tvl-depot/src/libstore/local-store.cc

#include "local-store.hh"
#include "util.hh"
#include "globals.hh"
#include "db.hh"
#include "archive.hh"
#include "pathlocks.hh"
#include "aterm.hh"
#include "derivations-ast.hh"
#include "config.h"
    
#include <iostream>
#include <algorithm>

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <utime.h>


namespace nix {

    
/* Nix database. */
static Database nixDB;


/* Database tables. */

/* dbValidPaths :: Path -> ()

   The existence of a key $p$ indicates that path $p$ is valid (that
   is, produced by a succesful build). */
static TableId dbValidPaths = 0;

/* dbReferences :: Path -> [Path]

   This table lists the outgoing file system references for each
   output path that has been built by a Nix derivation.  These are
   found by scanning the path for the hash components of input
   paths. */
static TableId dbReferences = 0;

/* dbReferrers :: Path -> Path

   This table is just the reverse mapping of dbReferences.  This table
   can have duplicate keys, each corresponding value denoting a single
   referrer. */
static TableId dbReferrers = 0;

/* dbSubstitutes :: Path -> [[Path]]

   Each pair $(p, subs)$ tells Nix that it can use any of the
   substitutes in $subs$ to build path $p$.  Each substitute defines a
   command-line invocation of a program (i.e., the first list element
   is the full path to the program, the remaining elements are
   arguments).

   The main purpose of this is for distributed caching of derivates.
   One system can compute a derivate and put it on a website (as a Nix
   archive), for instance, and then another system can register a
   substitute for that derivate.  The substitute in this case might be
   a Nix derivation that fetches the Nix archive.
*/
static TableId dbSubstitutes = 0;

/* dbDerivers :: Path -> [Path]

   This table lists the derivation used to build a path.  There can
   only be multiple such paths for fixed-output derivations (i.e.,
   derivations specifying an expected hash). */
static TableId dbDerivers = 0;


bool Substitute::operator == (const Substitute & sub) const
{
    return program == sub.program
        && args == sub.args;
}


static void upgradeStore07();
static void upgradeStore09();


void checkStoreNotSymlink()
{
    if (getEnv("NIX_IGNORE_SYMLINK_STORE") == "1") return;
    Path path = nixStore;
    struct stat st;
    while (path != "/") {
        if (lstat(path.c_str(), &st))
            throw SysError(format("getting status of `%1%'") % path);
        if (S_ISLNK(st.st_mode))
            throw Error(format(
                "the path `%1%' is a symlink; "
                "this is not allowed for the Nix store and its parent directories")
                % path);
        path = dirOf(path);
    }
}


LocalStore::LocalStore(bool reserveSpace)
{
    if (readOnlyMode) return;

    checkStoreNotSymlink();

    try {
        Path reservedPath = nixDBPath + "/reserved";
        string s = querySetting("gc-reserved-space", "");
        int reservedSize;
        if (!string2Int(s, reservedSize)) reservedSize = 1024 * 1024;
        if (reserveSpace) {
            struct stat st;
            if (stat(reservedPath.c_str(), &st) == -1 ||
                st.st_size != reservedSize)
                writeFile(reservedPath, string(reservedSize, 'X'));
        }
        else
            deletePath(reservedPath);
    } catch (SysError & e) { /* don't care about errors */
    }

    try {
        nixDB.open(nixDBPath);
    } catch (DbNoPermission & e) {
        printMsg(lvlTalkative, "cannot access Nix database; continuing anyway");
        readOnlyMode = true;
        return;
    }
    dbValidPaths = nixDB.openTable("validpaths");
    dbReferences = nixDB.openTable("references");
    dbReferrers = nixDB.openTable("referrers", true); /* must be sorted */
    dbSubstitutes = nixDB.openTable("substitutes");
    dbDerivers = nixDB.openTable("derivers");

