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.

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 */
 

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);
 

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) {

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