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.

1 files changed, 23 insertions, 2 deletions

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) {