diff -r 275a4428c13b -r 4277359896dc librazor/atomic-emulate.c
--- a/librazor/atomic-emulate.c	Tue Mar 27 21:32:46 2012 +0100
+++ b/librazor/atomic-emulate.c	Tue Sep 09 15:04:24 2014 +0100
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2012  J. Ali Harlow <ali@juiblex.co.uk>
+ * Copyright (C) 2012, 2014  J. Ali Harlow <ali@juiblex.co.uk>
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -29,6 +29,7 @@
 #include <fcntl.h>
 #include <dirent.h>
 #include <errno.h>
+#include <unistd.h>
 #include "razor-internal.h"
 
 /*
@@ -119,21 +120,70 @@
 	free(atomic);
 }
 
+#ifndef MSWIN_API
+static char *absolute_path(const char *path)
+{
+	int len;
+	char *result, *subpath, *p, *s, *t;
+
+	result = realpath(path, NULL);
+
+	if (!result && errno == ENOENT) {
+		p = strdup(path);
+		s = strrchr(p, '/');
+
+		while (s) {
+			if (s == p) {
+				result = strdup("/");
+				break;
+			}
+
+			*s = '\0';
+			subpath = realpath(p, NULL);
+
+			if (subpath) {
+				*s = '/';
+				len = strlen(subpath);
+				result = malloc(len + strlen(s) + 1);
+				memcpy(result, subpath, len);
+				strcpy(result + len, s);
+				break;
+			} else if (errno != ENOENT)
+				break;
+
+			t = strrchr(p, '/');
+			*s = '/';
+			s = t;
+		}
+
+		if (!s)
+			result = realpath(".", NULL);
+
+		free(p);
+	}
+
+	return result;
+}
+#endif
+
 /*
  * We need a toplevel directory in which to hold temporary files
  * before they are committed. Since we can generally assume that
- * we have write permissions anywhere on the disk in question,
- * the best location is in the relevant root directory. The most
- * common case where this assumption fails is when testing, when
- * the current directory is a good choice.
- * It might be even better to find a mount point above path instead
- * but this is hard to do, and probably not worth the effort.
+ * we have write permissions anywhere on the filesystem in
+ * question, the best location is at the relevant mount point.
+ * The most common case where this assumption fails is when
+ * testing, when the current directory is a good choice.
  */
 
 static int
 razor_atomic_set_toplevel_from_path(struct razor_atomic *atomic,
 				    const char *path)
 {
+#ifndef MSWIN_API
+	dev_t filesystem;
+	struct stat buf;
+#endif
+
 	if (razor_atomic_in_error_state(atomic))
 		return -1;
 
@@ -183,7 +233,66 @@
 		free(buf);
 	}
 #else
-	atomic->toplevel = strdup("/.atomic-XXXXXX");
+	{
+		/*
+		 * Find the mount point (assuming we can write to the
+		 * whole filesystem). Otherwise stop at the first
+		 * unwritable directory and take one step back.
+		 */
+		char *s, *abspath, saved;
+		int len;
+
+		abspath = absolute_path(path);
+		if (!abspath) {
+			atomic->error = razor_error_new_str(path,
+							    strerror(errno));
+			return -1;
+		}
+
+		if (stat(abspath, &buf) < 0) {
+			atomic->error = razor_error_new_str(abspath,
+							    strerror(errno));
+			free(abspath);
+			return -1;
+		}
+		filesystem = buf.st_dev;
+
+		len = strlen(abspath);
+		while(len > 1 && (s = strrchr(abspath, '/'))) {
+			if (s == abspath) {
+				saved = s[1];
+				s[1] = '\0';
+				len = s + 1 - abspath;
+			} else {
+				s[0] = '\0';
+				len = s - abspath;
+			}
+
+			if (stat(abspath, &buf) < 0) {
+				atomic->error =
+				  razor_error_new_str(abspath, strerror(errno));
+				free(abspath);
+				return -1;
+			}
+
+			if (buf.st_dev != filesystem || access(abspath, W_OK)) {
+				if (s == abspath)
+					s[1] = saved;
+				else
+					s[0] = '/';
+				len = strlen(abspath);
+				break;
+			}
+		}
+
+		if (len == 1)
+			len = 0;	/* Avoid an unslightly double slash. */
+		atomic->toplevel = malloc(len + strlen("/.atomic-XXXXXX") + 1);
+		memcpy(atomic->toplevel, abspath, len);
+		strcpy(atomic->toplevel + len, "/.atomic-XXXXXX");
+
+		free(abspath);
+	}
 #endif
 
 	if (!mkdtemp(atomic->toplevel)) {
@@ -193,6 +302,24 @@
 		if (err == EACCES) {
 			char *s = strdup("atomic-XXXXXX");
 
+#ifndef MSWIN_API
+			if (stat(".", &buf) < 0) {
+				atomic->error =
+				  razor_error_new_str(".", strerror(errno));
+				free(s);
+				free(atomic->toplevel);
+				atomic->toplevel = NULL;
+				return -1;
+			}
+			if (buf.st_dev != filesystem)
+				/*
+				 * Don't use a different filesystem. It will
+				 * only fail later on (in rename) and cause
+				 * an unhelpful error message (EXDEV).
+				 */
+				free(s);
+			else
+#endif
 			if (mkdtemp(s)) {
 				free(atomic->toplevel);
 				atomic->toplevel = s;