    int curSchema = 0;
    Path schemaFN = nixDBPath + "/schema";
    if (pathExists(schemaFN)) {
        string s = readFile(schemaFN);
        if (!string2Int(s, curSchema))
            throw Error(format("`%1%' is corrupt") % schemaFN);
    }

    if (curSchema > nixSchemaVersion)
        throw Error(format("current Nix store schema is version %1%, but I only support %2%")
            % curSchema % nixSchemaVersion);

    if (curSchema < nixSchemaVersion) {
        if (curSchema <= 1)
            upgradeStore07();
        if (curSchema == 2)
            upgradeStore09();
        writeFile(schemaFN, (format("%1%") % nixSchemaVersion).str());
    }
}


LocalStore::~LocalStore()
{
    /* If the database isn't open, this is a NOP. */
    nixDB.close();
}


void createStoreTransaction(Transaction & txn)
{
    Transaction txn2(nixDB);
    txn2.moveTo(txn);
}


void copyPath(const Path & src, const Path & dst, PathFilter & filter)
{
    debug(format("copying `%1%' to `%2%'") % src % dst);

    /* Dump an archive of the path `src' into a string buffer, then
       restore the archive to `dst'.  This is not a very good method
       for very large paths, but `copyPath' is mainly used for small
       files. */ 

    StringSink sink;
    dumpPath(src, sink, filter);

    StringSource source(sink.s);
    restorePath(dst, source);
}


static void _canonicalisePathMetaData(const Path & path)
{
    checkInterrupt();

    struct stat st;
    if (lstat(path.c_str(), &st))
	throw SysError(format("getting attributes of path `%1%'") % path);

    /* Change ownership to the current uid.  If its a symlink, use
       lchown if available, otherwise don't bother.  Wrong ownership
       of a symlink doesn't matter, since the owning user can't change
       the symlink and can't delete it because the directory is not
       writable.  The only exception is top-level paths in the Nix
       store (since that directory is group-writable for the Nix build
       users group); we check for this case below. */
    if (st.st_uid != geteuid()) {
#if HAVE_LCHOWN
        if (lchown(path.c_str(), geteuid(), (gid_t) -1) == -1)
#else
        if (!S_ISLNK(st.st_mode) &&
            chown(path.c_str(), geteuid(), (gid_t) -1) == -1)
#endif
            throw SysError(format("changing owner of `%1%' to %2%")
                % path % geteuid());
    }
    
    if (!S_ISLNK(st.st_mode)) {

        /* Mask out all type related bits. */
        mode_t mode = st.st_mode & ~S_IFMT;
        
        if (mode != 0444 && mode != 0555) {
            mode = (st.st_mode & S_IFMT)
                 | 0444
                 | (st.st_mode & S_IXUSR ? 0111 : 0);
            if (chmod(path.c_str(), mode) == -1)
                throw SysError(format("changing mode of `%1%' to %2$o") % path % mode);
        }

        if (st.st_mtime != 0) {
            struct utimbuf utimbuf;
            utimbuf.actime = st.st_atime;
            utimbuf.modtime = 0;
            if (utime(path.c_str(), &utimbuf) == -1) 
                throw SysError(format("changing modification time of `%1%'") % path);
        }

    }

    if (S_ISDIR(st.st_mode)) {
        Strings names = readDirectory(path);
	for (Strings::iterator i = names.begin(); i != names.end(); ++i)
	    _canonicalisePathMetaData(path + "/" + *i);
    }
}


void canonicalisePathMetaData(const Path & path)
{
    _canonicalisePathMetaData(path);

