoot, storePath, true);
store.addIndirectRoot(gcRoot);
}
else {
if (!allowOutsideRootsDir) {
Path rootsDir = canonPath((format("%1%/%2%") % nixStateDir % gcRootsDir).str());
if (string(gcRoot, 0, rootsDir.size() + 1) != rootsDir + "/")
throw Error(format(
"path `%1%' is not a valid garbage collector root; "
"it's not in the directory `%2%'")
% gcRoot % rootsDir);
}
createSymlink(gcRoot, storePath, false);
}
/* Check that the root can be found by the garbage collector.
!!! This can be very slow on machines that have many roots.
Instead of reading all the roots, it would be more efficient to
check if the root is in a directory in or linked from the
gcroots directory. */
if (queryBoolSetting("gc-check-reachability", false)) {
Roots roots = store.findRoots();
if (roots.find(gcRoot) == roots.end())
printMsg(lvlError,
format(
"warning: `%1%' is not in a directory where the garbage collector looks for roots; "
"therefore, `%2%' might be removed by the garbage collector")
% gcRoot % storePath);
}
/* Grab the global GC root, causing us to block while a GC is in
progress. This prevents the set of permanent roots from
increasing while a GC is in progress. */
store.syncWithGC();
return gcRoot;
}
/* The file to which we write our temporary roots. */
static Path fnTempRoots;
static AutoCloseFD fdTempRoots;
void LocalStore::addTempRoot(const Path & path)
{
/* Create the temporary roots file for this process. */
if (fdTempRoots == -1) {
while (1) {
Path dir = (format("%1%/%2%") % nixStateDir % tempRootsDir).str();
createDirs(dir);
fnTempRoots = (format("%1%/%2%")
% dir % getpid()).str();
AutoCloseFD fdGCLock = openGCLock(ltRead);
if (pathExists(fnTempRoots))
/* It *must* be stale, since there can be no two
processes with the same pid. */
deletePath(fnTempRoots);
fdTempRoots = openLockFile(fnTempRoots, true);
fdGCLock.close();
debug(format("acquiring read lock on `%1%'") % fnTempRoots);
lockFile(fdTempRoots, ltRead, true);
/* Check whether the garbage collector didn't get in our
way. */
struct stat st;
if (fstat(fdTempRoots, &st) == -1)
throw SysError(format("statting `%1%'") % fnTempRoots);
if (st.st_size == 0) break;
/* The garbage collector deleted this file before we could
get a lock. (It won't delete the file after we get a
lock.) Try again. */
}
}
/* Upgrade the lock to a write lock. This will cause us to block
if the garbage collector is holding our lock. */
debug(format("acquiring write lock on `%1%'") % fnTempRoots);
lockFile(fdTempRoots, ltWrite, true);
string s = path + '\0';
writeFull(fdTempRoots, (const unsigned char *) s.c_str(), s.size());
/* Downgrade to a read lock. */
debug(format("downgrading to read lock on `%1%'") % fnTempRoots);
lockFile(fdTempRoots, ltRead, true);
}
void removeTempRoots()
{
if (fdTempRoots != -1) {
fdTempRoots.close();
unlink(fnTempRoots.c_str());
}
}
typedef boost::shared_ptr<AutoCloseFD> FDPtr;
typedef list<FDPtr> FDs;
static void readTempRoots(PathSet & tempRoots, FDs & fds)
{
/* Read the `temproots' directory for per-process temporary root
files. */
Strings tempRootFiles = readDirectory(
(format("%1%/%2%") % nixStateDir % tempRootsDir).str());
foreach (Strings::iterator, i, tempRootFiles) {
Path path = (format("%1%/%2%/%3%") % nixStateDir % tempRootsDir % *i).str();
debug(format("reading temporary root file `%1%'") % path);
FDPtr fd(new AutoCloseFD(open(path.c_str(), O_RDWR, 0666)));
if (*fd == -1) {
/* It's okay if the file has disappeared. */
if (errno == ENOENT) continue;
throw SysError(format("opening temporary roots file `%1%'") % path);
}
/* This should work, but doesn't, for some reason. */
//FDPtr fd(new AutoCloseFD(openLockFile(path, false)));
//if (*fd == -1) continue;
/* Try to acquire a write lock without blocking. This can
only succeed if the owning process has died. In that case
we don't care about its temporary roots. */
if (lockFile(*fd, ltWrite, false)) {
printMsg(lvlError, format("removing stale temporary roots file `%1%'")
