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Use BSD instead of POSIX file locks

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POSIX file locks are essentially incompatible with multithreading. BSD
locks have much saner semantics. We need this now that there can be
multiple concurrent LocalStore::buildPaths() invocations.
This commit is contained in:
Eelco Dolstra 2018-10-04 15:03:03 +02:00
parent ec415d7166
commit e349f2c0a3
No known key found for this signature in database
GPG key ID: 8170B4726D7198DE
5 changed files with 45 additions and 107 deletions
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17
src/libstore/build.cc
View file

@ -3984,17 +3984,6 @@ void SubstitutionGoal::tryToRun()
return;
}
/* If the store path is already locked (probably by a
DerivationGoal), then put this goal to sleep. Note: we don't
acquire a lock here since that breaks addToStore(), so below we
handle an AlreadyLocked exception from addToStore(). The check
here is just an optimisation to prevent having to redo a
download due to a locked path. */
if (pathIsLockedByMe(worker.store.toRealPath(storePath))) {
worker.waitForAWhile(shared_from_this());
return;
}
maintainRunningSubstitutions = std::make_unique<MaintainCount<uint64_t>>(worker.runningSubstitutions);
worker.updateProgress();
@ -4034,12 +4023,6 @@ void SubstitutionGoal::finished()
try {
promise.get_future().get();
} catch (AlreadyLocked & e) {
/* Probably a DerivationGoal is already building this store
path. Sleep for a while and try again. */
state = &SubstitutionGoal::init;
worker.waitForAWhile(shared_from_this());
return;
} catch (std::exception & e) {
printError(e.what());

4
src/libstore/gc.cc
View file

@ -29,7 +29,7 @@ static string gcRootsDir = "gcroots";
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)
AutoCloseFD LocalStore::openGCLock(LockType lockType)
{
Path fnGCLock = (format("%1%/%2%")
% stateDir % gcLockName).str();
@ -49,7 +49,7 @@ int LocalStore::openGCLock(LockType lockType)
process that can open the file for reading can DoS the
collector. */
return fdGCLock.release();
return fdGCLock;
}

2
src/libstore/local-store.hh
View file

@ -263,7 +263,7 @@ private:
bool isActiveTempFile(const GCState & state,
const Path & path, const string & suffix);
int openGCLock(LockType lockType);
AutoCloseFD openGCLock(LockType lockType);
void findRoots(const Path & path, unsigned char type, Roots & roots);

