Allow remote builds without sending the derivation closure

Previously, to build a derivation remotely, we had to copy the entire
closure of the .drv file to the remote machine, even though we only
need the top-level derivation. This is very wasteful: the closure can
contain thousands of store paths, and in some Hydra use cases, include
source paths that are very large (e.g. Git/Mercurial checkouts).

So now there is a new operation, StoreAPI::buildDerivation(), that
performs a build from an in-memory representation of a derivation
(BasicDerivation) rather than from a on-disk .drv file. The only files
that need to be in the Nix store are the sources of the derivation
(drv.inputSrcs), and the needed output paths of the dependencies (as
described by drv.inputDrvs). "nix-store --serve" exposes this
interface.

Note that this is a privileged operation, because you can construct a
derivation that builds any store path whatsoever. Fixing this will
require changing the hashing scheme (i.e., the output paths should be
computed from the other fields in BasicDerivation, allowing them to be
verified without access to other derivations). However, this would be
quite nice because it would allow .drv-free building (e.g. "nix-env
-i" wouldn't have to write any .drv files to disk).

Fixes #173.

8 files changed, 300 insertions, 143 deletions

diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 8c4412f11..5e2e7d89a 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -275,6 +275,7 @@ public:
 
     /* Make a goal (with caching). */
     GoalPtr makeDerivationGoal(const Path & drvPath, const StringSet & wantedOutputs, BuildMode buildMode = bmNormal);
+    GoalPtr makeBasicDerivationGoal(const Path & drvPath, const BasicDerivation & drv, BuildMode buildMode = bmNormal);
     GoalPtr makeSubstitutionGoal(const Path & storePath, bool repair = false);
 
     /* Remove a dead goal. */
@@ -690,6 +691,9 @@ class SubstitutionGoal;
 class DerivationGoal : public Goal
 {
 private:
+    /* Whether to use an on-disk .drv file. */
+    bool useDerivation;
+
     /* The path of the derivation. */
     Path drvPath;
 
@@ -698,14 +702,14 @@ private:
     StringSet wantedOutputs;
 
     /* Whether additional wanted outputs have been added. */
-    bool needRestart;
+    bool needRestart = false;
 
     /* Whether to retry substituting the outputs after building the
        inputs. */
-    bool retrySubstitution;
+    bool retrySubstitution = false;
 
     /* The derivation stored at drvPath. */
-    Derivation drv;
+    std::unique_ptr<BasicDerivation> drv;
 
     /* The remainder is state held during the build. */
 
@@ -736,8 +740,8 @@ private:
     Path tmpDir;
 
     /* File descriptor for the log file. */
-    FILE * fLogFile;
-    BZFILE * bzLogFile;
+    FILE * fLogFile = 0;
+    BZFILE * bzLogFile = 0;
     AutoCloseFD fdLogFile;
 
     /* Number of bytes received from the builder's stdout/stderr. */
@@ -750,7 +754,7 @@ private:
     std::shared_ptr<HookInstance> hook;
 
     /* Whether we're currently doing a chroot build. */
-    bool useChroot;
+    bool useChroot = false;
 
     Path chrootRootDir;
 
@@ -790,7 +794,10 @@ private:
     InodesSeen inodesSeen;
 
 public:
-    DerivationGoal(const Path & drvPath, const StringSet & wantedOutputs, Worker & worker, BuildMode buildMode = bmNormal);
+    DerivationGoal(const Path & drvPath, const StringSet & wantedOutputs,
+        Worker & worker, BuildMode buildMode = bmNormal);
+    DerivationGoal(const Path & drvPath, const BasicDerivation & drv,
+        Worker & worker, BuildMode buildMode = bmNormal);
     ~DerivationGoal();
 
     void cancel(bool timeout);
@@ -816,7 +823,8 @@ public:
 
 private:
     /* The states. */
-    void init();
+    void getDerivation();
+    void loadDerivation();
     void haveDerivation();
     void outputsSubstituted();
     void closureRepaired();
@@ -867,23 +875,34 @@ private:
 };
 
 
-DerivationGoal::DerivationGoal(const Path & drvPath, const StringSet & wantedOutputs, Worker & worker, BuildMode buildMode)
+DerivationGoal::DerivationGoal(const Path & drvPath, const StringSet & wantedOutputs,
+    Worker & worker, BuildMode buildMode)
     : Goal(worker)
+    , useDerivation(true)
+    , drvPath(drvPath)
     , wantedOutputs(wantedOutputs)
-    , needRestart(false)
-    , retrySubstitution(false)
-    , fLogFile(0)
-    , bzLogFile(0)
-    , useChroot(false)
     , buildMode(buildMode)
 {
-    this->drvPath = drvPath;
-    state = &DerivationGoal::init;
+    state = &DerivationGoal::getDerivation;
     name = (format("building of ‘%1%’") % drvPath).str();
     trace("created");
 }
 
 
+DerivationGoal::DerivationGoal(const Path & drvPath, const BasicDerivation & drv,
+    Worker & worker, BuildMode buildMode)
+    : Goal(worker)
+    , useDerivation(false)
+    , drvPath(drvPath)
+    , buildMode(buildMode)
+{
+    this->drv = std::unique_ptr<BasicDerivation>(new BasicDerivation(drv));
+    state = &DerivationGoal::haveDerivation;
+    name = (format("building of %1%") % showPaths(outputPaths(drv))).str();
+    trace("created");
+}
+
+
 DerivationGoal::~DerivationGoal()
 {
     /* Careful: we should never ever throw an exception from a
@@ -951,28 +970,25 @@ void DerivationGoal::addWantedOutputs(const StringSet & outputs)
 }
 
