Temporary file creation & renaming.

lib/misc/lvm-file.c

@@ -0,0 +1,101 @@
+/*
+ * Copyright (C) 2001 Sistina Software (UK) Limited.
+ *
+ * This file is released under the LGPL.
+ */
+
+#include "log.h"
+#include "lvm-file.h"
+#include "lvm-string.h"
+
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/file.h>
+#include <fcntl.h>
+
+/*
+ * Creates a temporary filename, and opens a descriptor to the
+ * file.  Both the filename and descriptor are needed so we can
+ * rename the file after successfully writing it.  Grab
+ * NFS-supported exclusive fcntl discretionary lock.
+ */
+int create_temp_name(const char *dir, char *buffer, size_t len,
+			     int *fd)
+{
+	int i, num;
+	pid_t pid;
+	char hostname[255];
+	struct flock lock = {
+		l_type: F_WRLCK,
+		l_whence: 0,
+		l_start: 0,
+		l_len: 0
+	};
+
+	num = rand();
+	pid = getpid();
+	if (gethostname(hostname, sizeof(hostname)) < 0) {
+		log_sys_error("gethostname", "");
+		strcpy(hostname, "nohostname");
+	}
+
+	for (i = 0; i < 20; i++, num++) {
+
+		if (lvm_snprintf(buffer, len, "%s/.lvm_%s_%d_%d",
+				 dir, hostname, pid, num) == -1) {
+			log_err("Not enough space to build temporary file "
+				"string.");
+			return 0;
+		}
+
+		*fd = open(buffer, O_CREAT | O_EXCL | O_WRONLY | O_APPEND,
+			   S_IRUSR | S_IRGRP | S_IROTH |
+			   S_IWUSR | S_IWGRP | S_IWOTH);
+		if (*fd < 0)
+			continue;
+
+		if (!fcntl(*fd, F_SETLK, &lock))
+			return 1;
+
+		close(*fd);
+	}
+
+	return 0;
+}
+
+/*
+ * NFS-safe rename of a temporary file to a common name, designed
+ * to avoid race conditions and not overwrite the destination if
+ * it exists.
+ *
+ * Try to create the new filename as a hard link to the original.
+ * Check the link count of the original file to see if it worked.
+ * (Assumes nothing else touches our temporary file!)  If it
+ * worked, unlink the old filename.
+ */
+int lvm_rename(const char *old, const char *new)
+{
+	struct stat buf;
+
+	link(old, new);
+
+	if (stat(old, &buf)) {
+		log_sys_error("stat", old);
+		return 0;
+	}
+
+	if (buf.st_nlink != 2) {
+		log_error("%s: rename to %s failed", old, new);
+		return 0;
+	}
+
+	if (unlink(old)) {
+		log_sys_error("unlink", old);
+		return 0;
+	}
+
+	return 1;
+}
+

lib/misc/lvm-file.h

@@ -0,0 +1,18 @@
+/*
+ * Copyright (C) 2001 Sistina Software (UK) Limited.
+ *
+ * This file is released under the LGPL.
+ */
+
+/*
+ * Create a temporary filename, and opens a descriptor to the file.
+ */
+int create_temp_name(const char *dir, char *buffer, size_t len, int *fd);
+
+/*
+ * NFS-safe rename of a temporary file to a common name, designed
+ * to avoid race conditions and not overwrite the destination if
+ * it exists.
+ */
+int lvm_rename(const char *old, const char *new);
+