    /* On platforms that don't have lchown(), the top-level path can't
       be a symlink, since we can't change its ownership. */
    struct stat st;
    if (lstat(path.c_str(), &st))
	throw SysError(format("getting attributes of path `%1%'") % path);

    if (st.st_uid != geteuid()) {
        assert(S_ISLNK(st.st_mode));
        throw Error(format("wrong ownership of top-level store path `%1%'") % path);
    }
}


bool isValidPathTxn(const Transaction & txn, const Path & path)
{
    string s;
    return nixDB.queryString(txn, dbValidPaths, path, s);
}


bool LocalStore::isValidPath(const Path & path)
{
    return isValidPathTxn(noTxn, path);
}


static Substitutes readSubstitutes(const Transaction & txn,
    const Path & srcPath);


static bool isRealisablePath(const Transaction & txn, const Path & path)
{
    return isValidPathTxn(txn, path)
        || readSubstitutes(txn, path).size() > 0;
}


static string addPrefix(const string & prefix, const string & s)
{
    return prefix + string(1, (char) 0) + s;
}


static string stripPrefix(const string & prefix, const string & s)
{
    if (s.size() <= prefix.size() ||
        string(s, 0, prefix.size()) != prefix ||
        s[prefix.size()] != 0)
        throw Error(format("string `%1%' is missing prefix `%2%'")
            % s % prefix);
    return string(s, prefix.size() + 1);
}


static PathSet getReferrers(const Transaction & txn, const Path & storePath)
{
    PathSet referrers;
    Strings keys;
    nixDB.enumTable(txn, dbReferrers, keys, storePath + string(1, (char) 0));
    for (Strings::iterator i = keys.begin(); i != keys.end(); ++i)
        referrers.insert(stripPrefix(storePath, *i));
    return referrers;
}


void setReferences(const Transaction & txn, const Path & storePath,
    const PathSet & references)
{
    /* For unrealisable paths, we can only clear the references. */
    if (references.size() > 0 && !isRealisablePath(txn, storePath))
        throw Error(
            format("cannot set references for path `%1%' which is invalid and has no substitutes")
            % storePath);

    Paths oldReferences;
    nixDB.queryStrings(txn, dbReferences, storePath, oldReferences);

    PathSet oldReferences2(oldReferences.begin(), oldReferences.end());
    if (oldReferences2 == references) return;
    
    nixDB.setStrings(txn, dbReferences, storePath,
        Paths(references.begin(), references.end()));

    /* Update the referrers mappings of all new referenced paths. */
    for (PathSet::const_iterator i = references.begin();
         i != references.end(); ++i)
        if (oldReferences2.find(*i) == oldReferences2.end())
            nixDB.setString(txn, dbReferrers, addPrefix(*i, storePath), "");

    /* Remove referrer mappings from paths that are no longer
       references. */
    for (Paths::iterator i = oldReferences.begin();
         i != oldReferences.end(); ++i)
        if (references.find(*i) == references.end())
            nixDB.delPair(txn, dbReferrers, addPrefix(*i, storePath));
}


void queryReferences(const Transaction & txn,
    const Path & storePath, PathSet & references)
{
    Paths references2;
    if (!isRealisablePath(txn, storePath))
        throw Error(format("path `%1%' is not valid") % storePath);
    nixDB.queryStrings(txn, dbReferences, storePath, references2);
    references.insert(references2.begin(), references2.end());
}


void LocalStore::queryReferences(const Path & storePath,
    PathSet & references)
{
    nix::queryReferences(noTxn, storePath, references);
}


void queryReferrers(const Transaction & txn,
    const Path & storePath, PathSet & referrers)
{
    if (!isRealisablePath(txn, storePath))
        throw Error(format("path `%1%' is not valid") % storePath);
    PathSet referrers2 = getReferrers(txn, storePath);
    referrers.insert(referrers2.begin(), referrers2.end());
}


void LocalStore::queryReferrers(const Path & storePath,
    PathSet & referrers)
{
    nix::queryReferrers(noTxn, storePath, referrers);
}


void setDeriver(const Transaction & txn, const Path & storePath,
    const Path & deriver)
{
    assertStorePath(storePath);
    if (deriver == "") return;
    assertStorePath(deriver);
    if (!isRealisablePath(txn, storePath))
        throw Error(format("path `%1%' is not valid") % storePath);
    nixDB.setString(txn, dbDerivers, storePath, deriver);
}