125
src/libstore/pathlocks.cc
View file

@ -7,7 +7,7 @@
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/file.h>
namespace nix {
@ -40,17 +40,14 @@ void deleteLockFile(const Path & path, int fd)
bool lockFile(int fd, LockType lockType, bool wait)
{
struct flock lock;
if (lockType == ltRead) lock.l_type = F_RDLCK;
else if (lockType == ltWrite) lock.l_type = F_WRLCK;
else if (lockType == ltNone) lock.l_type = F_UNLCK;
int type;
if (lockType == ltRead) type = LOCK_SH;
else if (lockType == ltWrite) type = LOCK_EX;
else if (lockType == ltNone) type = LOCK_UN;
else abort();
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0; /* entire file */
if (wait) {
while (fcntl(fd, F_SETLKW, &lock) != 0) {
while (flock(fd, type) != 0) {
checkInterrupt();
if (errno != EINTR)
throw SysError(format("acquiring/releasing lock"));
@ -58,9 +55,9 @@ bool lockFile(int fd, LockType lockType, bool wait)
return false;
}
} else {
while (fcntl(fd, F_SETLK, &lock) != 0) {
while (flock(fd, type | LOCK_NB) != 0) {
checkInterrupt();
if (errno == EACCES || errno == EAGAIN) return false;
if (errno == EWOULDBLOCK) return false;
if (errno != EINTR)
throw SysError(format("acquiring/releasing lock"));
}
@ -70,14 +67,6 @@ bool lockFile(int fd, LockType lockType, bool wait)
}
/* This enables us to check whether are not already holding a lock on
a file ourselves. POSIX locks (fcntl) suck in this respect: if we
close a descriptor, the previous lock will be closed as well. And
there is no way to query whether we already have a lock (F_GETLK
only works on locks held by other processes). */
static Sync<StringSet> lockedPaths_;
PathLocks::PathLocks()
: deletePaths(false)
{
@ -91,7 +80,7 @@ PathLocks::PathLocks(const PathSet & paths, const string & waitMsg)
}
bool PathLocks::lockPaths(const PathSet & _paths,
bool PathLocks::lockPaths(const PathSet & paths,
const string & waitMsg, bool wait)
{
assert(fds.empty());
@ -99,75 +88,54 @@ bool PathLocks::lockPaths(const PathSet & _paths,
/* Note that `fds' is built incrementally so that the destructor
will only release those locks that we have already acquired. */
/* Sort the paths. This assures that locks are always acquired in
the same order, thus preventing deadlocks. */
Paths paths(_paths.begin(), _paths.end());
paths.sort();
/* Acquire the lock for each path. */
/* Acquire the lock for each path in sorted order. This ensures
that locks are always acquired in the same order, thus
preventing deadlocks. */
for (auto & path : paths) {
checkInterrupt();
Path lockPath = path + ".lock";
debug(format("locking path '%1%'") % path);
{
auto lockedPaths(lockedPaths_.lock());
if (lockedPaths->count(lockPath)) {
if (!wait) return false;
throw AlreadyLocked("deadlock: trying to re-acquire self-held lock '%s'", lockPath);
}
lockedPaths->insert(lockPath);
}
AutoCloseFD fd;
try {
while (1) {
AutoCloseFD fd;
/* Open/create the lock file. */
fd = openLockFile(lockPath, true);
while (1) {
/* Open/create the lock file. */
fd = openLockFile(lockPath, true);
/* Acquire an exclusive lock. */
if (!lockFile(fd.get(), ltWrite, false)) {
if (wait) {
if (waitMsg != "") printError(waitMsg);
lockFile(fd.get(), ltWrite, true);
} else {
/* Failed to lock this path; release all other
locks. */
unlock();
lockedPaths_.lock()->erase(lockPath);
return false;
}
/* Acquire an exclusive lock. */
if (!lockFile(fd.get(), ltWrite, false)) {
if (wait) {
if (waitMsg != "") printError(waitMsg);
lockFile(fd.get(), ltWrite, true);
} else {
/* Failed to lock this path; release all other
locks. */
unlock();
return false;
}
debug(format("lock acquired on '%1%'") % lockPath);
/* Check that the lock file hasn't become stale (i.e.,
hasn't been unlinked). */
struct stat st;
if (fstat(fd.get(), &st) == -1)
throw SysError(format("statting lock file '%1%'") % lockPath);
if (st.st_size != 0)
/* This lock file has been unlinked, so we're holding
a lock on a deleted file. This means that other
processes may create and acquire a lock on
`lockPath', and proceed. So we must retry. */
debug(format("open lock file '%1%' has become stale") % lockPath);
else
break;
}
/* Use borrow so that the descriptor isn't closed. */
fds.push_back(FDPair(fd.release(), lockPath));
debug(format("lock acquired on '%1%'") % lockPath);
} catch (...) {
lockedPaths_.lock()->erase(lockPath);
throw;
/* Check that the lock file hasn't become stale (i.e.,
hasn't been unlinked). */
struct stat st;
if (fstat(fd.get(), &st) == -1)
throw SysError(format("statting lock file '%1%'") % lockPath);
if (st.st_size != 0)
/* This lock file has been unlinked, so we're holding
a lock on a deleted file. This means that other
processes may create and acquire a lock on
`lockPath', and proceed. So we must retry. */
debug(format("open lock file '%1%' has become stale") % lockPath);
else
break;
}
/* Use borrow so that the descriptor isn't closed. */
fds.push_back(FDPair(fd.release(), lockPath));
}
return true;
@ -189,8 +157,6 @@ void PathLocks::unlock()
for (auto & i : fds) {
if (deletePaths) deleteLockFile(i.second, i.first);
lockedPaths_.lock()->erase(i.second);
if (close(i.first) == -1)
printError(
format("error (ignored): cannot close lock file on '%1%'") % i.second);
@ -208,11 +174,4 @@ void PathLocks::setDeletion(bool deletePaths)
}
bool pathIsLockedByMe(const Path & path)
{
Path lockPath = path + ".lock";
return lockedPaths_.lock()->count(lockPath);
}
}

4
src/libstore/pathlocks.hh
View file

@ -16,8 +16,6 @@ enum LockType { ltRead, ltWrite, ltNone };
bool lockFile(int fd, LockType lockType, bool wait);
MakeError(AlreadyLocked, Error);
class PathLocks
{
private:
@ -37,6 +35,4 @@ public:
void setDeletion(bool deletePaths);
};
bool pathIsLockedByMe(const Path & path);
}