SQLite: fsync db.sqlite-shm before opening the database

src/libexpr/eval-cache.cc

@@ -69,10 +69,10 @@ struct AttrDb
     {
         auto state(_state->lock());
 
-        Path cacheDir = getCacheDir() + "/eval-cache-v5";
+        auto cacheDir = std::filesystem::path(getCacheDir()) / "eval-cache-v5";
         createDirs(cacheDir);
 
-        Path dbPath = cacheDir + "/" + fingerprint.to_string(HashFormat::Base16, false) + ".sqlite";
+        auto dbPath = cacheDir / (fingerprint.to_string(HashFormat::Base16, false) + ".sqlite");
 
         state->db = SQLite(dbPath);
         state->db.isCache();

src/libstore/include/nix/store/sqlite.hh

@@ -1,6 +1,7 @@
 #pragma once
 ///@file
 
+#include <filesystem>
 #include <functional>
 #include <string>
 
@@ -41,7 +42,7 @@ struct SQLite
 
     SQLite() {}
 
-    SQLite(const Path & path, SQLiteOpenMode mode = SQLiteOpenMode::Normal);
+    SQLite(const std::filesystem::path & path, SQLiteOpenMode mode = SQLiteOpenMode::Normal);
     SQLite(const SQLite & from) = delete;
     SQLite & operator=(const SQLite & from) = delete;
 

src/libstore/local-store.cc

@@ -466,12 +466,11 @@ void LocalStore::openDB(State & state, bool create)
         throw SysError("Nix database directory '%1%' is not writable", dbDir);
 
     /* Open the Nix database. */
-    std::string dbPath = dbDir + "/db.sqlite";
     auto & db(state.db);
     auto openMode = config->readOnly ? SQLiteOpenMode::Immutable
                     : create         ? SQLiteOpenMode::Normal
                                      : SQLiteOpenMode::NoCreate;
-    state.db = SQLite(dbPath, openMode);
+    state.db = SQLite(std::filesystem::path(dbDir) / "db.sqlite", openMode);
 
 #ifdef __CYGWIN__
     /* The cygwin version of sqlite3 has a patch which calls

src/libstore/sqlite.cc

@@ -4,6 +4,10 @@
 #include "nix/util/url.hh"
 #include "nix/util/signals.hh"
 
+#ifdef __linux__
+#  include <sys/vfs.h>
+#endif
+
 #include <sqlite3.h>
 
 #include <atomic>
@@ -58,8 +62,28 @@ static void traceSQL(void * x, const char * sql)
     notice("SQL<[%1%]>", sql);
 };
 
-SQLite::SQLite(const Path & path, SQLiteOpenMode mode)
+SQLite::SQLite(const std::filesystem::path & path, SQLiteOpenMode mode)
 {
+#ifdef __linux__
+    // Work around a ZFS issue where SQLite's truncate() call on
+    // db.sqlite-shm can randomly take up to a few seconds. See
+    // https://github.com/openzfs/zfs/issues/14290#issuecomment-3074672917.
+    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'", shmFile);
+            if (fs.f_type == /* ZFS_SUPER_MAGIC */ 801189825 && fdatasync(fd.get()) != 0)
+                throw SysError("fsync() on '%s'", shmFile);
+        }
+    } catch (...) {
+        throw;
+    }
+#endif
+
     // useSQLiteWAL also indicates what virtual file system we need.  Using
     // `unix-dotfile` is needed on NFS file systems and on Windows' Subsystem
     // for Linux (WSL) where useSQLiteWAL should be false by default.
@@ -68,7 +92,7 @@ SQLite::SQLite(const Path & path, SQLiteOpenMode mode)
     int flags = immutable ? SQLITE_OPEN_READONLY : SQLITE_OPEN_READWRITE;
     if (mode == SQLiteOpenMode::Normal)
         flags |= SQLITE_OPEN_CREATE;
-    auto uri = "file:" + percentEncode(path) + "?immutable=" + (immutable ? "1" : "0");
+    auto uri = "file:" + percentEncode(path.string()) + "?immutable=" + (immutable ? "1" : "0");
     int ret = sqlite3_open_v2(uri.c_str(), &db, SQLITE_OPEN_URI | flags, vfs);
     if (ret != SQLITE_OK) {
         const char * err = sqlite3_errstr(ret);