 
-void DerivationGoal::init()
+void DerivationGoal::getDerivation()
 {
     trace("init");
 
-    if (settings.readOnlyMode)
-        throw Error(format("cannot build derivation ‘%1%’ - no write access to the Nix store") % drvPath);
-
     /* The first thing to do is to make sure that the derivation
        exists.  If it doesn't, it may be created through a
        substitute. */
     if (buildMode == bmNormal && worker.store.isValidPath(drvPath)) {
-        haveDerivation();
+        loadDerivation();
         return;
     }
 
     addWaitee(worker.makeSubstitutionGoal(drvPath));
 
-    state = &DerivationGoal::haveDerivation;
+    state = &DerivationGoal::loadDerivation;
 }
 
 
-void DerivationGoal::haveDerivation()
+void DerivationGoal::loadDerivation()
 {
     trace("loading derivation");
 
@@ -990,10 +1006,18 @@ void DerivationGoal::haveDerivation()
     assert(worker.store.isValidPath(drvPath));
 
     /* Get the derivation. */
-    drv = derivationFromPath(worker.store, drvPath);
+    drv = std::unique_ptr<BasicDerivation>(new Derivation(derivationFromPath(worker.store, drvPath)));
+
+    haveDerivation();
+}
 
-    foreach (DerivationOutputs::iterator, i, drv.outputs)
-        worker.store.addTempRoot(i->second.path);
+
+void DerivationGoal::haveDerivation()
+{
+    trace("have derivation");
+
+    for (auto & i : drv->outputs)
+        worker.store.addTempRoot(i.second.path);
 
     /* Check what outputs paths are not already valid. */
     PathSet invalidOutputs = checkPathValidity(false, buildMode == bmRepair);
@@ -1006,15 +1030,15 @@ void DerivationGoal::haveDerivation()
 
     /* Check whether any output previously failed to build.  If so,
        don't bother. */
-    foreach (PathSet::iterator, i, invalidOutputs)
-        if (pathFailed(*i)) return;
+    for (auto & i : invalidOutputs)
+        if (pathFailed(i)) return;
 
     /* We are first going to try to create the invalid output paths
        through substitutes.  If that doesn't work, we'll build
        them. */
-    if (settings.useSubstitutes && substitutesAllowed(drv))
-        foreach (PathSet::iterator, i, invalidOutputs)
-            addWaitee(worker.makeSubstitutionGoal(*i, buildMode == bmRepair));
+    if (settings.useSubstitutes && substitutesAllowed(*drv))
+        for (auto & i : invalidOutputs)
+            addWaitee(worker.makeSubstitutionGoal(i, buildMode == bmRepair));
 
     if (waitees.empty()) /* to prevent hang (no wake-up event) */
         outputsSubstituted();
@@ -1063,11 +1087,17 @@ void DerivationGoal::outputsSubstituted()
     wantedOutputs = PathSet();
 
     /* The inputs must be built before we can build this goal. */
-    foreach (DerivationInputs::iterator, i, drv.inputDrvs)
-        addWaitee(worker.makeDerivationGoal(i->first, i->second, buildMode == bmRepair ? bmRepair : bmNormal));
+    if (useDerivation)
+        for (auto & i : dynamic_cast<Derivation *>(drv.get())->inputDrvs)
+            addWaitee(worker.makeDerivationGoal(i.first, i.second, buildMode == bmRepair ? bmRepair : bmNormal));
 
-    foreach (PathSet::iterator, i, drv.inputSrcs)
-        addWaitee(worker.makeSubstitutionGoal(*i));
+    for (auto & i : drv->inputSrcs) {
+        if (worker.store.isValidPath(i)) continue;
+        if (!settings.useSubstitutes)
+            throw Error(format("dependency of ‘%1%’ of ‘%2%’ does not exist, and substitution is disabled")
+                % i % drvPath);
+        addWaitee(worker.makeSubstitutionGoal(i));
+    }
 
     if (waitees.empty()) /* to prevent hang (no wake-up event) */
         inputsRealised();
@@ -1085,34 +1115,34 @@ void DerivationGoal::repairClosure()
 
     /* Get the output closure. */
     PathSet outputClosure;
-    foreach (DerivationOutputs::iterator, i, drv.outputs)
-        computeFSClosure(worker.store, i->second.path, outputClosure);
+    for (auto & i : drv->outputs)
+        computeFSClosure(worker.store, i.second.path, outputClosure);
 
     /* Filter out our own outputs (which we have already checked). */
-    foreach (DerivationOutputs::iterator, i, drv.outputs)
-        outputClosure.erase(i->second.path);
+    for (auto & i : drv->outputs)
+        outputClosure.erase(i.second.path);
 
     /* Get all dependencies of this derivation so that we know which
        derivation is responsible for which path in the output
        closure. */
     PathSet inputClosure;
-    computeFSClosure(worker.store, drvPath, inputClosure);
+    if (useDerivation) computeFSClosure(worker.store, drvPath, inputClosure);
     std::map<Path, Path> outputsToDrv;
-    foreach (PathSet::iterator, i, inputClosure)
-        if (isDerivation(*i)) {
-            Derivation drv = derivationFromPath(worker.store, *i);
-            foreach (DerivationOutputs::iterator, j, drv.outputs)
-                outputsToDrv[j->second.path] = *i;
+    for (auto & i : inputClosure)
+        if (isDerivation(i)) {
+            Derivation drv = derivationFromPath(worker.store, i);
+            for (auto & j : drv.outputs)
+                outputsToDrv[j.second.path] = i;
         }
 
