Eventually add write permission when setting extended attributes

When we need to set extended atributes of file which does not
allow write then temporarily add write permission and after
attributes are set, remove it again.

Resolves #208

xattrs.c

@@ -23,6 +23,8 @@
 #include "ifuncs.h"
 #include "inums.h"
 #include "lib/sysxattrs.h"
+#include <unistd.h>
+#include <sys/stat.h>
 
 #ifdef SUPPORT_XATTRS
 
@@ -1056,6 +1058,8 @@ int set_xattr(const char *fname, const struct file_struct *file, const char *fna
 	rsync_xa_list *glst = rsync_xal_l.items;
 	item_list *lst;
 	int ndx;
+	int added_write_perm = 0;
+	struct stat current_stat;
 
 	if (dry_run)
 		return 1; /* FIXME: --dry-run needs to compute this value */
@@ -1084,10 +1088,34 @@ int set_xattr(const char *fname, const struct file_struct *file, const char *fna
 	}
 #endif
 
+	// if target file has not write permission
+	// then add it temporarily so we can change extended attributes
+	if (access(fname, W_OK) != 0) {
+		if (do_stat(fname, &current_stat) != 0) {
+			rsyserr(FERROR_XFER, errno, "set_xattr: was not able to access file %s",
+				full_fname(fname));
+		}
+		if (current_stat.st_uid == (uid_t)F_OWNER(file)) {
+			do_chmod(fname, current_stat.st_mode | S_IWUSR);
+			added_write_perm = 1;
+		} else if (current_stat.st_gid == (gid_t)F_GROUP(file)) {
+			do_chmod(fname, current_stat.st_mode | S_IWGRP);
+			added_write_perm = 1;
+		} else {
+			rprintf(FERROR_XFER, "set_xattr: file %s is not accessible for writing",
+				full_fname(fname));
+		}
+	}
+
 	ndx = F_XATTR(file);
 	glst += ndx;
 	lst = &glst->xa_items;
-	return rsync_xal_set(fname, lst, fnamecmp, sxp);
+	int return_value = rsync_xal_set(fname, lst, fnamecmp, sxp);
+	if (added_write_perm) {
+		// remove the temporary write permission
+		do_chmod(fname, current_stat.st_mode);
+	}
+	return return_value;
 }
 
 #ifdef SUPPORT_ACLS