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* nix-store, nix-instantiate: added an option `--add-root' to

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immediately add the result as a permanent GC root.  This is the only
  way to prevent a race with the garbage collector.  For instance, the
  old style

    ln -s $(nix-store -r $(nix-instantiate foo.nix)) \
      /nix/var/nix/gcroots/result

  has two time windows in which the garbage collector can interfere
  (by GC'ing the derivation and the output, respectively).  On the
  other hand,

    nix-store --add-root /nix/var/nix/gcroots/result -r \
      $(nix-instantiate --add-root /nix/var/nix/gcroots/drv \
        foo.nix)

  is safe.

* nix-build: use `--add-root' to prevent GC races.
This commit is contained in:
Eelco Dolstra 2005-02-01 12:36:25 +00:00
parent a6b65fd5e1
commit dcc37c236c
11 changed files with 165 additions and 17 deletions
Download patch file Download diff file Expand all files Collapse all files

46
src/libstore/gc.cc
View file

@ -39,6 +39,48 @@ static int openGCLock(LockType lockType)
}
static void createDirs(const Path & path)
{
if (path == "") return;
createDirs(dirOf(path));
if (!pathExists(path))
if (mkdir(path.c_str(), 0777) == -1)
throw SysError(format("creating directory `%1%'") % path);
}
Path addPermRoot(const Path & _storePath, const Path & _gcRoot)
{
Path storePath(canonPath(_storePath));
Path gcRoot(canonPath(_gcRoot));
Path rootsDir = canonPath((format("%1%/%2%") % nixStateDir % "gcroots").str());
if (string(gcRoot, 0, rootsDir.size() + 1) != rootsDir + "/")
throw Error(format(
"path `%1%' is not a valid garbage collector root; "
"it's not in the `%1%' directory")
% gcRoot % rootsDir);
/* Grab the global GC root. This prevents the set of permanent
roots from increasing while a GC is in progress. */
AutoCloseFD fdGCLock = openGCLock(ltRead);
/* Create directories up to `gcRoot'. */
createDirs(dirOf(gcRoot));
/* Remove the old symlink. */
unlink(gcRoot.c_str());
/* And create the new own. */
if (symlink(storePath.c_str(), gcRoot.c_str()) == -1)
throw SysError(format("symlinking `%1%' to `%2%'")
% gcRoot % storePath);
return gcRoot;
}
static string tempRootsDir = "temproots";
/* The file to which we write our temporary roots. */
@ -210,6 +252,9 @@ void collectGarbage(const PathSet & roots, GCAction action,
b) Processes from creating new temporary root files. */
AutoCloseFD fdGCLock = openGCLock(ltWrite);
/* !!! Find the roots here, after we've grabbed the GC lock, since
the set of permanent roots cannot increase now. */
/* Determine the live paths which is just the closure of the
roots under the `references' relation. */
PathSet livePaths;
@ -264,6 +309,7 @@ void collectGarbage(const PathSet & roots, GCAction action,
will not work anymore because we get cycles. */
storePaths = topoSort(storePaths2);
/* Try to delete store paths in the topologically sorted order. */
for (Paths::iterator i = storePaths.begin(); i != storePaths.end(); ++i) {
debug(format("considering deletion of `%1%'") % *i);