     /* Check each path (slow!). */
     PathSet broken;
-    foreach (PathSet::iterator, i, outputClosure) {
-        if (worker.store.pathContentsGood(*i)) continue;
-        printMsg(lvlError, format("found corrupted or missing path ‘%1%’ in the output closure of ‘%2%’") % *i % drvPath);
-        Path drvPath2 = outputsToDrv[*i];
+    for (auto & i : outputClosure) {
+        if (worker.store.pathContentsGood(i)) continue;
+        printMsg(lvlError, format("found corrupted or missing path ‘%1%’ in the output closure of ‘%2%’") % i % drvPath);
+        Path drvPath2 = outputsToDrv[i];
         if (drvPath2 == "")
-            addWaitee(worker.makeSubstitutionGoal(*i, true));
+            addWaitee(worker.makeSubstitutionGoal(i, true));
         else
             addWaitee(worker.makeDerivationGoal(drvPath2, PathSet(), bmRepair));
     }
@@ -1140,6 +1170,8 @@ void DerivationGoal::inputsRealised()
     trace("all inputs realised");
 
     if (nrFailed != 0) {
+        if (!useDerivation)
+            throw Error(format("some dependencies of ‘%1%’ are missing") % drvPath);
         printMsg(lvlError,
             format("cannot build derivation ‘%1%’: %2% dependencies couldn't be built")
             % drvPath % nrFailed);
@@ -1156,32 +1188,33 @@ void DerivationGoal::inputsRealised()
        running the build hook. */
 
     /* The outputs are referenceable paths. */
-    foreach (DerivationOutputs::iterator, i, drv.outputs) {
-        debug(format("building path ‘%1%’") % i->second.path);
-        allPaths.insert(i->second.path);
+    for (auto & i : drv->outputs) {
+        debug(format("building path ‘%1%’") % i.second.path);
+        allPaths.insert(i.second.path);
     }
 
     /* Determine the full set of input paths. */
 
     /* First, the input derivations. */
-    foreach (DerivationInputs::iterator, i, drv.inputDrvs) {
-        /* Add the relevant output closures of the input derivation
-           `*i' as input paths.  Only add the closures of output paths
-           that are specified as inputs. */
-        assert(worker.store.isValidPath(i->first));
-        Derivation inDrv = derivationFromPath(worker.store, i->first);
-        foreach (StringSet::iterator, j, i->second)
-            if (inDrv.outputs.find(*j) != inDrv.outputs.end())
-                computeFSClosure(worker.store, inDrv.outputs[*j].path, inputPaths);
-            else
-                throw Error(
-                    format("derivation ‘%1%’ requires non-existent output ‘%2%’ from input derivation ‘%3%’")
-                    % drvPath % *j % i->first);
-    }
+    if (useDerivation)
+        for (auto & i : dynamic_cast<Derivation *>(drv.get())->inputDrvs) {
+            /* Add the relevant output closures of the input derivation
+               `i' as input paths.  Only add the closures of output paths
+               that are specified as inputs. */
+            assert(worker.store.isValidPath(i.first));
+            Derivation inDrv = derivationFromPath(worker.store, i.first);
+            for (auto & j : i.second)
+                if (inDrv.outputs.find(j) != inDrv.outputs.end())
+                    computeFSClosure(worker.store, inDrv.outputs[j].path, inputPaths);
+                else
+                    throw Error(
+                        format("derivation ‘%1%’ requires non-existent output ‘%2%’ from input derivation ‘%3%’")
+                        % drvPath % j % i.first);
+        }
 
     /* Second, the input sources. */
-    foreach (PathSet::iterator, i, drv.inputSrcs)
-        computeFSClosure(worker.store, *i, inputPaths);
+    for (auto & i : drv->inputSrcs)
+        computeFSClosure(worker.store, i, inputPaths);
 
     debug(format("added input paths %1%") % showPaths(inputPaths));
 
@@ -1189,8 +1222,8 @@ void DerivationGoal::inputsRealised()
 
     /* Is this a fixed-output derivation? */
     fixedOutput = true;
-    foreach (DerivationOutputs::iterator, i, drv.outputs)
-        if (i->second.hash == "") fixedOutput = false;
+    for (auto & i : drv->outputs)
+        if (i.second.hash == "") fixedOutput = false;
 
     /* Okay, try to build.  Note that here we don't wait for a build
        slot to become available, since we don't need one if there is a
@@ -1217,13 +1250,13 @@ static string get(const StringPairs & map, const string & key, const string & de
 }
 
 
-bool willBuildLocally(const Derivation & drv)
+bool willBuildLocally(const BasicDerivation & drv)
 {
     return get(drv.env, "preferLocalBuild") == "1" && canBuildLocally(drv.platform);
 }
 
