util: Add helper to write() all of a buffer

write(2) might not write all the data it is given.  Add a write_all_buf()
helper to keep calling it until all the given data is written, or we get an
error.

Currently we use write_remainder() to do this operation in pcap_frame().
That's a little awkward since it requires constructing an iovec, and future
changes we want to make to write_remainder() will be easier in terms of
this single buffer helper.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

pcap.c

@@ -86,9 +86,8 @@ static void pcap_frame(const struct iovec *iov, size_t iovcnt,
 		.caplen = l2len,
 		.len = l2len
 	};
-	struct iovec hiov = { &h, sizeof(h) };
 
-	if (write_remainder(pcap_fd, &hiov, 1, 0) < 0 ||
+	if (write_all_buf(pcap_fd, &h, sizeof(h)) < 0 ||
 	    write_remainder(pcap_fd, iov, iovcnt, offset) < 0)
 		debug_perror("Cannot log packet, length %zu", l2len);
 }

util.c

@@ -582,6 +582,31 @@ int do_clone(int (*fn)(void *), char *stack_area, size_t stack_size, int flags,
 #endif
 }
 
+/* write_all_buf() - write all of a buffer to an fd
+ * @fd:		File descriptor
+ * @buf:	Pointer to base of buffer
+ * @len:	Length of buffer
+ *
+ * Return: 0 on success, -1 on error (with errno set)
+ *
+ * #syscalls write
+ */
+int write_all_buf(int fd, const void *buf, size_t len)
+{
+	const char *p = buf;
+	size_t left = len;
+
+	while (left) {
+		ssize_t rc = write(fd, p, left);
+
+		if (rc < 0)
+			return -1;
+		p += rc;
+		left -= rc;
+	}
+	return 0;
+}
+
 /* write_remainder() - write the tail of an IO vector to an fd
  * @fd:		File descriptor
  * @iov:	IO vector

util.h

@@ -200,6 +200,7 @@ void pidfile_write(int fd, pid_t pid);
 int __daemon(int pidfile_fd, int devnull_fd);
 int fls(unsigned long x);
 int write_file(const char *path, const char *buf);
+int write_all_buf(int fd, const void *buf, size_t len);
 int write_remainder(int fd, const struct iovec *iov, size_t iovcnt, size_t skip);
 void close_open_files(int argc, char **argv);