% path);
unlink(path.c_str());
writeFull(*fd, (const unsigned char *) "d", 1);
continue;
}
/* Acquire a read lock. This will prevent the owning process
from upgrading to a write lock, therefore it will block in
addTempRoot(). */
debug(format("waiting for read lock on `%1%'") % path);
lockFile(*fd, ltRead, true);
/* Read the entire file. */
string contents = readFile(*fd);
/* Extract the roots. */
string::size_type pos = 0, end;
while ((end = contents.find((char) 0, pos)) != string::npos) {
Path root(contents, pos, end - pos);
debug(format("got temporary root `%1%'") % root);
assertStorePath(root);
tempRoots.insert(root);
pos = end + 1;
}
fds.push_back(fd); /* keep open */
}
}
static void findRoots(StoreAPI & store, const Path & path,
bool recurseSymlinks, bool deleteStale, Roots & roots)
{
try {
struct stat st;
if (lstat(path.c_str(), &st) == -1)
throw SysError(format("statting `%1%'") % path);
printMsg(lvlVomit, format("looking at `%1%'") % path);
if (S_ISDIR(st.st_mode)) {
Strings names = readDirectory(path);
foreach (Strings::iterator, i, names)
findRoots(store, path + "/" + *i, recurseSymlinks, deleteStale, roots);
}
else if (S_ISLNK(st.st_mode)) {
Path target = absPath(readLink(path), dirOf(path));
if (isInStore(target)) {
debug(format("found root `%1%' in `%2%'")
% target % path);
Path storePath = toStorePath(target);
if (store.isValidPath(storePath))
roots[path] = storePath;
else
printMsg(lvlInfo, format("skipping invalid root from `%1%' to `%2%'")
% path % storePath);
}
else if (recurseSymlinks) {
if (pathExists(target))
findRoots(store, target, false, deleteStale, roots);
else if (deleteStale) {
printMsg(lvlInfo, format("removing stale link from `%1%' to `%2%'") % path % target);
/* Note that we only delete when recursing, i.e.,
when we are still in the `gcroots' tree. We
never delete stuff outside that tree. */
unlink(path.c_str());
}
}
}
}
catch (SysError & e) {
/* We only ignore permanent failures. */
if (e.errNo == EACCES || e.errNo == ENOENT || e.errNo == ENOTDIR)
printMsg(lvlInfo, format("cannot read potential root `%1%'") % path);
else
throw;
}
}
static Roots findRoots(StoreAPI & store, bool deleteStale)
{
Roots roots;
Path rootsDir = canonPath((format("%1%/%2%") % nixStateDir % gcRootsDir).str());
findRoots(store, rootsDir, true, deleteStale, roots);
return roots;
}
Roots LocalStore::findRoots()
{
return nix::findRoots(*this, false);
}
static void addAdditionalRoots(StoreAPI & store, PathSet & roots)
{
Path rootFinder = getEnv("NIX_ROOT_FINDER",
nixLibexecDir + "/nix/find-runtime-roots.pl");
if (rootFinder.empty()) return;
debug(format("executing `%1%' to find additional roots") % rootFinder);
string result = runProgram(rootFinder);
Strings paths = tokenizeString(result, "\n");
foreach (Strings::iterator, i, paths) {
if (isInStore(*i)) {
Path path = toStorePath(*i);
if (roots.find(path) == roots.end() && store.isValidPath(path)) {
debug(format("got additional root `%1%'") % path);
roots.insert(path);
}
}
}
}
static void dfsVisit(StoreAPI & store, const PathSet & paths,
const Path & path, PathSet & visited, Paths & sorted)
{
if (visited.find(path) != visited.end()) return;
visited.insert(path);
PathSet references;
if (store.isValidPath(path))
store.queryReferences(path, references);
foreach (PathSet::iterator, i, references)
/* Don't traverse into paths that don't exist. That can
happen due to substitutes for non-existent paths. */
if (*i != path && paths.find(*i) != paths.end())
dfsVisit(store, paths, *i, visited, sorted);
sorted.push_front(path);
}
Paths topoSortPaths(StoreAPI & store, const PathSet & paths)
{
Paths sorted;
PathSet visited;
foreach (PathSet::const_iterator, i, paths)
dfsVisit(store, paths, *i, visited, sorted);
return sorted;
}
struct GCLimitReached { };
struct LocalStore::GCState
{
GCOptions options;
GCResults & results;
PathSet roots;
PathSet tempRoots;
PathSet deleted;
PathSet live;
PathSet busy;
