Add flake evaluation cache

This exploits the hermetic nature of flake evaluation to speed up
repeated evaluations of a flake output attribute.

For example (doing 'nix build' on an already present package):

  $ time nix build nixpkgs:firefox

  real    0m1.497s
  user    0m1.160s
  sys     0m0.139s

  $ time nix build nixpkgs:firefox

  real    0m0.052s
  user    0m0.038s
  sys     0m0.007s

The cache is ~/.cache/nix/eval-cache-v1.sqlite, which has entries like

  INSERT INTO Attributes VALUES(
    X'92a907d4efe933af2a46959b082cdff176aa5bfeb47a98fabd234809a67ab195',
    'packages.firefox',
    1,
    '/nix/store/pbalzf8x19hckr8cwdv62rd6g0lqgc38-firefox-67.0.drv /nix/store/g6q0gx0v6xvdnizp8lrcw7c4gdkzana0-firefox-67.0 out');

where the hash 92a9... is a fingerprint over the flake store path and
the contents of the lockfile. Because flakes are evaluated in pure
mode, this uniquely identifies the evaluation result.

9 files changed, 293 insertions, 49 deletions

diff --git a/src/libexpr/flake/eval-cache.cc b/src/libexpr/flake/eval-cache.cc
new file mode 100644
index 000000000..fece1a2b5
--- /dev/null
+++ b/src/libexpr/flake/eval-cache.cc
@@ -0,0 +1,111 @@
+#include "eval-cache.hh"
+#include "sqlite.hh"
+
+#include <set>
+
+namespace nix::flake {
+
+static const char * schema = R"sql(
+
+create table if not exists Fingerprints (
+    fingerprint blob primary key not null,
+    timestamp   integer not null
+);
+
+create table if not exists Attributes (
+    fingerprint blob not null,
+    attrPath    text not null,
+    type        integer,
+    value       text,
+    primary key (fingerprint, attrPath),
+    foreign key (fingerprint) references Fingerprints(fingerprint) on delete cascade
+);
+)sql";
+
+struct EvalCache::State
+{
+    SQLite db;
+    SQLiteStmt insertFingerprint;
+    SQLiteStmt insertAttribute;
+    SQLiteStmt queryAttribute;
+    std::set<Fingerprint> fingerprints;
+};
+
+EvalCache::EvalCache()
+    : _state(std::make_unique<Sync<State>>())
+{
+    auto state(_state->lock());
+
+    Path dbPath = getCacheDir() + "/nix/eval-cache-v1.sqlite";
+    createDirs(dirOf(dbPath));
+
+    state->db = SQLite(dbPath);
+    state->db.isCache();
+    state->db.exec(schema);
+
+    state->insertFingerprint.create(state->db,
+        "insert or ignore into Fingerprints(fingerprint, timestamp) values (?, ?)");
+
+    state->insertAttribute.create(state->db,
+        "insert or replace into Attributes(fingerprint, attrPath, type, value) values (?, ?, ?, ?)");
+
+    state->queryAttribute.create(state->db,
+        "select type, value from Attributes where fingerprint = ? and attrPath = ?");
+}
+
+enum ValueType {
+    Derivation = 1,
+};
+
+void EvalCache::addDerivation(
+    const Fingerprint & fingerprint,
+    const std::string & attrPath,
+    const Derivation & drv)
+{
+    auto state(_state->lock());
+
+    if (state->fingerprints.insert(fingerprint).second)
+        // FIXME: update timestamp
+        state->insertFingerprint.use()
+            (fingerprint.hash, fingerprint.hashSize)
+            (time(0)).exec();
+
+    state->insertAttribute.use()
+        (fingerprint.hash, fingerprint.hashSize)
+        (attrPath)
+        (ValueType::Derivation)
+        (drv.drvPath + " " + drv.outPath + " " + drv.outputName).exec();
+}
+
+std::optional<EvalCache::Derivation> EvalCache::getDerivation(
+    const Fingerprint & fingerprint,
+    const std::string & attrPath)
+{
+    auto state(_state->lock());
+
+    auto queryAttribute(state->queryAttribute.use()
+        (fingerprint.hash, fingerprint.hashSize)
+        (attrPath));
+    if (!queryAttribute.next()) return {};
+
+    // FIXME: handle negative results
+
+    auto type = (ValueType) queryAttribute.getInt(0);
+    auto s = queryAttribute.getStr(1);
+
+    if (type != ValueType::Derivation) return {};
+
+    auto ss = tokenizeString<std::vector<std::string>>(s, " ");
+
+    debug("evaluation cache hit for '%s'", attrPath);
+
+    return Derivation { ss[0], ss[1], ss[2] };
+}
+
+EvalCache & EvalCache::singleton()
+{
+    static std::unique_ptr<EvalCache> evalCache(new EvalCache());
+    return *evalCache;
+}
+
+}
diff --git a/src/libexpr/flake/eval-cache.hh b/src/libexpr/flake/eval-cache.hh
new file mode 100644
index 000000000..03aea142e
--- /dev/null
+++ b/src/libexpr/flake/eval-cache.hh
@@ -0,0 +1,39 @@
+#pragma once
+
+#include "sync.hh"
+#include "flake.hh"
+
+namespace nix { struct SQLite; struct SQLiteStmt; }
+
+namespace nix::flake {
+
+class EvalCache
+{
+    struct State;
+
+    std::unique_ptr<Sync<State>> _state;
+
+    EvalCache();
+
+public:
+
+    struct Derivation
+    {
+        Path drvPath;
+        Path outPath;
+        std::string outputName;
+    };
+
+    void addDerivation(
+        const Fingerprint & fingerprint,
+        const std::string & attrPath,
+        const Derivation & drv);
+
+    std::optional<Derivation> getDerivation(
+        const Fingerprint & fingerprint,
+        const std::string & attrPath);
+
+    static EvalCache & singleton();
+};
+
+}
diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index bb0543541..0018a0d07 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -601,4 +601,13 @@ const Registries EvalState::getFlakeRegistries()
     return registries;
 }
 