Path queryDeriver(const Transaction & txn, const Path & storePath)
{
    if (!isRealisablePath(txn, storePath))
        throw Error(format("path `%1%' is not valid") % storePath);
    Path deriver;
    if (nixDB.queryString(txn, dbDerivers, storePath, deriver))
        return deriver;
    else
        return "";
}


const int substituteVersion = 2;


static Substitutes readSubstitutes(const Transaction & txn,
    const Path & srcPath)
{
    Strings ss;
    nixDB.queryStrings(txn, dbSubstitutes, srcPath, ss);

    Substitutes subs;
    
    for (Strings::iterator i = ss.begin(); i != ss.end(); ++i) {
        if (i->size() < 4 || (*i)[3] != 0) {
            /* Old-style substitute.  !!! remove this code
               eventually? */
            break;
        }
        Strings ss2 = unpackStrings(*i);
        if (ss2.size() == 0) continue;
        int version;
        if (!string2Int(ss2.front(), version)) continue;
        if (version != substituteVersion) continue;
        if (ss2.size() != 4) throw Error("malformed substitute");
        Strings::iterator j = ss2.begin();
        j++;
        Substitute sub;
        sub.deriver = *j++;
        sub.program = *j++;
        sub.args = unpackStrings(*j++);
        subs.push_back(sub);
    }

    return subs;
}


static void writeSubstitutes(const Transaction & txn,
    const Path & srcPath, const Substitutes & subs)
{
    Strings ss;

    for (Substitutes::const_iterator i = subs.begin();
         i != subs.end(); ++i)
    {
        Strings ss2;
        ss2.push_back((format("%1%") % substituteVersion).str());
        ss2.push_back(i->deriver);
        ss2.push_back(i->program);
        ss2.push_back(packStrings(i->args));
        ss.push_back(packStrings(ss2));
    }

    nixDB.setStrings(txn, dbSubstitutes, srcPath, ss);
}


void registerSubstitute(const Transaction & txn,
    const Path & srcPath, const Substitute & sub)
{
    assertStorePath(srcPath);
    
    Substitutes subs = readSubstitutes(txn, srcPath);

    if (find(subs.begin(), subs.end(), sub) != subs.end())
        return;

    /* New substitutes take precedence over old ones.  If the
       substitute is already present, it's moved to the front. */
    remove(subs.begin(), subs.end(), sub);
    subs.push_front(sub);
        
    writeSubstitutes(txn, srcPath, subs);
}


Substitutes querySubstitutes(const Transaction & txn, const Path & path)
{
    return readSubstitutes(txn, path);
}


Substitutes LocalStore::querySubstitutes(const Path & path)
{
    return nix::querySubstitutes(noTxn, path);
}


static void invalidatePath(Transaction & txn, const Path & path);


void clearSubstitutes()
{
    Transaction txn(nixDB);
    
    /* Iterate over all paths for which there are substitutes. */
    Paths subKeys;
    nixDB.enumTable(txn, dbSubstitutes, subKeys);
    for (Paths::iterator i = subKeys.begin(); i != subKeys.end(); ++i) {
        
        /* Delete all substitutes for path *i. */
        nixDB.delPair(txn, dbSubstitutes, *i);
        
        /* Maintain the cleanup invariant. */
        if (!isValidPathTxn(txn, *i))
            invalidatePath(txn, *i);
    }