bool gcKeepOutputs;
bool gcKeepDerivations;
GCState(GCResults & results_) : results(results_)
{
}
};
static bool shouldDelete(GCOptions::GCAction action)
{
return action == GCOptions::gcDeleteDead
|| action == GCOptions::gcDeleteSpecific;
}
bool LocalStore::isActiveTempFile(const GCState & state,
const Path & path, const string & suffix)
{
return hasSuffix(path, suffix)
&& state.tempRoots.find(string(path, 0, path.size() - suffix.size())) != state.tempRoots.end();
}
bool LocalStore::tryToDelete(GCState & state, const Path & path)
{
checkInterrupt();
if (!pathExists(path)) return true;
if (state.deleted.find(path) != state.deleted.end()) return true;
if (state.live.find(path) != state.live.end()) return false;
startNest(nest, lvlDebug, format("considering whether to delete `%1%'") % path);
if (state.roots.find(path) != state.roots.end()) {
printMsg(lvlDebug, format("cannot delete `%1%' because it's a root") % path);
goto isLive;
}
if (isValidPath(path)) {
/* Recursively try to delete the referrers of this path. If
any referrer can't be deleted, then this path can't be
deleted either. */
PathSet referrers;
queryReferrers(path, referrers);
foreach (PathSet::iterator, i, referrers)
if (*i != path && !tryToDelete(state, *i)) {
printMsg(lvlDebug, format("cannot delete `%1%' because it has live referrers") % path);
goto isLive;
}
/* If gc-keep-derivations is set and this is a derivation,
then don't delete the derivation if any of the outputs are
live. */
if (state.gcKeepDerivations && isDerivation(path)) {
PathSet outputs = queryDerivationOutputs(path);
foreach (PathSet::iterator, i, outputs)
if (!tryToDelete(state, *i)) {
printMsg(lvlDebug, format("cannot delete derivation `%1%' because its output `%2%' is alive") % path % *i);
goto isLive;
}
}
/* If gc-keep-derivations and gc-keep-outputs are both set,
it's possible that the path has already been deleted (due
to the recursion below), so bail out. */
if (!pathExists(path)) return true;
/* If gc-keep-outputs is set, then don't delete this path if
there are derivers of this path that are not garbage. */
if (state.gcKeepOutputs) {
PathSet derivers = queryValidDerivers(path);
foreach (PathSet::iterator, deriver, derivers) {
/* Break an infinite recursion if gc-keep-derivations
and gc-keep-outputs are both set by tentatively
assuming that this path is garbage. This is a safe
assumption because at this point, the only thing
that can prevent it from being garbage is the
deriver. Since tryToDelete() works "upwards"
through the dependency graph, it won't encouter
this path except in the call to tryToDelete() in
the gc-keep-derivation branch. */
state.deleted.insert(path);
if (!tryToDelete(state, *deriver)) {
state.deleted.erase(path);
printMsg(lvlDebug, format("cannot delete `%1%' because its deriver `%2%' is alive") % path % *deriver);
goto isLive;
}
}
}
}
else {
/* A lock file belonging to a path that we're building right
now isn't garbage. */
if (isActiveTempFile(state, path, ".lock")) return false;
/* Don't delete .chroot directories for derivations that are
currently being built. */
if (isActiveTempFile(state, path, ".chroot")) return false;
}
/* The path is garbage, so delete it. */
if (shouldDelete(state.options.action)) {
printMsg(lvlInfo, format("deleting `%1%'") % path);
unsigned long long bytesFreed, blocksFreed;
deleteFromStore(path, bytesFreed, blocksFreed);
state.results.bytesFreed += bytesFreed;
state.results.blocksFreed += blocksFreed;
if (state.options.maxFreed && state.results.bytesFreed > state.options.maxFreed) {
printMsg(lvlInfo, format("deleted more than %1% bytes; stopping") % state.options.maxFreed);
throw GCLimitReached();
}
if (state.options.maxLinks) {
struct stat st;
if (stat(nixStore.c_str(), &st) == -1)
throw SysError(format("statting `%1%'") % nixStore);
if (st.st_nlink < state.options.maxLinks) {
printMsg(lvlInfo, format("link count on the store has dropped below %1%; stopping") % state.options.maxLinks);