+Fingerprint ResolvedFlake::getFingerprint() const
+{
+    // FIXME: as an optimization, if the flake contains a lockfile and
+    // we haven't changed it, then it's sufficient to use
+    // flake.sourceInfo.storePath for the fingerprint.
+    return hashString(htSHA256,
+        fmt("%s;%s", flake.sourceInfo.storePath, lockFile));
+}
+
 }
diff --git a/src/libexpr/flake/flake.hh b/src/libexpr/flake/flake.hh
index b8d0da252..81b6541f0 100644
--- a/src/libexpr/flake/flake.hh
+++ b/src/libexpr/flake/flake.hh
@@ -83,12 +83,18 @@ struct NonFlake
 
 Flake getFlake(EvalState &, const FlakeRef &, bool impureIsAllowed);
 
+/* Fingerprint of a locked flake; used as a cache key. */
+typedef Hash Fingerprint;
+
 struct ResolvedFlake
 {
     Flake flake;
     LockFile lockFile;
+
     ResolvedFlake(Flake && flake, LockFile && lockFile)
         : flake(flake), lockFile(lockFile) {}
+
+    Fingerprint getFingerprint() const;
 };
 
 ResolvedFlake resolveFlake(EvalState &, const FlakeRef &, HandleLockFile);
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 485fdd691..f39c73b23 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -294,9 +294,7 @@ void LocalStore::openDB(State & state, bool create)
     /* Open the Nix database. */
     string dbPath = dbDir + "/db.sqlite";
     auto & db(state.db);
-    if (sqlite3_open_v2(dbPath.c_str(), &db.db,
-            SQLITE_OPEN_READWRITE | (create ? SQLITE_OPEN_CREATE : 0), 0) != SQLITE_OK)
-        throw Error(format("cannot open Nix database '%1%'") % dbPath);
+    state.db = SQLite(dbPath, create);
 
