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Fix race condition when two processes create a hard link to a file in .links

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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
This commit is contained in:
Eelco Dolstra 2012-08-05 21:41:44 -04:00
parent 108e14bb18
commit b6c989b801
1 changed files with 37 additions and 27 deletions
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64
src/libstore/optimise-store.cc
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@ -123,9 +123,6 @@ void LocalStore::optimisePath_(OptimiseStats & stats, const Path & path)
printMsg(lvlTalkative, format("linking `%1%' to `%2%'") % path % linkPath); printMsg(lvlTalkative, format("linking `%1%' to `%2%'") % path % linkPath);
Path tempLink = (format("%1%/.tmp-link-%2%-%3%")
% nixStore % getpid() % rand()).str();
/* Make the containing directory writable, but only if it's not /* Make the containing directory writable, but only if it's not
the store itself (we don't want or need to mess with its the store itself (we don't want or need to mess with its
permissions). */ permissions). */
@ -140,40 +137,53 @@ void LocalStore::optimisePath_(OptimiseStats & stats, const Path & path)
so make it mutable first (and make it immutable again when so make it mutable first (and make it immutable again when
we're done). We also have to make ‘path’ mutable, otherwise we're done). We also have to make ‘path’ mutable, otherwise
rename() will fail to delete it. */ rename() will fail to delete it. */
makeMutable(linkPath);
MakeImmutable mk1(linkPath);
makeMutable(path); makeMutable(path);
MakeImmutable mk2(path); MakeImmutable mk2(path);
if (link(linkPath.c_str(), tempLink.c_str()) == -1) { /* Another process might be doing the same thing (creating a new
if (errno == EMLINK) { link to ‘linkPath’) and make ‘linkPath’ immutable before we're
/* Too many links to the same file (>= 32000 on most file done. In that case, just retry. */
systems). This is likely to happen with empty files. unsigned int retries = 1024;
Just shrug and ignore. */ while (--retries > 0) {
printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath); makeMutable(linkPath);
return; MakeImmutable mk1(linkPath);
Path tempLink = (format("%1%/.tmp-link-%2%-%3%")
% nixStore % getpid() % rand()).str();
if (link(linkPath.c_str(), tempLink.c_str()) == -1) {
if (errno == EMLINK) {
/* Too many links to the same file (>= 32000 on most
file systems). This is likely to happen with empty
files. Just shrug and ignore. */
printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
return;
}
if (errno == EPERM) continue;
throw SysError(format("cannot link `%1%' to `%2%'") % tempLink % linkPath);
} }
throw SysError(format("cannot link `%1%' to `%2%'") % tempLink % linkPath);
}
/* Atomically replace the old file with the new hard link. */ /* Atomically replace the old file with the new hard link. */
if (rename(tempLink.c_str(), path.c_str()) == -1) { if (rename(tempLink.c_str(), path.c_str()) == -1) {
if (errno == EMLINK) {
/* Some filesystems generate too many links on the rename,
rather than on the original link. (Probably it
temporarily increases the st_nlink field before
decreasing it again.) */
printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
/* Unlink the temp link. */
if (unlink(tempLink.c_str()) == -1) if (unlink(tempLink.c_str()) == -1)
printMsg(lvlError, format("unable to unlink `%1%'") % tempLink); printMsg(lvlError, format("unable to unlink `%1%'") % tempLink);
return; if (errno == EMLINK) {
/* Some filesystems generate too many links on the
rename, rather than on the original link.
(Probably it temporarily increases the st_nlink
field before decreasing it again.) */
printMsg(lvlInfo, format("`%1%' has maximum number of links") % linkPath);
return;
}
if (errno == EPERM) continue;
throw SysError(format("cannot rename `%1%' to `%2%'") % tempLink % path);
} }
throw SysError(format("cannot rename `%1%' to `%2%'") % tempLink % path);
break;
} }
if (retries == 0) throw Error(format("cannot link `%1%' to `%2%'") % path % linkPath);
stats.filesLinked++; stats.filesLinked++;
stats.bytesFreed += st.st_size; stats.bytesFreed += st.st_size;
stats.blocksFreed += st.st_blocks; stats.blocksFreed += st.st_blocks;