throw GCLimitReached();
}
}
} else
printMsg(lvlTalkative, format("would delete `%1%'") % path);
state.deleted.insert(path);
if (state.options.action != GCOptions::gcReturnLive)
state.results.paths.insert(path);
return true;
isLive:
state.live.insert(path);
if (state.options.action == GCOptions::gcReturnLive)
state.results.paths.insert(path);
return false;
}
void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
{
GCState state(results);
state.options = options;
state.gcKeepOutputs = queryBoolSetting("gc-keep-outputs", false);
state.gcKeepDerivations = queryBoolSetting("gc-keep-derivations", true);
/* Using `--ignore-liveness' with `--delete' can have unintended
consequences if `gc-keep-outputs' or `gc-keep-derivations' are
true (the garbage collector will recurse into deleting the
outputs or derivers, respectively). So disable them. */
if (options.action == GCOptions::gcDeleteSpecific && options.ignoreLiveness) {
state.gcKeepOutputs = false;
state.gcKeepDerivations = false;
}
/* Acquire the global GC root. This prevents
a) New roots from being added.
b) Processes from creating new temporary root files. */
AutoCloseFD fdGCLock = openGCLock(ltWrite);
/* Find the roots. Since we've grabbed the GC lock, the set of
permanent roots cannot increase now. */
printMsg(lvlError, format("finding garbage collector roots..."));
Roots rootMap = options.ignoreLiveness ? Roots() : nix::findRoots(*this, true);
foreach (Roots::iterator, i, rootMap) state.roots.insert(i->second);
/* Add additional roots returned by the program specified by the
NIX_ROOT_FINDER environment variable. This is typically used
to add running programs to the set of roots (to prevent them
from being garbage collected). */
if (!options.ignoreLiveness)
addAdditionalRoots(*this, state.roots);
/* Read the temporary roots. This acquires read locks on all
per-process temporary root files. So after this point no paths
can be added to the set of temporary roots. */
FDs fds;
readTempRoots(state.tempRoots, fds);
state.roots.insert(state.tempRoots.begin(), state.tempRoots.end());
/* After this point the set of roots or temporary roots cannot
increase, since we hold locks on everything. So everything
that is not reachable from `roots'. */
/* Now either delete all garbage paths, or just the specified
paths (for gcDeleteSpecific). */
if (options.action == GCOptions::gcDeleteSpecific) {
foreach (PathSet::iterator, i, options.pathsToDelete) {
assertStorePath(*i);
if (!tryToDelete(state, *i))
throw Error(format("cannot delete path `%1%' since it is still alive") % *i);
}
} else {
if (shouldDelete(state.options.action))
printMsg(lvlError, format("deleting garbage..."));
else
printMsg(lvlError, format("determining live/dead paths..."));
try {
AutoCloseDir dir = opendir(nixStore.c_str());
if (!dir) throw SysError(format("opening directory `%1%'") % nixStore);
/* Read the store and immediately delete all paths that
aren't valid. When using --max-freed etc., deleting
invalid paths is preferred over deleting unreachable
paths, since unreachable paths could become reachable
again. We don't use readDirectory() here so that GCing
can start faster. */
Paths entries;
struct dirent * dirent;
while (errno = 0, dirent = readdir(dir)) {
checkInterrupt();
string name = dirent->d_name;
if (name == "." || name == "..") continue;
Path path = nixStore + "/" + name;
if (isValidPath(path))
entries.push_back(path);
else
tryToDelete(state, path);
}
dir.close();
/* Now delete the unreachable valid paths. Randomise the
order in which we delete entries to make the collector
less biased towards deleting paths that come
alphabetically first (e.g. /nix/store/000...). This
matters when using --max-freed etc. */
vector<Path> entries_(entries.begin(), entries.end());
random_shuffle(entries_.begin(), entries_.end());
foreach (vector<Path>::iterator, i, entries_)
tryToDelete(state, nixStore + "/" + *i);
} catch (GCLimitReached & e) {
}
}
}
}