system/FileDescriptor: add method FullWrite()

src/system/FileDescriptor.cxx

@@ -298,6 +298,24 @@ FileDescriptor::FullRead(void *_buffer, size_t length)
 	}
 }
 
+void
+FileDescriptor::FullWrite(const void *_buffer, size_t length)
+{
+	const uint8_t *buffer = (const uint8_t *)_buffer;
+
+	while (length > 0) {
+		ssize_t nbytes = Write(buffer, length);
+		if (nbytes <= 0) {
+			if (nbytes < 0)
+				throw MakeErrno("Failed to write");
+			throw std::runtime_error("Failed to write");
+		}
+
+		buffer += nbytes;
+		length -= nbytes;
+	}
+}
+
 #ifndef _WIN32
 
 int

src/system/FileDescriptor.hxx

@@ -237,6 +237,12 @@ public:
 		return ::write(fd, buffer, length);
 	}
 
+	/**
+	 * Write until all of the given buffer has been written.
+	 * Throws on error.
+	 */
+	void FullWrite(const void *buffer, size_t length);
+
 #ifndef _WIN32
 	int Poll(short events, int timeout) const noexcept;