    /* !!! there should be no referrers to any of the invalid
       substitutable paths.  This should be the case by construction
       (the only referrers can be other invalid substitutable paths,
       which have all been removed now). */
    
    txn.commit();
}


static void setHash(const Transaction & txn, const Path & storePath,
    const Hash & hash)
{
    assert(hash.type == htSHA256);
    nixDB.setString(txn, dbValidPaths, storePath, "sha256:" + printHash(hash));
}


static Hash queryHash(const Transaction & txn, const Path & storePath)
{
    string s;
    nixDB.queryString(txn, dbValidPaths, storePath, s);
    string::size_type colon = s.find(':');
    if (colon == string::npos)
        throw Error(format("corrupt hash `%1%' in valid-path entry for `%2%'")
            % s % storePath);
    HashType ht = parseHashType(string(s, 0, colon));
    if (ht == htUnknown)
        throw Error(format("unknown hash type `%1%' in valid-path entry for `%2%'")
            % string(s, 0, colon) % storePath);
    return parseHash(ht, string(s, colon + 1));
}


Hash LocalStore::queryPathHash(const Path & path)
{
    if (!isValidPath(path))
        throw Error(format("path `%1%' is not valid") % path);
    return queryHash(noTxn, path);
}


void registerValidPath(const Transaction & txn,
    const Path & path, const Hash & hash, const PathSet & references,
    const Path & deriver)
{
    ValidPathInfo info;
    info.path = path;
    info.hash = hash;
    info.references = references;
    info.deriver = deriver;
    ValidPathInfos infos;
    infos.push_back(info);
    registerValidPaths(txn, infos);
}


void registerValidPaths(const Transaction & txn,
    const ValidPathInfos & infos)
{
    PathSet newPaths;
    for (ValidPathInfos::const_iterator i = infos.begin();
         i != infos.end(); ++i)
        newPaths.insert(i->path);
        
    for (ValidPathInfos::const_iterator i = infos.begin();
         i != infos.end(); ++i)
    {
        assertStorePath(i->path);

        debug(format("registering path `%1%'") % i->path);
        setHash(txn, i->path, i->hash);

        setReferences(txn, i->path, i->references);
    
        /* Check that all referenced paths are also valid (or about to
           become valid). */
        for (PathSet::iterator j = i->references.begin();
             j != i->references.end(); ++j)
            if (!isValidPathTxn(txn, *j) && newPaths.find(*j) == newPaths.end())
                throw Error(format("cannot register path `%1%' as valid, since its reference `%2%' is invalid")
                    % i->path % *j);

        setDeriver(txn, i->path, i->deriver);
    }
}


/* Invalidate a path.  The caller is responsible for checking that
   there are no referrers. */
static void invalidatePath(Transaction & txn, const Path & path)
{
    debug(format("unregistering path `%1%'") % path);

    /* Clear the `references' entry for this path, as well as the
       inverse `referrers' entries, and the `derivers' entry; but only
       if there are no substitutes for this path.  This maintains the
       cleanup invariant. */
    if (querySubstitutes(txn, path).size() == 0) {
        setReferences(txn, path, PathSet());
        nixDB.delPair(txn, dbDerivers, path);
    }
    
    nixDB.delPair(txn, dbValidPaths, path);
}


Path LocalStore::addToStore(const Path & _srcPath, bool fixed,
    bool recursive, string hashAlgo, PathFilter & filter)
{
    Path srcPath(absPath(_srcPath));
    debug(format("adding `%1%' to the store") % srcPath);

    std::pair<Path, Hash> pr =
        computeStorePathForPath(srcPath, fixed, recursive, hashAlgo, filter);
    Path & dstPath(pr.first);
    Hash & h(pr.second);

    addTempRoot(dstPath);

    if (!isValidPath(dstPath)) {

        /* The first check above is an optimisation to prevent
           unnecessary lock acquisition. */

        PathLocks outputLock(singleton<PathSet, Path>(dstPath));

        if (!isValidPath(dstPath)) {

            if (pathExists(dstPath)) deletePathWrapped(dstPath);

            copyPath(srcPath, dstPath, filter);

            Hash h2 = hashPath(htSHA256, dstPath, filter);
            if (h != h2)
                throw Error(format("contents of `%1%' changed while copying it to `%2%' (%3% -> %4%)")
                    % srcPath % dstPath % printHash(h) % printHash(h2));

            canonicalisePathMetaData(dstPath);
            
