forked from lix-project/lix
Fix race condition when two processes create a hard link to a file in .links
This is a problem because one process may set the immutable bit before the second process has created its link. Addressed random Hydra failures such as: error: cannot rename `/nix/store/.tmp-link-17397-1804289383' to `/nix/store/rsvzm574rlfip3830ac7kmaa028bzl6h-nixos-0.1pre-git/upstart-interface-version': Operation not permitted
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@ -123,9 +123,6 @@ void LocalStore::optimisePath_(OptimiseStats & stats, const Path & path)
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printMsg(lvlTalkative, format("linking `%1%' to `%2%'") % path % linkPath);
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Path tempLink = (format("%1%/.tmp-link-%2%-%3%")
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% nixStore % getpid() % rand()).str();
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/* Make the containing directory writable, but only if it's not
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the store itself (we don't want or need to mess with its
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permissions). */
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@ -140,40 +137,53 @@ void LocalStore::optimisePath_(OptimiseStats & stats, const Path & path)
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so make it mutable first (and make it immutable again when
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we're done). We also have to make ‘path’ mutable, otherwise
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rename() will fail to delete it. */
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makeMutable(linkPath);
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MakeImmutable mk1(linkPath);
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makeMutable(path);
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MakeImmutable mk2(path);
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if (link(linkPath.c_str(), tempLink.c_str()) == -1) {
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if (errno == EMLINK) {
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/* Too many links to the same file (>= 32000 on most file
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systems). This is likely to happen with empty files.
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Just shrug and ignore. */
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printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
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return;
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/* Another process might be doing the same thing (creating a new
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link to ‘linkPath’) and make ‘linkPath’ immutable before we're
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done. In that case, just retry. */
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unsigned int retries = 1024;
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while (--retries > 0) {
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makeMutable(linkPath);
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MakeImmutable mk1(linkPath);
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Path tempLink = (format("%1%/.tmp-link-%2%-%3%")
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% nixStore % getpid() % rand()).str();
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if (link(linkPath.c_str(), tempLink.c_str()) == -1) {
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if (errno == EMLINK) {
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/* Too many links to the same file (>= 32000 on most
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file systems). This is likely to happen with empty
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files. Just shrug and ignore. */
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printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
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return;
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}
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if (errno == EPERM) continue;
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throw SysError(format("cannot link `%1%' to `%2%'") % tempLink % linkPath);
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}
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throw SysError(format("cannot link `%1%' to `%2%'") % tempLink % linkPath);
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}
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/* Atomically replace the old file with the new hard link. */
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if (rename(tempLink.c_str(), path.c_str()) == -1) {
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if (errno == EMLINK) {
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/* Some filesystems generate too many links on the rename,
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rather than on the original link. (Probably it
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temporarily increases the st_nlink field before
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decreasing it again.) */
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printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
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/* Unlink the temp link. */
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/* Atomically replace the old file with the new hard link. */
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if (rename(tempLink.c_str(), path.c_str()) == -1) {
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if (unlink(tempLink.c_str()) == -1)
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printMsg(lvlError, format("unable to unlink `%1%'") % tempLink);
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return;
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if (errno == EMLINK) {
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/* Some filesystems generate too many links on the
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rename, rather than on the original link.
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(Probably it temporarily increases the st_nlink
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field before decreasing it again.) */
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printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
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return;
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}
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if (errno == EPERM) continue;
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throw SysError(format("cannot rename `%1%' to `%2%'") % tempLink % path);
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}
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throw SysError(format("cannot rename `%1%' to `%2%'") % tempLink % path);
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break;
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}
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if (retries == 0) throw Error(format("cannot link `%1%' to `%2%'") % path % linkPath);
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stats.filesLinked++;
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stats.bytesFreed += st.st_size;
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stats.blocksFreed += st.st_blocks;
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