io/FileOutputStream: add method Sync()

src/io/FileOutputStream.cxx

@@ -138,6 +138,15 @@ FileOutputStream::Write(const void *data, size_t size)
 				      GetPath().c_str());
 }
 
+void
+FileOutputStream::Sync()
+{
+	assert(IsDefined());
+
+	if (!FlushFileBuffers(handle))
+		throw FormatLastError("Failed to sync %s", GetPath().c_str());
+}
+
 void
 FileOutputStream::Commit()
 {
@@ -240,6 +249,15 @@ FileOutputStream::Write(const void *data, size_t size)
 				  GetPath().c_str());
 }
 
+void
+FileOutputStream::Sync()
+{
+	assert(IsDefined());
+
+	if (fdatasync(fd.Get()) < 0)
+		throw FormatErrno("Failed to sync %s", GetPath().c_str());
+}
+
 void
 FileOutputStream::Commit()
 {

src/io/FileOutputStream.hxx

@@ -148,6 +148,15 @@ public:
 	/* virtual methods from class OutputStream */
 	void Write(const void *data, size_t size) override;
 
+	/**
+	 * Flush all data written to this object to disk (but does not
+	 * commit to the final path).  This method blocks until this
+	 * flush is complete.  It can be called repeatedly.
+	 *
+	 * Throws on error.
+	 */
+	void Sync();
+
 	/**
 	 * Commit all data written to the file and make the file
 	 * visible on the specified path.