            Transaction txn(nixDB);
            registerValidPath(txn, dstPath, h, PathSet(), "");
            txn.commit();
        }

        outputLock.setDeletion(true);
    }

    return dstPath;
}


Path LocalStore::addTextToStore(const string & suffix, const string & s,
    const PathSet & references)
{
    Path dstPath = computeStorePathForText(suffix, s, references);
    
    addTempRoot(dstPath);

    if (!isValidPath(dstPath)) {

        PathLocks outputLock(singleton<PathSet, Path>(dstPath));

        if (!isValidPath(dstPath)) {

            if (pathExists(dstPath)) deletePathWrapped(dstPath);

            writeStringToFile(dstPath, s);

            canonicalisePathMetaData(dstPath);
            
            Transaction txn(nixDB);
            registerValidPath(txn, dstPath,
                hashPath(htSHA256, dstPath), references, "");
            txn.commit();
        }

        outputLock.setDeletion(true);
    }

    return dstPath;
}


void deleteFromStore(const Path & _path, unsigned long long & bytesFreed)
{
    bytesFreed = 0;
    Path path(canonPath(_path));

    assertStorePath(path);

    Transaction txn(nixDB);
    if (isValidPathTxn(txn, path)) {
        PathSet referrers = getReferrers(txn, path);
        for (PathSet::iterator i = referrers.begin();
             i != referrers.end(); ++i)
            if (*i != path && isValidPathTxn(txn, *i))
                throw PathInUse(format("cannot delete path `%1%' because it is in use by path `%2%'") % path % *i);
        invalidatePath(txn, path);
    }
    txn.commit();

    deletePathWrapped(path, bytesFreed);
}


void verifyStore(bool checkContents)
{
    Transaction txn(nixDB);

    
    printMsg(lvlInfo, "checking path existence");

    Paths paths;
    PathSet validPaths;
    nixDB.enumTable(txn, dbValidPaths, paths);

    for (Paths::iterator i = paths.begin(); i != paths.end(); ++i) {
        if (!pathExists(*i)) {
            printMsg(lvlError, format("path `%1%' disappeared") % *i);
            invalidatePath(txn, *i);
        } else if (!isStorePath(*i)) {
            printMsg(lvlError, format("path `%1%' is not in the Nix store") % *i);
            invalidatePath(txn, *i);
        } else {
            if (checkContents) {
                debug(format("checking contents of `%1%'") % *i);
                Hash expected = queryHash(txn, *i);
                Hash current = hashPath(expected.type, *i);
                if (current != expected) {
                    printMsg(lvlError, format("path `%1%' was modified! "
                                 "expected hash `%2%', got `%3%'")
                        % *i % printHash(expected) % printHash(current));
                }
            }
            validPaths.insert(*i);
        }
    }


    printMsg(lvlInfo, "checking path realisability");
    
    /* "Realisable" paths are those that are valid or have a
       substitute. */
    PathSet realisablePaths(validPaths);

    /* Check that the values of the substitute mappings are valid
       paths. */ 
    Paths subKeys;
    nixDB.enumTable(txn, dbSubstitutes, subKeys);
    for (Paths::iterator i = subKeys.begin(); i != subKeys.end(); ++i) {
        Substitutes subs = readSubstitutes(txn, *i);
        if (!isStorePath(*i)) {
            printMsg(lvlError, format("removing substitutes for non-store path `%1%'") % *i);
            nixDB.delPair(txn, dbSubstitutes, *i);
        }
        else if (subs.size() == 0)
            nixDB.delPair(txn, dbSubstitutes, *i);
        else
	    realisablePaths.insert(*i);
    }
    

    /* Check the cleanup invariant: only realisable paths can have
       `references', `referrers', or `derivers' entries. */


