From e492c64c8e2d905dd97dc9e9870f0eb18f4a8313 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Tue, 22 Jul 2025 12:18:52 +0200
Subject: [PATCH] SQLite: fsync db.sqlite-shm before opening the database

This is a workaround for https://github.com/NixOS/nix/issues/13515
(opening the SQLite DB randomly taking a couple of seconds on ZFS).

(cherry picked from commit a7fceb5eec404eabf461d4f1281bf4163c5d8ad0)
---
 src/libstore/sqlite.cc | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/src/libstore/sqlite.cc b/src/libstore/sqlite.cc
index 56a69470a..5f0b3ce51 100644
--- a/src/libstore/sqlite.cc
+++ b/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>
@@ -60,6 +64,28 @@ static void traceSQL(void * x, const char * sql)
 
 SQLite::SQLite(const std::filesystem::path & path, SQLiteOpenMode mode)
 {
+    // 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.
+    // 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
+#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'", 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.