Make app setting file writes durable

The atomic write replaces the old with the new file, but unless we sync
the parent directory, that replacement is not permanent yet.

src/linkapp.cpp

@@ -348,7 +348,17 @@ bool LinkApp::save() {
 
 	if (out.tellp() > 0) {
 		DEBUG("Saving app settings: %s\n", file.c_str());
-		return writeStringToFile(file, out.str());
+		if (writeStringToFile(file, out.str())) {
+			string dir = parentDir(file);
+			if (!syncDir(dir)) {
+				ERROR("Failed to sync dir: %s\n", dir.c_str());
+				// Note: Even if syncDir fails, the app settings have been
+				//       written, so have save() return true.
+			}
+			return true;
+		} else {
+			return false;
+		}
 	} else {
 		DEBUG("Empty app settings: %s\n", file.c_str());
 		return unlink(file.c_str()) == 0 || errno == ENOENT;

src/utilities.cpp

@@ -112,6 +112,31 @@ bool writeStringToFile(string const& filename, string const& data) {
 	return ok;
 }
 
+constexpr int dirOpenFlags =
+#ifdef O_DIRECTORY
+	O_DIRECTORY | O_RDONLY; // Linux specific
+#else
+	O_RDONLY;
+#endif
+
+bool syncDir(string const& dirname)
+{
+	int fd = open(dirname.c_str(), dirOpenFlags);
+	if (fd < 0) {
+		return false;
+	}
+
+	bool ok = fsync(fd) == 0;
+
+	while (close(fd)) {
+		if (errno != EINTR) {
+			return false;
+		}
+	}
+
+	return ok;
+}
+
 string parentDir(string const& dir) {
 	// Note that size() is unsigned, so for short strings the '- 2' wraps
 	// around and as a result the entire string is searched, which is fine.

src/utilities.h

@@ -57,6 +57,12 @@ std::string readFileAsString(std::string const& filename);
  */
 bool writeStringToFile(std::string const& filename, std::string const& data);
 
+/**
+ * Tells the file system to commit the given directory to disk.
+ * @return True iff the sync was successful.
+ */
+bool syncDir(std::string const& dirname);
+
 std::string strreplace(std::string orig, const std::string &search, const std::string &replace);
 std::string cmdclean(std::string cmdline);