lix/src/libstore/gc.cc

#include "globals.hh"
#include "gc.hh"
#include "build.hh"
#include "pathlocks.hh"

#include <boost/shared_ptr.hpp>

#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>


static string tempRootsDir = "temproots";

/* The file to which we write our temporary roots. */
Path fnTempRoots;
static AutoCloseFD fdTempRoots;


void addTempRoot(const Path & path)
{
    /* Create the temporary roots file for this process. */
    if (fdTempRoots == -1) {

        while (1) {
            fnTempRoots = (format("%1%/%2%/%3%")
                % nixStateDir % tempRootsDir % getpid()).str();
        
            fdTempRoots = open(fnTempRoots.c_str(), O_RDWR | O_CREAT | O_TRUNC, 0600);
            if (fdTempRoots == -1)
                throw SysError(format("opening temporary roots file `%1%'") % fnTempRoots);

            debug(format("acquiring read lock on `%1%'") % fnTempRoots);
            lockFile(fdTempRoots, ltRead, true);

            /* Check whether the garbage collector didn't get in our
               way. */
            struct stat st;
            if (fstat(fdTempRoots, &st) == -1)
                throw SysError(format("statting `%1%'") % fnTempRoots);
            if (st.st_size == 0) break;
            
            /* The garbage collector deleted this file before we could
               get a lock.  (It won't delete the file after we get a
               lock.)  Try again. */
        }

    }

    /* Upgrade the lock to a write lock.  This will cause us to block
       if the garbage collector is holding our lock. */
    debug(format("acquiring write lock on `%1%'") % fnTempRoots);
    lockFile(fdTempRoots, ltWrite, true);

    string s = path + '\0';
    writeFull(fdTempRoots, (const unsigned char *) s.c_str(), s.size());

    /* Downgrade to a read lock. */
    debug(format("downgrading to read lock on `%1%'") % fnTempRoots);
    lockFile(fdTempRoots, ltRead, true);
}


typedef shared_ptr<AutoCloseFD> FDPtr;
typedef list<FDPtr> FDs;


static void readTempRoots(PathSet & tempRoots, FDs & fds)
{
    /* Read the `temproots' directory for per-process temporary root
       files. */
    Strings tempRootFiles = readDirectory(
        (format("%1%/%2%") % nixStateDir % tempRootsDir).str());

    for (Strings::iterator i = tempRootFiles.begin();
         i != tempRootFiles.end(); ++i)
    {
        Path path = (format("%1%/%2%/%3%") % nixStateDir % tempRootsDir % *i).str();

        debug(format("reading temporary root file `%1%'") % path);
            
        FDPtr fd(new AutoCloseFD(open(path.c_str(), O_RDWR, 0666)));
        if (*fd == -1) {
            /* It's okay if the file has disappeared. */
            if (errno == ENOENT) continue;
            throw SysError(format("opening temporary roots file `%1%'") % path);
        }

        /* Try to acquire a write lock without blocking.  This can
           only succeed if the owning process has died.  In that case
           we don't care about its temporary roots. */
        if (lockFile(*fd, ltWrite, false)) {
            printMsg(lvlError, format("removing stale temporary roots file `%1%'")
                % path);
            /* !!! write token, unlink */
            continue;
        }

        /* Acquire a read lock.  This will prevent the owning process
           from upgrading to a write lock, therefore it will block in
           addTempRoot(). */
        debug(format("waiting for read lock on `%1%'") % path);
        lockFile(*fd, ltRead, true);

        /* Read the entire file. */
        struct stat st;
        if (fstat(*fd, &st) == -1)
            throw SysError(format("statting `%1%'") % path);
        unsigned char buf[st.st_size]; /* !!! stack space */
        readFull(*fd, buf, st.st_size);
        debug(format("FILE SIZE %1%") % st.st_size);

        /* Extract the roots. */
        string contents((char *) buf, st.st_size);
        unsigned int pos = 0, end;

        while ((end = contents.find((char) 0, pos)) != string::npos) {
            Path root(contents, pos, end - pos);
            debug(format("got temporary root `%1%'") % root);
            assertStorePath(root);
            tempRoots.insert(root);
            pos = end + 1;
        }

        fds.push_back(fd); /* keep open */
    }
}


void collectGarbage(const PathSet & roots, GCAction action,
    PathSet & result)
{
    result.clear();
    
    /* !!! TODO: Acquire the global GC root.  This prevents
       a) New roots from being added.
       b) Processes from creating new temporary root files. */

    /* !!! Restrict read permission on the GC root.  Otherwise any
       process that can open the file for reading can DoS the
       collector. */
    
    /* Determine the live paths which is just the closure of the
       roots under the `references' relation. */
    PathSet livePaths;
    for (PathSet::const_iterator i = roots.begin(); i != roots.end(); ++i)
        computeFSClosure(canonPath(*i), livePaths);

    if (action == gcReturnLive) {
        result = livePaths;
        return;
    }

    /* Read the temporary roots.  This acquires read locks on all
       per-process temporary root files.  So after this point no paths
       can be added to the set of temporary roots. */
    PathSet tempRoots;
    FDs fds;
    readTempRoots(tempRoots, fds);

    for (FDs::iterator i = fds.begin(); i != fds.end(); ++i)
        debug(format("FD %1%") % (int) **i);
    
    /* !!! TODO: Try to acquire (without blocking) exclusive locks on
       the files in the `pending' directory.  Delete all files for
       which we managed to acquire such a lock (since if we could get
       such a lock, that means that the process that owned the file
       has died). */

    /* !!! TODO: Acquire shared locks on all files in the pending
       directories.  This prevents the set of pending paths from
       increasing while we are garbage-collecting.  Read the set of
       pending paths from those files. */

    /* Read the Nix store directory to find all currently existing
       paths. */
    Strings storeNames = readDirectory(nixStore);

    for (Strings::iterator i = storeNames.begin(); i != storeNames.end(); ++i) {
        Path path = canonPath(nixStore + "/" + *i);

        if (livePaths.find(path) != livePaths.end()) {
            debug(format("live path `%1%'") % path);
            continue;
        }

        if (tempRoots.find(path) != tempRoots.end()) {
            debug(format("temporary root `%1%'") % path);
            continue;
        }

        debug(format("dead path `%1%'") % path);
        result.insert(path);

        AutoCloseFD fdLock;

        if (action == gcDeleteDead) {
            printMsg(lvlInfo, format("deleting `%1%'") % path);

            /* Only delete a lock file if we can acquire a write lock
               on it.  That means that it's either stale, or the
               process that created it hasn't locked it yet.  In the
               latter case the other process will detect that we
               deleted the lock, and retry (see pathlocks.cc). */
            if (path.size() >= 5 && string(path, path.size() - 5) == ".lock") {

                fdLock = open(path.c_str(), O_RDWR);
                if (fdLock == -1) {
                    if (errno == ENOENT) continue;
                    throw SysError(format("opening lock file `%1%'") % path);
                }

                if (!lockFile(fdLock, ltWrite, false)) {
                    debug(format("skipping active lock `%1%'") % path);
                    continue;
                }
            }
            
            deleteFromStore(path);

            if (fdLock != -1)
                /* Write token to stale (deleted) lock file. */
                writeFull(fdLock, (const unsigned char *) "d", 1);
        }

        /* Only delete lock files if the path is belongs to doesn't
           exist and isn't a temporary root and we can acquire an
           exclusive lock on it. */
        /* !!! */
    }
}