 
-bool substitutesAllowed(const Derivation & drv)
+bool substitutesAllowed(const BasicDerivation & drv)
 {
     return get(drv.env, "allowSubstitutes", "1") == "1";
 }
@@ -1238,10 +1271,10 @@ void DerivationGoal::tryToBuild()
        (It can't happen between here and the lockPaths() call below
        because we're not allowing multi-threading.)  If so, put this
        goal to sleep until another goal finishes, then try again. */
-    foreach (DerivationOutputs::iterator, i, drv.outputs)
-        if (pathIsLockedByMe(i->second.path)) {
+    for (auto & i : drv->outputs)
+        if (pathIsLockedByMe(i.second.path)) {
             debug(format("putting derivation ‘%1%’ to sleep because ‘%2%’ is locked by another goal")
-                % drvPath % i->second.path);
+                % drvPath % i.second.path);
             worker.waitForAnyGoal(shared_from_this());
             return;
         }
@@ -1251,7 +1284,7 @@ void DerivationGoal::tryToBuild()
        can't acquire the lock, then continue; hopefully some other
        goal can start a build, and if not, the main loop will sleep a
        few seconds and then retry this goal. */
-    if (!outputLocks.lockPaths(outputPaths(drv), "", false)) {
+    if (!outputLocks.lockPaths(outputPaths(*drv), "", false)) {
         worker.waitForAWhile(shared_from_this());
         return;
     }
@@ -1264,22 +1297,22 @@ void DerivationGoal::tryToBuild()
        now hold the locks on the output paths, no other process can
        build this derivation, so no further checks are necessary. */
     validPaths = checkPathValidity(true, buildMode == bmRepair);
-    assert(buildMode != bmCheck || validPaths.size() == drv.outputs.size());
-    if (buildMode != bmCheck && validPaths.size() == drv.outputs.size()) {
+    assert(buildMode != bmCheck || validPaths.size() == drv->outputs.size());
+    if (buildMode != bmCheck && validPaths.size() == drv->outputs.size()) {
         debug(format("skipping build of derivation ‘%1%’, someone beat us to it") % drvPath);
         outputLocks.setDeletion(true);
         amDone(ecSuccess);
         return;
     }
 
-    missingPaths = outputPaths(drv);
+    missingPaths = outputPaths(*drv);
     if (buildMode != bmCheck)
         foreach (PathSet::iterator, i, validPaths) missingPaths.erase(*i);
 
     /* If any of the outputs already exist but are not valid, delete
        them. */
-    foreach (DerivationOutputs::iterator, i, drv.outputs) {
-        Path path = i->second.path;
+    for (auto & i : drv->outputs) {
+        Path path = i.second.path;
         if (worker.store.isValidPath(path)) continue;
         if (!pathExists(path)) continue;
         debug(format("removing invalid path ‘%1%’") % path);
@@ -1289,13 +1322,13 @@ void DerivationGoal::tryToBuild()
     /* Check again whether any output previously failed to build,
        because some other process may have tried and failed before we
        acquired the lock. */
-    foreach (DerivationOutputs::iterator, i, drv.outputs)
-        if (pathFailed(i->second.path)) return;
+    for (auto & i : drv->outputs)
+        if (pathFailed(i.second.path)) return;
 
     /* Don't do a remote build if the derivation has the attribute
        `preferLocalBuild' set.  Also, check and repair modes are only
        supported for local builds. */
-    bool buildLocally = buildMode != bmNormal || willBuildLocally(drv);
+    bool buildLocally = buildMode != bmNormal || willBuildLocally(*drv);
 
     /* Is the build hook willing to accept this job? */
     if (!buildLocally) {
@@ -1504,8 +1537,8 @@ void DerivationGoal::buildDone()
                Hook errors (like communication problems with the
                remote machine) shouldn't be cached either. */
             if (settings.cacheFailure && !fixedOutput && !diskFull)
-                foreach (DerivationOutputs::iterator, i, drv.outputs)
-                    worker.store.registerFailedPath(i->second.path);
+                for (auto & i : drv->outputs)
+                    worker.store.registerFailedPath(i.second.path);
         }
 
         amDone(ecFailed);
@@ -1524,21 +1557,21 @@ void DerivationGoal::buildDone()
 
 HookReply DerivationGoal::tryBuildHook()
 {
-    if (!settings.useBuildHook || getEnv("NIX_BUILD_HOOK") == "") return rpDecline;
+    if (!settings.useBuildHook || getEnv("NIX_BUILD_HOOK") == "" || !useDerivation) return rpDecline;
 
     if (!worker.hook)
-        worker.hook = std::shared_ptr<HookInstance>(new HookInstance);
+        worker.hook = std::make_shared<HookInstance>();
 
     /* Tell the hook about system features (beyond the system type)
        required from the build machine.  (The hook could parse the
        drv file itself, but this is easier.) */
-    Strings features = tokenizeString<Strings>(get(drv.env, "requiredSystemFeatures"));
+    Strings features = tokenizeString<Strings>(get(drv->env, "requiredSystemFeatures"));
     foreach (Strings::iterator, i, features) checkStoreName(*i); /* !!! abuse */
 
     /* Send the request to the hook. */
     writeLine(worker.hook->toHook.writeSide, (format("%1% %2% %3% %4%")
         % (worker.getNrLocalBuilds() < settings.maxBuildJobs ? "1" : "0")
-        % drv.platform % drvPath % concatStringsSep(",", features)).str());
+        % drv->platform % drvPath % concatStringsSep(",", features)).str());
 
     /* Read the first line of input, which should be a word indicating
        whether the hook wishes to perform the build. */
@@ -1596,7 +1629,7 @@ HookReply DerivationGoal::tryBuildHook()
 
     if (settings.printBuildTrace)
         printMsg(lvlError, format("@ build-started %1% - %2% %3%")
-            % drvPath % drv.platform % logFile);
+            % drvPath % drv->platform % logFile);
 
     return rpAccept;
 }
@@ -1624,12 +1657,12 @@ void DerivationGoal::startBuilder()
             "building path(s) %1%") % showPaths(missingPaths));
 
     /* Right platform? */
-    if (!canBuildLocally(drv.platform)) {
+    if (!canBuildLocally(drv->platform)) {
         if (settings.printBuildTrace)
-            printMsg(lvlError, format("@ unsupported-platform %1% %2%") % drvPath % drv.platform);
+            printMsg(lvlError, format("@ unsupported-platform %1% %2%") % drvPath % drv->platform);
         throw Error(
             format("a ‘%1%’ is required to build ‘%3%’, but I am a ‘%2%’")
-            % drv.platform % settings.thisSystem % drvPath);
+            % drv->platform % settings.thisSystem % drvPath);
     }
 