 #ifdef __CYGWIN__
     /* The cygwin version of sqlite3 has a patch which calls
@@ -308,11 +306,6 @@ void LocalStore::openDB(State & state, bool create)
     SetDllDirectoryW(L"");
 #endif
 
-    if (sqlite3_busy_timeout(db, 60 * 60 * 1000) != SQLITE_OK)
-        throwSQLiteError(db, "setting timeout");
-
-    db.exec("pragma foreign_keys = 1");
-
     /* !!! check whether sqlite has been built with foreign key
        support */
 
diff --git a/src/libstore/nar-info-disk-cache.cc b/src/libstore/nar-info-disk-cache.cc
index 32ad7f2b2..3f6dbbcf5 100644
--- a/src/libstore/nar-info-disk-cache.cc
+++ b/src/libstore/nar-info-disk-cache.cc
@@ -78,12 +78,7 @@ public:
 
         state->db = SQLite(dbPath);
 
-        if (sqlite3_busy_timeout(state->db, 60 * 60 * 1000) != SQLITE_OK)
-            throwSQLiteError(state->db, "setting timeout");
-
-        // We can always reproduce the cache.
-        state->db.exec("pragma synchronous = off");
-        state->db.exec("pragma main.journal_mode = truncate");
+        state->db.isCache();
 
         state->db.exec(schema);
 
diff --git a/src/libstore/sqlite.cc b/src/libstore/sqlite.cc
index a061d64f3..eb1daafc5 100644
--- a/src/libstore/sqlite.cc
+++ b/src/libstore/sqlite.cc
@@ -25,11 +25,16 @@ namespace nix {
         throw SQLiteError("%s: %s (in '%s')", fs.s, sqlite3_errstr(exterr), path);
 }
 
-SQLite::SQLite(const Path & path)
+SQLite::SQLite(const Path & path, bool create)
 {
     if (sqlite3_open_v2(path.c_str(), &db,
-            SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE, 0) != SQLITE_OK)
+            SQLITE_OPEN_READWRITE | (create ? SQLITE_OPEN_CREATE : 0), 0) != SQLITE_OK)
         throw Error(format("cannot open SQLite database '%s'") % path);
+
+    if (sqlite3_busy_timeout(db, 60 * 60 * 1000) != SQLITE_OK)
+        throwSQLiteError(db, "setting timeout");
+
+    exec("pragma foreign_keys = 1");
 }
 
 SQLite::~SQLite()
@@ -42,6 +47,12 @@ SQLite::~SQLite()
     }
 }
 
+void SQLite::isCache()
+{
+    exec("pragma synchronous = off");
+    exec("pragma main.journal_mode = truncate");
+}
+
 void SQLite::exec(const std::string & stmt)
 {
     retrySQLite<void>([&]() {
@@ -94,6 +105,16 @@ SQLiteStmt::Use & SQLiteStmt::Use::operator () (const std::string & value, bool
     return *this;
 }
 
+SQLiteStmt::Use & SQLiteStmt::Use::operator () (const unsigned char * data, size_t len, bool notNull)
+{
+    if (notNull) {
+        if (sqlite3_bind_blob(stmt, curArg++, data, len, SQLITE_TRANSIENT) != SQLITE_OK)
+            throwSQLiteError(stmt.db, "binding argument");
+    } else
+        bind();
+    return *this;
+}
+
 SQLiteStmt::Use & SQLiteStmt::Use::operator () (int64_t value, bool notNull)
 {
     if (notNull) {
diff --git a/src/libstore/sqlite.hh b/src/libstore/sqlite.hh
index 115679b84..78e53fa32 100644
--- a/src/libstore/sqlite.hh
+++ b/src/libstore/sqlite.hh
@@ -15,13 +15,16 @@ struct SQLite
 {
     sqlite3 * db = 0;
     SQLite() { }
-    SQLite(const Path & path);
+    SQLite(const Path & path, bool create = true);
     SQLite(const SQLite & from) = delete;
     SQLite& operator = (const SQLite & from) = delete;
     SQLite& operator = (SQLite && from) { db = from.db; from.db = 0; return *this; }
     ~SQLite();
     operator sqlite3 * () { return db; }
 