    /* Check the `derivers' table. */
    printMsg(lvlInfo, "checking the derivers table");
    Paths deriversKeys;
    nixDB.enumTable(txn, dbDerivers, deriversKeys);
    for (Paths::iterator i = deriversKeys.begin();
         i != deriversKeys.end(); ++i)
    {
        if (realisablePaths.find(*i) == realisablePaths.end()) {
            printMsg(lvlError, format("removing deriver entry for unrealisable path `%1%'")
                % *i);
            nixDB.delPair(txn, dbDerivers, *i);
        }
        else {
            Path deriver = queryDeriver(txn, *i);
            if (!isStorePath(deriver)) {
                printMsg(lvlError, format("removing corrupt deriver `%1%' for `%2%'")
                    % deriver % *i);
                nixDB.delPair(txn, dbDerivers, *i);
            }
        }
    }


    /* Check the `references' table. */
    printMsg(lvlInfo, "checking the references table");
    Paths referencesKeys;
    nixDB.enumTable(txn, dbReferences, referencesKeys);
    for (Paths::iterator i = referencesKeys.begin();
         i != referencesKeys.end(); ++i)
    {
        if (realisablePaths.find(*i) == realisablePaths.end()) {
            printMsg(lvlError, format("removing references entry for unrealisable path `%1%'")
                % *i);
            setReferences(txn, *i, PathSet());
        }
        else {
            bool isValid = validPaths.find(*i) != validPaths.end();
            PathSet references;
            queryReferences(txn, *i, references);
            for (PathSet::iterator j = references.begin();
                 j != references.end(); ++j)
            {
                string dummy;
                if (!nixDB.queryString(txn, dbReferrers, addPrefix(*j, *i), dummy)) {
                    printMsg(lvlError, format("adding missing referrer mapping from `%1%' to `%2%'")
                        % *j % *i);
                    nixDB.setString(txn, dbReferrers, addPrefix(*j, *i), "");
                }
                if (isValid && validPaths.find(*j) == validPaths.end()) {
                    printMsg(lvlError, format("incomplete closure: `%1%' needs missing `%2%'")
                        % *i % *j);
                }
            }
        }
    }

    /* Check the `referrers' table. */
    printMsg(lvlInfo, "checking the referrers table");
    Strings referrers;
    nixDB.enumTable(txn, dbReferrers, referrers);
    for (Strings::iterator i = referrers.begin(); i != referrers.end(); ++i) {

        /* Decode the entry (it's a tuple of paths). */
        string::size_type nul = i->find((char) 0);
        if (nul == string::npos) {
            printMsg(lvlError, format("removing bad referrer table entry `%1%'") % *i);
            nixDB.delPair(txn, dbReferrers, *i);
            continue;
        }
        Path to(*i, 0, nul);
        Path from(*i, nul + 1);
        
        if (realisablePaths.find(to) == realisablePaths.end()) {
            printMsg(lvlError, format("removing referrer entry from `%1%' to unrealisable `%2%'")
                % from % to);
            nixDB.delPair(txn, dbReferrers, *i);
        }

        else if (realisablePaths.find(from) == realisablePaths.end()) {
            printMsg(lvlError, format("removing referrer entry from unrealisable `%1%' to `%2%'")
                % from % to);
            nixDB.delPair(txn, dbReferrers, *i);
        }
        
        else {
            PathSet references;
            queryReferences(txn, from, references);
            if (find(references.begin(), references.end(), to) == references.end()) {
                printMsg(lvlError, format("adding missing referrer mapping from `%1%' to `%2%'")
                    % from % to);
                references.insert(to);
                setReferences(txn, from, references);
            }
        }
        
    }

    
    txn.commit();
}


/* Upgrade from schema 1 (Nix <= 0.7) to schema 2 (Nix >= 0.8). */
static void upgradeStore07()
{
    printMsg(lvlError, "upgrading Nix store to new schema (this may take a while)...");

    Transaction txn(nixDB);

