files: improve the backup procedure to ensure no data is lost

The file-saving procedure that nano followed was not crash-safe
under ext4 (and perhaps other filesystems) when editing existing
files.  Specifically, modifying an existing file could lead to data
loss, since a modified file is not replaced atomically on disk.
Using "-B" did not ensure crash-safety either since the backup might
not have persisted on disk when the writeout of the file started.

In addition, if I/O pressure filled up the remaining disk space
after an existing file is truncated during save, nano would not
be able to finish saving the file, which would remain truncated.

This change addresses these issues by making nano do the following:

1) Make a temporary backup of the file being written so that there
is no time window such that (a) an existing file is truncated, and
(b) the buffer corresponding to said file has not been flushed to
disk.  Such time windows are dangerous because, under certain
circumstances, they can lead to data loss on some filesystems.

The backup is made by copying the original file, and a second
attempt to HOME is made in case that first copy fails.

2) Use fsync() so that, when the save finishes, it is certain
that the file has been successfully written to disk.

This addresses https://savannah.gnu.org/bugs/?58642.

Signed-off-by: Michalis Kokologiannakis <michalis@mpi-sws.org>

src/files.c

@@ -1571,6 +1571,23 @@ int copy_file(FILE *inn, FILE *out, bool close_out)
 	return retval;
 }
 
+/* Sync the contents of the given file to disk.  Return 0 on success, and
+ * a negative number on failure.  In case of failure, the file is closed. */
+int sync_file(FILE *thefile)
+{
+	if (fflush(thefile) != 0) {
+		fclose(thefile);
+		return -1;
+	}
+
+	if (fsync(fileno(thefile)) != 0) {
+		fclose(thefile);
+		return -2;
+	}
+
+	return 0;
+}
+
 /* Write the current buffer to disk.  If thefile isn't NULL, we write to a
  * temporary file that is already open.  If tmp is TRUE (when spell checking
  * or emergency dumping, for example), we set the umask to disallow anyone else
@@ -1589,6 +1606,8 @@ bool write_file(const char *name, FILE *thefile, bool tmp,
 		/* The status fields filled in by statting the file. */
 	char *backupname = NULL;
 		/* The name of the backup file, in case we make one. */
+	bool second_attempt = FALSE;
+		/* Whether a normal backup failed and we are resorting to a failsafe. */
 #endif
 	char *realname = real_dir_from_tilde(name);
 		/* The filename after tilde expansion. */
@@ -1622,7 +1641,7 @@ bool write_file(const char *name, FILE *thefile, bool tmp,
 	/* When the user requested a backup, we do this only if the file exists and
 	 * isn't temporary AND the file has not been modified by someone else since
 	 * we opened it (or we are appending/prepending or writing a selection). */
-	if (ISSET(MAKE_BACKUP) && is_existing_file && openfile->statinfo &&
+	if (is_existing_file && openfile->statinfo &&
 						(openfile->statinfo->st_mtime == st.st_mtime ||
 						method != OVERWRITE || openfile->mark)) {
 		static struct timespec filetime[2];
@@ -1633,20 +1652,18 @@ bool write_file(const char *name, FILE *thefile, bool tmp,
 		filetime[0].tv_sec = openfile->statinfo->st_atime;
 		filetime[1].tv_sec = openfile->statinfo->st_mtime;
 
-		/* Open the file of which a backup must be made. */
-		original = fopen(realname, "rb");
-
-		/* If we can't read from the original file, go on, since saving
-		 * only the current buffer is better than saving nothing. */
-		if (original == NULL) {
-			statusline(ALERT, _("Error reading %s: %s"), realname, strerror(errno));
-			goto skip_backup;
-		}
+		statusbar(_("Making backup..."));
 
 		/* If no backup directory was specified, we make a simple backup
 		 * by appending a tilde to the original file name.  Otherwise,
 		 * we create a numbered backup in the specified directory. */
-		if (backup_dir == NULL) {
+		if (!ISSET(MAKE_BACKUP)) {
+			backupname = charalloc(strlen(realname) + 8);
+			sprintf(backupname, "%s~XXXXXX", realname);
+
+			backup_fd = mkstemp(backupname);
+			goto try_backup;
+		} else if (backup_dir == NULL) {
 			backupname = charalloc(strlen(realname) + 2);
 			sprintf(backupname, "%s~", realname);
 		} else {
@@ -1674,21 +1691,13 @@ bool write_file(const char *name, FILE *thefile, bool tmp,
 			 * be fond of backups.  Thus, without one, do not go on. */
 			if (*backupname == '\0') {
 				statusline(ALERT, _("Too many existing backup files"));
-				fclose(original);
 				goto cleanup_and_exit;
 			}
 		}
 
 		/* Now first try to delete an existing backup file. */
-		if (unlink(backupname) < 0 && errno != ENOENT && !ISSET(INSECURE_BACKUP)) {
-			warn_and_briefly_pause(_("Cannot delete existing backup"));
-			fclose(original);
-			if (user_wants_to_proceed())
-				goto skip_backup;
-			statusline(HUSH, _("Cannot delete backup %s: %s"),
-								backupname, strerror(errno));
-			goto cleanup_and_exit;
-		}
+		if (unlink(backupname) < 0 && errno != ENOENT && !ISSET(INSECURE_BACKUP))
+			goto backup_error;
 
 		if (ISSET(INSECURE_BACKUP))
 			backup_cflags = O_WRONLY | O_CREAT | O_TRUNC;
@@ -1698,18 +1707,12 @@ bool write_file(const char *name, FILE *thefile, bool tmp,
 		/* Create the backup file (or truncate the existing one). */
 		backup_fd = open(backupname, backup_cflags, S_IRUSR|S_IWUSR);
 
