swapon: handle <=linux-2.6.19 bug in /proc/swaps

Linux <=2.6.19 contained a bug in the /proc/swaps code where the header
would not be displayed (the first line).  Most people report the issue as
a sequence of swapon/swapoff calls to trigger, but for some lucky people,
it triggers all the time at initial boot.  Since this throws up an error,
init systems don't actually activate any swap files.

First, swapon shouldn't whine about unexpected format if the file is empty
(the default at boot).  This is easy to do by putting the warning behind a
check to ferror().

Second, we can detect that the first line isn't actually the header but
instead is a valid swap line and so need to be processed.  This assumes
that the first line will always be the same format.  Looking quickly at
older versions shows that this header has retained its exact format since
at least Linux 2.2.0 and considering the concern that goes along with proc
files and the ABI, it's highly unlikely it will ever change.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

mount/swapon.c

@@ -135,11 +135,18 @@ read_proc_swaps(void) {
 
 	/* skip the first line */
 	if (!fgets(line, sizeof(line), swaps)) {
-		warnx(_("%s: unexpected file format"), _PATH_PROC_SWAPS);
+		/* do not whine about an empty file */
+		if (ferror(swaps))
+			warn(_("%s: unexpected file format"), _PATH_PROC_SWAPS);
 		fclose(swaps);
 		return;
 	}
+	/* make sure the first line is the header */
+	if (line[0] != '\0' && strncmp(line, "Filename\t", 9))
+		goto valid_first_line;
+
 	while (fgets(line, sizeof(line), swaps)) {
+ valid_first_line:
 		/*
 		 * Cut the line "swap_device  ... more info" after device.
 		 * This will fail with names with embedded spaces.