    Paths validPaths2;
    nixDB.enumTable(txn, dbValidPaths, validPaths2);
    PathSet validPaths(validPaths2.begin(), validPaths2.end());

    std::cerr << "hashing paths...";
    int n = 0;
    for (PathSet::iterator i = validPaths.begin(); i != validPaths.end(); ++i) {
        checkInterrupt();
        string s;
        nixDB.queryString(txn, dbValidPaths, *i, s);
        if (s == "") {
            Hash hash = hashPath(htSHA256, *i);
            setHash(txn, *i, hash);
            std::cerr << ".";
            if (++n % 1000 == 0) {
                txn.commit();
                txn.begin(nixDB);
            }
        }
    }
    std::cerr << std::endl;

    txn.commit();

    txn.begin(nixDB);
    
    std::cerr << "processing closures...";
    for (PathSet::iterator i = validPaths.begin(); i != validPaths.end(); ++i) {
        checkInterrupt();
        if (i->size() > 6 && string(*i, i->size() - 6) == ".store") {
            ATerm t = ATreadFromNamedFile(i->c_str());
            if (!t) throw Error(format("cannot read aterm from `%1%'") % *i);

            ATermList roots, elems;
            if (!matchOldClosure(t, roots, elems)) continue;

            for (ATermIterator j(elems); j; ++j) {

                ATerm path2;
                ATermList references2;
                if (!matchOldClosureElem(*j, path2, references2)) continue;

                Path path = aterm2String(path2);
                if (validPaths.find(path) == validPaths.end())
                    /* Skip this path; it's invalid.  This is a normal
                       condition (Nix <= 0.7 did not enforce closure
                       on closure store expressions). */
                    continue;

                PathSet references;
                for (ATermIterator k(references2); k; ++k) {
                    Path reference = aterm2String(*k);
                    if (validPaths.find(reference) == validPaths.end())
                        /* Bad reference.  Set it anyway and let the
                           user fix it. */
                        printMsg(lvlError, format("closure `%1%' contains reference from `%2%' "
                                     "to invalid path `%3%' (run `nix-store --verify')")
                            % *i % path % reference);
                    references.insert(reference);
                }

                PathSet prevReferences;
                queryReferences(txn, path, prevReferences);
                if (prevReferences.size() > 0 && references != prevReferences)
                    printMsg(lvlError, format("warning: conflicting references for `%1%'") % path);

                if (references != prevReferences)
                    setReferences(txn, path, references);
            }
            
            std::cerr << ".";
        }
    }
    std::cerr << std::endl;

    /* !!! maybe this transaction is way too big */
    txn.commit();
}


/* Upgrade from schema 2 (0.8 <= Nix <= 0.9) to schema 3 (Nix >=
   0.10).  The only thing to do here is to upgrade the old `referer'
   table (which causes quadratic complexity in some cases) to the new
   (and properly spelled) `referrer' table. */
static void upgradeStore09() 
{
    /* !!! we should disallow concurrent upgrades */
    
    printMsg(lvlError, "upgrading Nix store to new schema (this may take a while)...");

    if (!pathExists(nixDBPath + "/referers")) return;

    Transaction txn(nixDB);

    std::cerr << "converting referers to referrers...";

    TableId dbReferers = nixDB.openTable("referers"); /* sic! */

    Paths referersKeys;
    nixDB.enumTable(txn, dbReferers, referersKeys);

    int n = 0;
    for (Paths::iterator i = referersKeys.begin();
         i != referersKeys.end(); ++i)
    {
        Paths referers;
        nixDB.queryStrings(txn, dbReferers, *i, referers);
        for (Paths::iterator j = referers.begin();
             j != referers.end(); ++j)
            nixDB.setString(txn, dbReferrers, addPrefix(*i, *j), "");
        if (++n % 1000 == 0) {
            txn.commit();
            txn.begin(nixDB);
            std::cerr << "|";
        }
        std::cerr << ".";
    }

    txn.commit();
    
    std::cerr << std::endl;

    nixDB.closeTable(dbReferers);

    nixDB.deleteTable("referers");
}

 
}