Rename old 'xml' file as 'bak' file when saving

If you use the standard naming convention and end your subsurface
filename in ".xml", we will now save away any previous xml file as a
"bak" file before writing a new one.

This can be useful for:

- recovering from mistakes that deleted old dives

- seeing what changed (ie you can do things like "diff -u xyz.bak
  xyz.xml") after doing some operation and saving the result.

However, this does only a single level of backups - if you save twice,
you will obviously have lost the original. I'd strongly encourage some
external backup system in addition to this very simplistic backup.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

save-xml.c

@@ -577,12 +577,46 @@ void save_dives_buffer(struct membuffer *b, const bool select_only)
 	put_format(b, "</dives>\n</divelog>\n");
 }
 
+static void save_backup(const char *name, const char *ext, const char *new_ext)
+{
+	int len = strlen(name);
+	int a = strlen(ext), b = strlen(new_ext);
+	char *newname;
+
+	/* len up to and including the final '.' */
+	len -= a;
+	if (len <= 1)
+		return;
+	if (name[len-1] != '.')
+		return;
+	/* msvc doesn't have strncasecmp, has _strnicmp instead - crazy */
+	if (strncasecmp(name+len, ext, a))
+		return;
+
+	newname = malloc(len + b + 1);
+	if (!newname)
+		return;
+
+	memcpy(newname, name, len);
+	memcpy(newname+len, new_ext, b+1);
+
+	/*
+	 * Ignore errors. Maybe we can't create the backup file,
+	 * maybe no old file existed.  Regardless, we'll write the
+	 * new file.
+	 */
+	subsurface_rename(name, newname);
+	free(newname);
+}
+
 void save_dives_logic(const char *filename, const bool select_only)
 {
 	struct membuffer buf = {0};
 	FILE *f;
 
 	save_dives_buffer(&buf, select_only);
+	/* Maybe we might want to make this configurable? */
+	save_backup(filename, "xml", "bak");
 	f = subsurface_fopen(filename, "w");
 	if (f) {
 		flush_buffer(&buf, f);