+    /* Disable synchronous mode, set truncate journal mode. */
+    void isCache();
+
     void exec(const std::string & stmt);
 };
 
@@ -52,6 +55,7 @@ struct SQLiteStmt
 
         /* Bind the next parameter. */
         Use & operator () (const std::string & value, bool notNull = true);
+        Use & operator () (const unsigned char * data, size_t len, bool notNull = true);
         Use & operator () (int64_t value, bool notNull = true);
         Use & bind(); // null
 
diff --git a/src/nix/installables.cc b/src/nix/installables.cc
index a85295a09..86e601bc4 100644
--- a/src/nix/installables.cc
+++ b/src/nix/installables.cc
@@ -8,6 +8,7 @@
 #include "store-api.hh"
 #include "shared.hh"
 #include "flake/flake.hh"
+#include "flake/eval-cache.hh"
 
 #include <regex>
 #include <queue>
@@ -110,7 +111,7 @@ struct InstallableValue : Installable
 
     InstallableValue(SourceExprCommand & cmd) : cmd(cmd) { }
 
-    Buildables toBuildables() override
+    virtual std::vector<flake::EvalCache::Derivation> toDerivations()
     {
         auto state = cmd.getEvalState();
 
@@ -118,22 +119,36 @@ struct InstallableValue : Installable
 
         Bindings & autoArgs = *cmd.getAutoArgs(*state);
 
-        DrvInfos drvs;
-        getDerivations(*state, *v, "", autoArgs, drvs, false);
+        DrvInfos drvInfos;
+        getDerivations(*state, *v, "", autoArgs, drvInfos, false);
+
+        std::vector<flake::EvalCache::Derivation> res;
+        for (auto & drvInfo : drvInfos) {
+            res.push_back({
+                drvInfo.queryDrvPath(),
+                drvInfo.queryOutPath(),
+                drvInfo.queryOutputName()
+            });
+        }
+
+        return res;
+    }
 
+    Buildables toBuildables() override
+    {
         Buildables res;
 
         PathSet drvPaths;
 
-        for (auto & drv : drvs) {
-            Buildable b{drv.queryDrvPath()};
+        for (auto & drv : toDerivations()) {
+            Buildable b{drv.drvPath};
             drvPaths.insert(b.drvPath);
 
-            auto outputName = drv.queryOutputName();
+            auto outputName = drv.outputName;
             if (outputName == "")
                 throw Error("derivation '%s' lacks an 'outputName' attribute", b.drvPath);
 
-            b.outputs.emplace(outputName, drv.queryOutPath());
+            b.outputs.emplace(outputName, drv.outPath);
 
             res.push_back(std::move(b));
         }
@@ -254,11 +269,29 @@ struct InstallableFlake : InstallableValue
 
     std::string what() override { return flakeRef.to_string() + ":" + *attrPaths.begin(); }
 
-    Value * toValue(EvalState & state) override
+    std::vector<std::string> getActualAttrPaths()
     {
-        auto vFlake = state.allocValue();
+        std::vector<std::string> res;
 
