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SQLite: Only apply the ZFS -shm workaround on the first open of a database #664

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Merged
edolstra merged 1 commit into from
Oct 5, 2026
Merged

SQLite: Only apply the ZFS -shm workaround on the first open of a database #664

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42 changes: 30 additions & 12 deletions src/libstore/sqlite.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -3,13 +3,15 @@
#include "nix/util/util.hh"
#include "nix/util/url.hh"
#include "nix/util/signals.hh"
#include "nix/util/sync.hh"

#ifdef __linux__
# include <sys/vfs.h>
#endif

#include <sqlite3.h>

#include <set>
#include <thread>

namespace nix {
Expand Down Expand Up @@ -72,20 +74,36 @@ SQLite::SQLite(const std::filesystem::path & path, Settings && settings)
// https://github.com/openzfs/zfs/issues/14290#issuecomment-3074672917.
// Remove this workaround when a fix is widely installed, perhaps 2027? Candidate:
// https://github.com/search?q=repo%3Aopenzfs%2Fzfs+%22Linux%3A+zfs_putpage%3A+complete+async+page+writeback+immediately%22&type=commits
//
// Note: we only do this the first time that this process opens
// the database. Closing a file descriptor drops all POSIX locks
// that the process holds on that file, including the lock on
// db.sqlite-shm held by SQLite on behalf of other connections to
// the same database. Another process could then conclude that
// it's the only user of the database and truncate
// db.sqlite-shm, causing us to crash with SIGBUS.
//
// We hold the lock on `shmFilesSynced` until the file descriptor
// has been closed, so that a concurrent open of the same database
// in another thread cannot create a SQLite connection (and thus
// acquire POSIX locks on db.sqlite-shm) while we still have the
// file open.
#ifdef __linux__
try {
auto shmFile = path;
shmFile += "-shm";
AutoCloseFD fd = open(shmFile.string().c_str(), O_RDWR | O_CLOEXEC);
if (fd) {
struct statfs fs;
if (fstatfs(fd.get(), &fs))
throw SysError("statfs() on %s", PathFmt(shmFile));
if (fs.f_type == /* ZFS_SUPER_MAGIC */ 801189825 && fdatasync(fd.get()) != 0)
throw SysError("fsync() on %s", PathFmt(shmFile));
static Sync<std::set<std::filesystem::path>> shmFilesSynced;
{
auto shmFilesSynced_(shmFilesSynced.lock());
if (shmFilesSynced_->insert(path).second) {
auto shmFile = path;
shmFile += "-shm";
AutoCloseFD fd = open(shmFile.string().c_str(), O_RDWR | O_CLOEXEC);
if (fd) {
struct statfs fs;
if (fstatfs(fd.get(), &fs))
throw SysError("statfs() on %s", PathFmt(shmFile));
if (fs.f_type == /* ZFS_SUPER_MAGIC */ 801189825 && fdatasync(fd.get()) != 0)
throw SysError("fsync() on %s", PathFmt(shmFile));
}
}
} catch (...) {
throw;
}
#endif

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