lix/src/libstore/lock.cc

#include "lock.hh"
#include "globals.hh"
#include "pathlocks.hh"

#include <grp.h>
#include <pwd.h>

#include <fcntl.h>
#include <unistd.h>

namespace nix {

UserLock::UserLock()
{
    assert(settings.buildUsersGroup != "");
    createDirs(settings.nixStateDir + "/userpool");
}

bool UserLock::findFreeUser() {
    if (enabled()) return true;

    /* Get the members of the build-users-group. */
    struct group * gr = getgrnam(settings.buildUsersGroup.get().c_str());
    if (!gr)
        throw Error("the group '%1%' specified in 'build-users-group' does not exist",
            settings.buildUsersGroup);
    gid = gr->gr_gid;

    /* Copy the result of getgrnam. */
    Strings users;
    for (char * * p = gr->gr_mem; *p; ++p) {
        debug("found build user '%1%'", *p);
        users.push_back(*p);
    }

    if (users.empty())
        throw Error("the build users group '%1%' has no members",
            settings.buildUsersGroup);

    /* Find a user account that isn't currently in use for another
       build. */
    for (auto & i : users) {
        debug("trying user '%1%'", i);

        struct passwd * pw = getpwnam(i.c_str());
        if (!pw)
            throw Error("the user '%1%' in the group '%2%' does not exist",
                i, settings.buildUsersGroup);


        fnUserLock = (format("%1%/userpool/%2%") % settings.nixStateDir % pw->pw_uid).str();

        AutoCloseFD fd = open(fnUserLock.c_str(), O_RDWR | O_CREAT | O_CLOEXEC, 0600);
        if (!fd)
            throw SysError("opening user lock '%1%'", fnUserLock);

        if (lockFile(fd.get(), ltWrite, false)) {
            fdUserLock = std::move(fd);
            user = i;
            uid = pw->pw_uid;

            /* Sanity check... */
            if (uid == getuid() || uid == geteuid())
                throw Error("the Nix user should not be a member of '%1%'",
                    settings.buildUsersGroup);

#if __linux__
            /* Get the list of supplementary groups of this build user.  This
               is usually either empty or contains a group such as "kvm".  */
            int ngroups = 32; // arbitrary initial guess
            supplementaryGIDs.resize(ngroups);

            int err = getgrouplist(pw->pw_name, pw->pw_gid, supplementaryGIDs.data(),
                                 &ngroups);

            // Our initial size of 32 wasn't sufficient, the correct size has
            // been stored in ngroups, so we try again.
            if (err == -1) {
                supplementaryGIDs.resize(ngroups);
                err = getgrouplist(pw->pw_name, pw->pw_gid, supplementaryGIDs.data(),
                                   &ngroups);
            }

            // If it failed once more, then something must be broken.
            if (err == -1)
                throw Error("failed to get list of supplementary groups for '%1%'",
                            pw->pw_name);

            // Finally, trim back the GID list to its real size
            supplementaryGIDs.resize(ngroups);
#endif

            isEnabled = true;
            return true;
        }
    }

    return false;
}

void UserLock::kill()
{
    killUser(uid);
}

}