-        auto resFlake = resolveFlake(state, flakeRef, cmd.getLockFileMode());
+        if (searchPackages) {
+            // As a convenience, look for the attribute in
+            // 'outputs.packages'.
+            res.push_back("packages." + *attrPaths.begin());
+
+            // As a temporary hack until Nixpkgs is properly converted
+            // to provide a clean 'packages' set, look in 'legacyPackages'.
+            res.push_back("legacyPackages." + *attrPaths.begin());
+        }
+
+        for (auto & s : attrPaths)
+            res.push_back(s);
+
+        return res;
+    }
+
+    Value * getFlakeOutputs(EvalState & state, const flake::ResolvedFlake & resFlake)
+    {
+        auto vFlake = state.allocValue();
 
         callFlake(state, resFlake, *vFlake);
 
@@ -268,34 +301,67 @@ struct InstallableFlake : InstallableValue
 
         state.forceValue(*vOutputs);
 
-        auto emptyArgs = state.allocBindings(0);
+        return vOutputs;
+    }
 
-        if (searchPackages) {
-            // As a convenience, look for the attribute in
-            // 'outputs.packages'.
-            if (auto aPackages = *vOutputs->attrs->get(state.symbols.create("packages"))) {
-                try {
-                    auto * v = findAlongAttrPath(state, *attrPaths.begin(), *emptyArgs, *aPackages->value);
-                    state.forceValue(*v);
-                    return v;
-                } catch (AttrPathNotFound & e) {
-                }
+    std::vector<flake::EvalCache::Derivation> toDerivations() override
+    {
+        auto state = cmd.getEvalState();
+
+        auto resFlake = resolveFlake(*state, flakeRef, cmd.getLockFileMode());
+
+        Value * vOutputs = nullptr;
+
+        auto emptyArgs = state->allocBindings(0);
+
+        auto & evalCache = flake::EvalCache::singleton();
+
+        auto fingerprint = resFlake.getFingerprint();
+
+        for (auto & attrPath : getActualAttrPaths()) {
+            auto drv = evalCache.getDerivation(fingerprint, attrPath);
+            if (drv) {
+                if (state->store->isValidPath(drv->drvPath))
+                    return {*drv};
             }
 
-            // As a temporary hack until Nixpkgs is properly converted
-            // to provide a clean 'packages' set, look in 'legacyPackages'.
-            if (auto aPackages = *vOutputs->attrs->get(state.symbols.create("legacyPackages"))) {
-                try {
-                    auto * v = findAlongAttrPath(state, *attrPaths.begin(), *emptyArgs, *aPackages->value);
-                    state.forceValue(*v);
-                    return v;
-                } catch (AttrPathNotFound & e) {
-                }
+            if (!vOutputs)
+                vOutputs = getFlakeOutputs(*state, resFlake);
+
+            try {
+                auto * v = findAlongAttrPath(*state, attrPath, *emptyArgs, *vOutputs);
+                state->forceValue(*v);
+
+                auto drvInfo = getDerivation(*state, *v, false);
+                if (!drvInfo)
+                    throw Error("flake output attribute '%s' is not a derivation", attrPath);
+
+                auto drv = flake::EvalCache::Derivation{
+                    drvInfo->queryDrvPath(),
+                    drvInfo->queryOutPath(),
+                    drvInfo->queryOutputName()
+                };
+
+                evalCache.addDerivation(fingerprint, attrPath, drv);
+
+                return {drv};
+            } catch (AttrPathNotFound & e) {
             }
         }
 
-        // Otherwise, look for it in 'outputs'.
-        for (auto & attrPath : attrPaths) {
+        throw Error("flake '%s' does not provide attribute %s",
+            flakeRef, concatStringsSep(", ", quoteStrings(attrPaths)));
+    }
+
+    Value * toValue(EvalState & state) override
+    {
+        auto resFlake = resolveFlake(state, flakeRef, cmd.getLockFileMode());
+
+        auto vOutputs = getFlakeOutputs(state, resFlake);
+
+        auto emptyArgs = state.allocBindings(0);
+
+        for (auto & attrPath : getActualAttrPaths()) {
             try {
                 auto * v = findAlongAttrPath(state, attrPath, *emptyArgs, *vOutputs);
                 state.forceValue(*v);