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Fix obscure race condition in GC root creation 

This should fix rare Hydra errors of the form:

error: symlinking `/nix/var/nix/gcroots/per-user/hydra/hydra-roots/7sfhs5fdmjxm8sqgcpd0pgcsmz1kq0l0-nixos-iso-0.1pre33785-33795' to `/nix/store/7sfhs5fdmjxm8sqgcpd0pgcsmz1kq0l0-nixos-iso-0.1pre33785-33795': File exists
This commit is contained in:
Eelco Dolstra 2012-04-16 18:47:01 +02:00
parent 154aa7f71a
commit 1132dd27ea
1 changed files with 18 additions and 16 deletions
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34
src/libstore/gc.cc
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@ -56,24 +56,22 @@ int LocalStore::openGCLock(LockType lockType)
} }
void createSymlink(const Path & link, const Path & target, bool careful) void createSymlink(const Path & link, const Path & target)
{ {
/* Create directories up to `gcRoot'. */ /* Create directories up to `gcRoot'. */
createDirs(dirOf(link)); createDirs(dirOf(link));
/* !!! shouldn't removing and creating the symlink be atomic? */ /* Create the new symlink. */
Path tempLink = (format("%1%.tmp-%2%-%3%")
/* Remove the old symlink. */ % link % getpid() % rand()).str();
if (pathExists(link)) { if (symlink(target.c_str(), tempLink.c_str()) == -1)
if (careful && (!isLink(link) || !isInStore(readLink(link))))
throw Error(format("cannot create symlink `%1%'; already exists") % link);
unlink(link.c_str());
}
/* And create the new one. */
if (symlink(target.c_str(), link.c_str()) == -1)
throw SysError(format("symlinking `%1%' to `%2%'") throw SysError(format("symlinking `%1%' to `%2%'")
% link % target); % tempLink % target);
/* Atomically replace the old one. */
if (rename(tempLink.c_str(), link.c_str()) == -1)
throw SysError(format("cannot rename `%1%' to `%2%'")
% tempLink % link);
} }
@ -88,7 +86,7 @@ void LocalStore::addIndirectRoot(const Path & path)
string hash = printHash32(hashString(htSHA1, path)); string hash = printHash32(hashString(htSHA1, path));
Path realRoot = canonPath((format("%1%/%2%/auto/%3%") Path realRoot = canonPath((format("%1%/%2%/auto/%3%")
% nixStateDir % gcRootsDir % hash).str()); % nixStateDir % gcRootsDir % hash).str());
createSymlink(realRoot, path, false); createSymlink(realRoot, path);
} }
@ -105,7 +103,11 @@ Path addPermRoot(StoreAPI & store, const Path & _storePath,
"(are you running nix-build inside the store?)") % gcRoot); "(are you running nix-build inside the store?)") % gcRoot);
if (indirect) { if (indirect) {
createSymlink(gcRoot, storePath, true); /* Don't clobber the the link if it already exists and doesn't
point to the Nix store. */
if (pathExists(gcRoot) && (!isLink(gcRoot) || !isInStore(readLink(gcRoot))))
throw Error(format("cannot create symlink `%1%'; already exists") % gcRoot);
createSymlink(gcRoot, storePath);
store.addIndirectRoot(gcRoot); store.addIndirectRoot(gcRoot);
} }
@ -120,7 +122,7 @@ Path addPermRoot(StoreAPI & store, const Path & _storePath,
% gcRoot % rootsDir); % gcRoot % rootsDir);
} }
createSymlink(gcRoot, storePath, false); createSymlink(gcRoot, storePath);
} }
/* Check that the root can be found by the garbage collector. /* Check that the root can be found by the garbage collector.