Write app settings files atomically

This is safer than the original code, which didn't use a temporary file
and could therefore leave partial files. Also it avoids a full sync(),
which can take a long time if for example a big file transfer is going
on or recently ended.

src/linkapp.cpp

@@ -328,7 +328,7 @@ bool LinkApp::targetExists()
 }
 
 bool LinkApp::save() {
-	if (!edited) return false;
+	if (!edited) return true;
 
 	std::ostringstream out;
 	if (!isOpk()) {
@@ -348,16 +348,7 @@ bool LinkApp::save() {
 
 	if (out.tellp() > 0) {
 		DEBUG("Saving app settings: %s\n", file.c_str());
-		ofstream f(file.c_str());
-		if (f.is_open()) {
-			f << out.str();
-			f.close();
-			sync();
-			return true;
-		} else {
-			ERROR("Error while opening the file '%s' for write.\n", file.c_str());
-			return false;
-		}
+		return writeStringToFile(file, out.str());
 	} else {
 		DEBUG("Empty app settings: %s\n", file.c_str());
 		return unlink(file.c_str()) == 0 || errno == ENOENT;
@@ -560,7 +551,9 @@ void LinkApp::selector(int startSelection, const string &selectorDir) {
 }
 
 unique_ptr<Launcher> LinkApp::prepareLaunch(const string &selectedFile) {
-	save();
+	if (!save()) {
+		ERROR("Error saving app settings to '%s'.\n", file.c_str());
+	}
 
 	if (!isOpk()) {
 		//Set correct working directory

src/utilities.cpp

@@ -28,6 +28,7 @@
 //for browsing the filesystem
 #include <sys/stat.h>
 #include <sys/types.h>
+#include <fcntl.h>
 #include <dirent.h>
 #include <fstream>
 #include <iostream>
@@ -81,6 +82,36 @@ string readFileAsString(string const& filename) {
 	}
 }
 
+// Use C functions since STL doesn't seem to have any way of applying fsync().
+bool writeStringToFile(string const& filename, string const& data) {
+	// Open temporary file.
+	string tempname = filename + '~';
+	int fd = open(tempname.c_str(), O_CREAT | O_WRONLY | O_TRUNC | O_CLOEXEC);
+	if (fd < 0) {
+		return false;
+	}
+
+	// Write temporary file.
+	bool ok = write(fd, data.c_str(), data.size()) >= 0;
+	if (ok) {
+		ok = fsync(fd) == 0;
+	}
+
+	// Close temporary file.
+	while (close(fd)) {
+		if (errno != EINTR) {
+			return false;
+		}
+	}
+
+	// Replace actual output file with temporary file.
+	if (ok) {
+		ok = rename(tempname.c_str(), filename.c_str()) == 0;
+	}
+
+	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

@@ -49,6 +49,14 @@ std::string rtrim(const std::string& s);
 /** Returns the contents of the given file as a string. */
 std::string readFileAsString(std::string const& filename);
 
+/**
+ * Writes the given string to a file.
+ * The update is done atomically but not durably; if you need durability
+ * when fsync() the parent directory afterwards.
+ * @return True iff the file was written successfully.
+ */
+bool writeStringToFile(std::string const& filename, std::string const& data);
+
 std::string strreplace(std::string orig, const std::string &search, const std::string &replace);
 std::string cmdclean(std::string cmdline);