     /* Construct the environment passed to the builder. */
@@ -1666,9 +1699,9 @@ void DerivationGoal::startBuilder()
        attribute. Those are passed as file names pointing to
        temporary files containing the contents. */
     PathSet filesToChown;
-    StringSet passAsFile = tokenizeString<StringSet>(get(drv.env, "passAsFile"));
+    StringSet passAsFile = tokenizeString<StringSet>(get(drv->env, "passAsFile"));
     int fileNr = 0;
-    for (auto & i : drv.env) {
+    for (auto & i : drv->env) {
         if (passAsFile.find(i.first) == passAsFile.end()) {
             env[i.first] = i.second;
         } else {
@@ -1708,7 +1741,7 @@ void DerivationGoal::startBuilder()
        fixed-output derivations is by definition pure (since we
        already know the cryptographic hash of the output). */
     if (fixedOutput) {
-        Strings varNames = tokenizeString<Strings>(get(drv.env, "impureEnvVars"));
+        Strings varNames = tokenizeString<Strings>(get(drv->env, "impureEnvVars"));
         foreach (Strings::iterator, i, varNames) env[*i] = getEnv(*i);
     }
 
@@ -1719,7 +1752,7 @@ void DerivationGoal::startBuilder()
        temporary build directory.  The text files have the format used
        by `nix-store --register-validity'.  However, the deriver
        fields are left empty. */
-    string s = get(drv.env, "exportReferencesGraph");
+    string s = get(drv->env, "exportReferencesGraph");
     Strings ss = tokenizeString<Strings>(s);
     if (ss.size() % 2 != 0)
         throw BuildError(format("odd number of tokens in ‘exportReferencesGraph’: ‘%1%’") % s);
@@ -1748,8 +1781,8 @@ void DerivationGoal::startBuilder()
         foreach (PathSet::iterator, j, paths2) {
             if (isDerivation(*j)) {
                 Derivation drv = derivationFromPath(worker.store, *j);
-                foreach (DerivationOutputs::iterator, k, drv.outputs)
-                    computeFSClosure(worker.store, k->second.path, paths);
+                for (auto & k : drv.outputs)
+                    computeFSClosure(worker.store, k.second.path, paths);
             }
         }
 
@@ -1789,14 +1822,14 @@ void DerivationGoal::startBuilder()
         if (x != "true" && x != "false" && x != "relaxed")
             throw Error("option ‘build-use-chroot’ must be set to one of ‘true’, ‘false’ or ‘relaxed’");
         if (x == "true") {
-            if (get(drv.env, "__noChroot") == "1")
+            if (get(drv->env, "__noChroot") == "1")
                 throw Error(format("derivation ‘%1%’ has ‘__noChroot’ set, but that's not allowed when ‘build-use-chroot’ is ‘true’") % drvPath);
             useChroot = true;
         }
         else if (x == "false")
             useChroot = false;
         else if (x == "relaxed")
-            useChroot = !fixedOutput && get(drv.env, "__noChroot") != "1";
+            useChroot = !fixedOutput && get(drv->env, "__noChroot") != "1";
     }
 
     if (useChroot) {
@@ -1834,7 +1867,7 @@ void DerivationGoal::startBuilder()
         PathSet allowedPaths = tokenizeString<StringSet>(allowed);
 
         /* This works like the above, except on a per-derivation level */
-        Strings impurePaths = tokenizeString<Strings>(get(drv.env, "__impureHostDeps"));
+        Strings impurePaths = tokenizeString<Strings>(get(drv->env, "__impureHostDeps"));
 
         for (auto & i : impurePaths) {
             bool found = false;
@@ -1865,7 +1898,7 @@ void DerivationGoal::startBuilder()
         if (pathExists(chrootRootDir)) deletePath(chrootRootDir);
 
         /* Clean up the chroot directory automatically. */
-        autoDelChroot = std::shared_ptr<AutoDelete>(new AutoDelete(chrootRootDir));
+        autoDelChroot = std::make_shared<AutoDelete>(chrootRootDir);
 
         printMsg(lvlChatty, format("setting up chroot environment in ‘%1%’") % chrootRootDir);
 
@@ -1948,8 +1981,8 @@ void DerivationGoal::startBuilder()
            (typically the dependencies of /bin/sh).  Throw them
            out. */
         if (buildMode != bmNormal)
-            foreach (DerivationOutputs::iterator, i, drv.outputs)
-                dirsInChroot.erase(i->second.path);
+            for (auto & i : drv->outputs)
+                dirsInChroot.erase(i.second.path);
 
 #elif SANDBOX_ENABLED
         /* We don't really have any parent prep work to do (yet?)
@@ -2025,7 +2058,7 @@ void DerivationGoal::startBuilder()
     }
 
     /* Run the builder. */
-    printMsg(lvlChatty, format("executing builder ‘%1%’") % drv.builder);
+    printMsg(lvlChatty, format("executing builder ‘%1%’") % drv->builder);
 
     /* Create the log file. */
     Path logFile = openLogFile();
@@ -2111,7 +2144,7 @@ void DerivationGoal::startBuilder()
 
     if (settings.printBuildTrace) {
         printMsg(lvlError, format("@ build-started %1% - %2% %3%")
-            % drvPath % drv.platform % logFile);
+            % drvPath % drv->platform % logFile);
     }
 }
 