+  try_backup:
 		if (backup_fd >= 0)
 			backup_file = fdopen(backup_fd, "wb");
 
-		if (backup_file == NULL) {
-			warn_and_briefly_pause(_("Cannot create backup file"));
-			fclose(original);
-			if (user_wants_to_proceed())
-				goto skip_backup;
-			statusline(HUSH, _("Cannot create backup %s: %s"),
-								backupname, strerror(errno));
-			goto cleanup_and_exit;
-		}
+		if (backup_file == NULL)
+			goto backup_error;
 
 		/* Try to change owner and group to those of the original file;
 		 * ignore errors, as a normal user cannot change the owner. */
@@ -1721,6 +1724,16 @@ bool write_file(const char *name, FILE *thefile, bool tmp,
 		 * the file with just read and write permission for the owner. */
 		IGNORE_CALL_RESULT(fchmod(backup_fd, openfile->statinfo->st_mode));
 
+		original = fopen(realname, "rb");
+
+		/* If we can't read from the original file, go on, since saving
+		 * only the current buffer is better than saving nothing. */
+		if (original == NULL) {
+			statusline(ALERT, _("Error reading %s: %s"), realname, strerror(errno));
+			fclose(backup_file);
+			goto save_the_file;
+		}
+
 		/* Copy the existing file to the backup. */
 		verdict = copy_file(original, backup_file, FALSE);
 
@@ -1730,29 +1743,56 @@ bool write_file(const char *name, FILE *thefile, bool tmp,
 			goto cleanup_and_exit;
 		} else if (verdict > 0) {
 			fclose(backup_file);
-			warn_and_briefly_pause(_("Cannot write backup file"));
-			if (user_wants_to_proceed())
-				goto skip_backup;
-			statusline(HUSH, _("Cannot write backup %s: %s"),
-								backupname, strerror(errno));
-			goto cleanup_and_exit;
+			goto backup_error;
 		}
 
+		/* Since this backup is a newly created file, explicitly sync it to
+		 * permanent storage before starting to write out the actual file. */
+		if (sync_file(backup_file) != 0)
+			goto backup_error;
+
 		/* Set the backup's timestamps to those of the original file.
 		 * Failure is unimportant: saving the file apparently worked. */
 		IGNORE_CALL_RESULT(futimens(backup_fd, filetime));
 
-		if (fclose(backup_file) != 0) {
-			warn_and_briefly_pause(_("Cannot write backup"));
-			if (user_wants_to_proceed())
-				goto skip_backup;
+		if (fclose(backup_file) == 0)
+			goto save_the_file;
+
+  backup_error:
+		get_homedir();
+
+		/* If the first attempt of copying the file failed, try again to HOME. */
+		if (!second_attempt && homedir) {
+			unlink(backupname);
+			free(backupname);
+
+			backupname = charalloc(strlen(homedir) + strlen(tail(realname)) + 9);
+			sprintf(backupname, "%s/%s~XXXXXX", homedir, tail(realname));
+
+			backup_fd = mkstemp(backupname);
+			backup_file = NULL;
+
+			if (ISSET(MAKE_BACKUP)) {
+				warn_and_briefly_pause(_("Cannot make regular backup"));
+				warn_and_briefly_pause(_("Trying again in your home directory"));
+				currmenu = MMOST;
+			}
+
+			second_attempt = TRUE;
+			goto try_backup;
+		}
+
+		/* If all attempts failed, notify the user, because if something goes
+		 * wrong during the save, the contents of the file might be lost. */
+		warn_and_briefly_pause(_("Cannot make backup"));
+		if (!user_wants_to_proceed()) {
 			statusline(HUSH, _("Cannot write backup %s: %s"),
 								backupname, strerror(errno));
 			goto cleanup_and_exit;
 		}
 	}
 
-  skip_backup:
+  save_the_file:
 	/* When prepending, first copy the existing file to a temporary file. */
 	if (method == PREPEND) {
 		FILE *source = fopen(realname, "rb");
@@ -1894,24 +1934,37 @@ bool write_file(const char *name, FILE *thefile, bool tmp,
 			goto cleanup_and_exit;
 		}
 
-		verdict = copy_file(source, thefile, TRUE);
+		verdict = copy_file(source, thefile, FALSE);
 
 		if (verdict < 0) {
+			fclose(thefile);
 			statusline(ALERT, _("Error reading temp file: %s"), strerror(errno));
 			goto cleanup_and_exit;
 		} else if (verdict > 0) {
+			fclose(thefile);
 			statusline(ALERT, _("Error writing %s: %s"), realname, strerror(errno));
 			goto cleanup_and_exit;
 		}
 
 		unlink(tempname);
-	} else
+	}
 #endif
+
+	if (sync_file(thefile) != 0) {
+		statusline(ALERT, _("Error writing %s: %s"), realname, strerror(errno));
+		goto cleanup_and_exit;
+	}
+
 	if (fclose(thefile) != 0) {
 		statusline(ALERT, _("Error writing %s: %s"), realname, strerror(errno));
 		goto cleanup_and_exit;
 	}
 
+	/* If no backups were requested, delete the temporary backup file
+	 * that was created just to ensure a failsafe replacement. */
+	if (!ISSET(MAKE_BACKUP) && backupname != NULL)
+		unlink(backupname);
+
 	/* When having written an entire buffer, update some administrivia. */
 	if (fullbuffer && method == OVERWRITE && !tmp) {
 		/* If the filename was changed, write a new lockfile when needed,