Improve durability of schema version file writes

- call close explicitly in writeFile to prevent the close exception
  from being ignored
- fsync after writing schema file to flush data to disk
- fsync schema file parent to flush metadata to disk

https://github.com/NixOS/nix/issues/7064

src/libstore/local-store.cc

@@ -158,7 +158,7 @@ void migrateCASchema(SQLite& db, Path schemaPath, AutoCloseFD& lockFd)
             txn.commit();
         }
 
-        writeFile(schemaPath, fmt("%d", nixCASchemaVersion));
+        writeFile(schemaPath, fmt("%d", nixCASchemaVersion), 0666, true);
         lockFile(lockFd.get(), ltRead, true);
     }
 }
@@ -281,7 +281,7 @@ LocalStore::LocalStore(const Params & params)
     else if (curSchema == 0) { /* new store */
         curSchema = nixSchemaVersion;
         openDB(*state, true);
-        writeFile(schemaPath, (format("%1%") % nixSchemaVersion).str());
+        writeFile(schemaPath, (format("%1%") % nixSchemaVersion).str(), 0666, true);
     }
 
     else if (curSchema < nixSchemaVersion) {
@@ -329,7 +329,7 @@ LocalStore::LocalStore(const Params & params)
             txn.commit();
         }
 
-        writeFile(schemaPath, (format("%1%") % nixSchemaVersion).str());
+        writeFile(schemaPath, (format("%1%") % nixSchemaVersion).str(), 0666, true);
 
         lockFile(globalLock.get(), ltRead, true);
     }

src/libutil/util.cc

@@ -353,7 +353,7 @@ void readFile(const Path & path, Sink & sink)
 }
 
 
-void writeFile(const Path & path, std::string_view s, mode_t mode)
+void writeFile(const Path & path, std::string_view s, mode_t mode, bool sync)
 {
     AutoCloseFD fd = open(path.c_str(), O_WRONLY | O_TRUNC | O_CREAT | O_CLOEXEC, mode);
     if (!fd)
@@ -364,10 +364,16 @@ void writeFile(const Path & path, std::string_view s, mode_t mode)
         e.addTrace({}, "writing file '%1%'", path);
         throw;
     }
+    if (sync)
+        fd.fsync();
+    // Explicitly close to make sure exceptions are propagated.
+    fd.close();
+    if (sync)
+        syncParent(path);
 }
 
 
-void writeFile(const Path & path, Source & source, mode_t mode)
+void writeFile(const Path & path, Source & source, mode_t mode, bool sync)
 {
     AutoCloseFD fd = open(path.c_str(), O_WRONLY | O_TRUNC | O_CREAT | O_CLOEXEC, mode);
     if (!fd)
@@ -386,6 +392,20 @@ void writeFile(const Path & path, Source & source, mode_t mode)
         e.addTrace({}, "writing file '%1%'", path);
         throw;
     }
+    if (sync)
+        fd.fsync();
+    // Explicitly close to make sure exceptions are propagated.
+    fd.close();
+    if (sync)
+        syncParent(path);
+}
+
+void syncParent(const Path & path)
+{
+    AutoCloseFD fd = open(dirOf(path).c_str(), O_RDONLY, 0);
+    if (!fd)
+        throw SysError("opening file '%1%'", path);
+    fd.fsync();
 }
 
 std::string readLine(int fd)
@@ -841,6 +861,20 @@ void AutoCloseFD::close()
     }
 }
 
+void AutoCloseFD::fsync()
+{
+  if (fd != -1) {
+      int result;
+#if __APPLE__
+      result = ::fcntl(fd, F_FULLFSYNC);
+#else
+      result = ::fsync(fd);
+#endif
+      if (result == -1)
+          throw SysError("fsync file descriptor %1%", fd);
+  }
+}
+
 
 AutoCloseFD::operator bool() const
 {

src/libutil/util.hh

@@ -115,9 +115,12 @@ std::string readFile(const Path & path);
 void readFile(const Path & path, Sink & sink);
 
 /* Write a string to a file. */
-void writeFile(const Path & path, std::string_view s, mode_t mode = 0666);
+void writeFile(const Path & path, std::string_view s, mode_t mode = 0666, bool sync = false);
 
-void writeFile(const Path & path, Source & source, mode_t mode = 0666);
+void writeFile(const Path & path, Source & source, mode_t mode = 0666, bool sync = false);
+
+/* Flush a file's parent directory to disk */
+void syncParent(const Path & path);
 
 /* Read a line from a file descriptor. */
 std::string readLine(int fd);
@@ -231,6 +234,7 @@ public:
     explicit operator bool() const;
     int release();
     void close();
+    void fsync();
 };