@@ -2285,7 +2318,7 @@ void DerivationGoal::runChild()
            i686-linux build on an x86_64-linux machine. */
         struct utsname utsbuf;
         uname(&utsbuf);
-        if (drv.platform == "i686-linux" &&
+        if (drv->platform == "i686-linux" &&
             (settings.thisSystem == "x86_64-linux" ||
              (!strcmp(utsbuf.sysname, "Linux") && !strcmp(utsbuf.machine, "x86_64")))) {
             if (personality(PER_LINUX32) == -1)
@@ -2294,7 +2327,7 @@ void DerivationGoal::runChild()
 
         /* Impersonate a Linux 2.6 machine to get some determinism in
            builds that depend on the kernel version. */
-        if ((drv.platform == "i686-linux" || drv.platform == "x86_64-linux") && settings.impersonateLinux26) {
+        if ((drv->platform == "i686-linux" || drv->platform == "x86_64-linux") && settings.impersonateLinux26) {
             int cur = personality(0xffffffff);
             if (cur != -1) personality(cur | 0x0020000 /* == UNAME26 */);
         }
@@ -2449,15 +2482,15 @@ void DerivationGoal::runChild()
             args.push_back("sandbox-exec");
             args.push_back("-p");
             args.push_back(sandboxProfile);
-            args.push_back(drv.builder);
+            args.push_back(drv->builder);
         } else {
-            builder = drv.builder.c_str();
-            string builderBasename = baseNameOf(drv.builder);
+            builder = drv->builder.c_str();
+            string builderBasename = baseNameOf(drv->builder);
             args.push_back(builderBasename);
         }
 
-        foreach (Strings::iterator, i, drv.args)
-            args.push_back(rewriteHashes(*i, rewritesToTmp));
+        for (auto & i : drv->args)
+            args.push_back(rewriteHashes(i, rewritesToTmp));
 
         restoreSIGPIPE();
 
@@ -2473,7 +2506,7 @@ void DerivationGoal::runChild()
         /* Execute the program.  This should not return. */
         execve(builder, stringsToCharPtrs(args).data(), stringsToCharPtrs(envStrs).data());
 
-        throw SysError(format("executing ‘%1%’") % drv.builder);
+        throw SysError(format("executing ‘%1%’") % drv->builder);
 
     } catch (std::exception & e) {
         writeFull(STDERR_FILENO, "while setting up the build environment: " + string(e.what()) + "\n");
@@ -2485,7 +2518,7 @@ void DerivationGoal::runChild()
 /* Parse a list of reference specifiers.  Each element must either be
    a store path, or the symbolic name of the output of the derivation
    (such as `out'). */
-PathSet parseReferenceSpecifiers(const Derivation & drv, string attr)
+PathSet parseReferenceSpecifiers(const BasicDerivation & drv, string attr)
 {
     PathSet result;
     Paths paths = tokenizeString<Paths>(attr);
@@ -2509,8 +2542,8 @@ void DerivationGoal::registerOutputs()
        to do anything here. */
     if (hook) {
         bool allValid = true;
-        foreach (DerivationOutputs::iterator, i, drv.outputs)
-            if (!worker.store.isValidPath(i->second.path)) allValid = false;
+        for (auto & i : drv->outputs)
+            if (!worker.store.isValidPath(i.second.path)) allValid = false;
         if (allValid) return;
     }
 
@@ -2519,8 +2552,8 @@ void DerivationGoal::registerOutputs()
     /* Check whether the output paths were created, and grep each
        output path to determine what other paths it references.  Also make all
        output paths read-only. */
-    foreach (DerivationOutputs::iterator, i, drv.outputs) {
-        Path path = i->second.path;
+    for (auto & i : drv->outputs) {
+        Path path = i.second.path;
         if (missingPaths.find(path) == missingPaths.end()) continue;
 
         Path actualPath = path;
@@ -2591,10 +2624,10 @@ void DerivationGoal::registerOutputs()
         /* Check that fixed-output derivations produced the right
            outputs (i.e., the content hash should match the specified
            hash). */
-        if (i->second.hash != "") {
+        if (i.second.hash != "") {
 
             bool recursive; HashType ht; Hash h;
-            i->second.parseHashInfo(recursive, ht, h);
+            i.second.parseHashInfo(recursive, ht, h);
 
             if (!recursive) {
                 /* The output path should be a regular file without
@@ -2609,7 +2642,7 @@ void DerivationGoal::registerOutputs()
             if (h != h2)
                 throw BuildError(
                     format("output path ‘%1%’ should have %2% hash ‘%3%’, instead has ‘%4%’")
-                    % path % i->second.hashAlgo % printHash16or32(h) % printHash16or32(h2));
+                    % path % i.second.hashAlgo % printHash16or32(h) % printHash16or32(h2));
         }
 
         /* Get rid of all weird permissions.  This also checks that
@@ -2643,9 +2676,9 @@ void DerivationGoal::registerOutputs()
 
         /* Enforce `allowedReferences' and friends. */
         auto checkRefs = [&](const string & attrName, bool allowed, bool recursive) {
-            if (drv.env.find(attrName) == drv.env.end()) return;
+            if (drv->env.find(attrName) == drv->env.end()) return;
 
-            PathSet spec = parseReferenceSpecifiers(drv, get(drv.env, attrName));
+            PathSet spec = parseReferenceSpecifiers(*drv, get(drv->env, attrName));
 
             PathSet used;
             if (recursive) {
@@ -2806,12 +2839,12 @@ void DerivationGoal::handleEOF(int fd)
 PathSet DerivationGoal::checkPathValidity(bool returnValid, bool checkHash)
 {
     PathSet result;
-    foreach (DerivationOutputs::iterator, i, drv.outputs) {
-        if (!wantOutput(i->first, wantedOutputs)) continue;
+    for (auto & i : drv->outputs) {
+        if (!wantOutput(i.first, wantedOutputs)) continue;
         bool good =
-            worker.store.isValidPath(i->second.path) &&
-            (!checkHash || worker.store.pathContentsGood(i->second.path));
-        if (good == returnValid) result.insert(i->second.path);
+            worker.store.isValidPath(i.second.path) &&
+            (!checkHash || worker.store.pathContentsGood(i.second.path));
+        if (good == returnValid) result.insert(i.second.path);
     }
     return result;
 }
@@ -3066,7 +3099,7 @@ void SubstitutionGoal::tryToRun()
     }
 
     /* Acquire a lock on the output path. */
-    outputLock = std::shared_ptr<PathLocks>(new PathLocks);
+    outputLock = std::make_shared<PathLocks>();
     if (!outputLock->lockPaths(singleton<PathSet>(storePath), "", false)) {
         worker.waitForAWhile(shared_from_this());
         return;
@@ -3270,7 +3303,7 @@ GoalPtr Worker::makeDerivationGoal(const Path & path, const StringSet & wantedOu
 {
     GoalPtr goal = derivationGoals[path].lock();
     if (!goal) {
-        goal = GoalPtr(new DerivationGoal(path, wantedOutputs, *this, buildMode));
+        goal = std::make_shared<DerivationGoal>(path, wantedOutputs, *this, buildMode);
         derivationGoals[path] = goal;
         wakeUp(goal);
     } else
@@ -3279,11 +3312,19 @@ GoalPtr Worker::makeDerivationGoal(const Path & path, const StringSet & wantedOu
 }
 
 
+GoalPtr Worker::makeBasicDerivationGoal(const Path & drvPath, const BasicDerivation & drv, BuildMode buildMode)
+{
+    auto goal = std::make_shared<DerivationGoal>(drvPath, drv, *this, buildMode);
+    wakeUp(goal);
+    return goal;
+}
+
+
 GoalPtr Worker::makeSubstitutionGoal(const Path & path, bool repair)
 {
     GoalPtr goal = substitutionGoals[path].lock();
     if (!goal) {
-        goal = GoalPtr(new SubstitutionGoal(path, *this, repair));
+        goal = std::make_shared<SubstitutionGoal>(path, *this, repair);
         substitutionGoals[path] = goal;
         wakeUp(goal);
     }
@@ -3604,8 +3645,7 @@ unsigned int Worker::exitStatus()
 
 void LocalStore::buildPaths(const PathSet & drvPaths, BuildMode buildMode)
 {
-    startNest(nest, lvlDebug,
-        format("building %1%") % showPaths(drvPaths));
+    startNest(nest, lvlDebug, format("building %1%") % showPaths(drvPaths));
 
     Worker worker(*this);
 
@@ -3633,6 +3673,35 @@ void LocalStore::buildPaths(const PathSet & drvPaths, BuildMode buildMode)
 }
 
 
+BuildResult LocalStore::buildDerivation(const Path & drvPath, const BasicDerivation & drv,
+    BuildMode buildMode)
+{
+    startNest(nest, lvlDebug, format("building %1%") % showPaths({drvPath}));
+
+    Worker worker(*this);
+    auto goal = worker.makeBasicDerivationGoal(drvPath, drv, buildMode);
+
+    BuildResult result;
+
+    try {
+        worker.run(Goals{goal});
+        if (goal->getExitCode() == Goal::ecSuccess)
+            result.status = BuildResult::Success;
+        else if (worker.permanentFailure)
+            result.status = BuildResult::PermanentFailure;
+        else if (worker.timedOut)
+            result.status = BuildResult::TimedOut;
+        else
+            result.status = BuildResult::MiscFailure;
+    } catch (Error & e) {
+        result.status = BuildResult::MiscFailure;
+        result.errorMsg = e.msg();
+    }
+
+    return result;
+}
+
+
 void LocalStore::ensurePath(const Path & path)
 {
     /* If the path is already valid, we're done. */
diff --git a/src/libstore/derivations.cc b/src/libstore/derivations.cc
index fbc1d99f3..db35af6cb 100644
--- a/src/libstore/derivations.cc
+++ b/src/libstore/derivations.cc
@@ -3,6 +3,7 @@
 #include "globals.hh"
 #include "util.hh"
 #include "misc.hh"
+#include "worker-protocol.hh"
 
 
 namespace nix {
@@ -285,7 +286,7 @@ bool wantOutput(const string & output, const std::set<string> & wanted)
 }
 
 
-PathSet outputPaths(const Derivation & drv)
+PathSet outputPaths(const BasicDerivation & drv)
 {
     PathSet paths;
     for (auto & i : drv.outputs)
@@ -294,4 +295,44 @@ PathSet outputPaths(const Derivation & drv)
 }
 
 
+Source & operator >> (Source & in, BasicDerivation & drv)
+{
+    drv.outputs.clear();
+    auto nr = readInt(in);
+    for (unsigned int n = 0; n < nr; n++) {
+        auto name = readString(in);
+        DerivationOutput o;
+        in >> o.path >> o.hashAlgo >> o.hash;
+        assertStorePath(o.path);
+        drv.outputs[name] = o;
+    }
+
+    drv.inputSrcs = readStorePaths<PathSet>(in);
+    in >> drv.platform >> drv.builder;
+    drv.args = readStrings<Strings>(in);
+
+    nr = readInt(in);
+    for (unsigned int n = 0; n < nr; n++) {
+        auto key = readString(in);
+        auto value = readString(in);
+        drv.env[key] = value;
+    }
+
+    return in;
+}
+
+
+Sink & operator << (Sink & out, const BasicDerivation & drv)
+{
+    out << drv.outputs.size();
+    for (auto & i : drv.outputs)
+        out << i.first << i.second.path << i.second.hashAlgo << i.second.hash;
+    out << drv.inputSrcs << drv.platform << drv.builder << drv.args;
+    out << drv.env.size();
+    for (auto & i : drv.env)
+        out << i.first << i.second;
+    return out;
+}
+
+
 }
diff --git a/src/libstore/derivations.hh b/src/libstore/derivations.hh
index 8d5e4d05d..f0842045f 100644
--- a/src/libstore/derivations.hh
+++ b/src/libstore/derivations.hh
@@ -40,15 +40,21 @@ typedef std::map<Path, StringSet> DerivationInputs;
 
 typedef std::map<string, string> StringPairs;
 
-struct Derivation
+struct BasicDerivation
 {
     DerivationOutputs outputs; /* keyed on symbolic IDs */
-    DerivationInputs inputDrvs; /* inputs that are sub-derivations */
     PathSet inputSrcs; /* inputs that are sources */
     string platform;
     Path builder;
     Strings args;
     StringPairs env;
+
+    virtual ~BasicDerivation() { };
+};
+
+struct Derivation : BasicDerivation
+{
+    DerivationInputs inputDrvs; /* inputs that are sub-derivations */
 };
 
 
@@ -89,6 +95,12 @@ Path makeDrvPathWithOutputs(const Path & drvPath, const std::set<string> & outpu
 
 bool wantOutput(const string & output, const std::set<string> & wanted);
 
-PathSet outputPaths(const Derivation & drv);
+PathSet outputPaths(const BasicDerivation & drv);
+
+struct Source;
+struct Sink;
+
+Source & operator >> (Source & in, BasicDerivation & drv);
+Sink & operator << (Sink & out, const BasicDerivation & drv);
 
 }
diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index 819f59327..105f3592c 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -151,6 +151,9 @@ public:
 
     void buildPaths(const PathSet & paths, BuildMode buildMode);
 
+    BuildResult buildDerivation(const Path & drvPath, const BasicDerivation & drv,
+        BuildMode buildMode) override;
+
     void ensurePath(const Path & path);
 
     void addTempRoot(const Path & path);
diff --git a/src/libstore/misc.hh b/src/libstore/misc.hh
index d3e31d51f..495c52875 100644
--- a/src/libstore/misc.hh
+++ b/src/libstore/misc.hh
@@ -32,9 +32,9 @@ void queryMissing(StoreAPI & store, const PathSet & targets,
     PathSet & willBuild, PathSet & willSubstitute, PathSet & unknown,
     unsigned long long & downloadSize, unsigned long long & narSize);
 
-bool willBuildLocally(const Derivation & drv);
+bool willBuildLocally(const BasicDerivation & drv);
 
-bool substitutesAllowed(const Derivation & drv);
+bool substitutesAllowed(const BasicDerivation & drv);
 
 
 }
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index ab87d9d8b..acabd6cf0 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -482,6 +482,13 @@ void RemoteStore::buildPaths(const PathSet & drvPaths, BuildMode buildMode)
 }
 
 
+BuildResult RemoteStore::buildDerivation(const Path & drvPath, const BasicDerivation & drv,
+    BuildMode buildMode)
+{
+    throw Error("not implemented");
+}
+
+
 void RemoteStore::ensurePath(const Path & path)
 {
     openConnection();
diff --git a/src/libstore/remote-store.hh b/src/libstore/remote-store.hh
index 030120db4..09e250386 100644
--- a/src/libstore/remote-store.hh
+++ b/src/libstore/remote-store.hh
@@ -67,6 +67,9 @@ public:
 
     void buildPaths(const PathSet & paths, BuildMode buildMode);
 
+    BuildResult buildDerivation(const Path & drvPath, const BasicDerivation & drv,
+        BuildMode buildMode) override;
+
     void ensurePath(const Path & path);
 
     void addTempRoot(const Path & path);
diff --git a/src/libstore/store-api.hh b/src/libstore/store-api.hh
index 3764f3e54..afc7376fe 100644
--- a/src/libstore/store-api.hh
+++ b/src/libstore/store-api.hh
@@ -100,6 +100,22 @@ typedef list<ValidPathInfo> ValidPathInfos;
 enum BuildMode { bmNormal, bmRepair, bmCheck };
 
 
+struct BuildResult
+{
+    enum Status {
+        Success = 0,
+        PermanentFailure = 1,
+        TimedOut = 2,
+        MiscFailure = 3
+    } status = MiscFailure;
+    std::string errorMsg;
+    //time_t startTime = 0, stopTime = 0;
+};
+
+
+struct BasicDerivation;
+
+
 class StoreAPI
 {
 public:
@@ -194,6 +210,12 @@ public:
        not derivations, substitute them. */
     virtual void buildPaths(const PathSet & paths, BuildMode buildMode = bmNormal) = 0;
 
+    /* Build a single non-materialized derivation (i.e. not from an
+       on-disk .drv file). Note that ‘drvPath’ is only used for
+       informational purposes. */
+    virtual BuildResult buildDerivation(const Path & drvPath, const BasicDerivation & drv,
+        BuildMode buildMode = bmNormal) = 0;
+
     /* Ensure that a path is valid.  If it is not currently valid, it
        may be made valid by running a substitute (if defined for the
        path). */