Use the daemon when we don't have write access to the Nix database

doc/manual/release-notes/release-notes.xml

@@ -12,6 +12,7 @@
 </partintro>
 -->
 
+<xi:include href="rl-1.12.xml" />
 <xi:include href="rl-1.11.xml" />
 <xi:include href="rl-1.10.xml" />
 <xi:include href="rl-1.9.xml" />

doc/manual/release-notes/rl-1.12.xml

@@ -0,0 +1,24 @@
+<section xmlns="http://docbook.org/ns/docbook"
+      xmlns:xlink="http://www.w3.org/1999/xlink"
+      xmlns:xi="http://www.w3.org/2001/XInclude"
+      version="5.0"
+      xml:id="ssec-relnotes-1.12">
+
+<title>Release 1.12 (TBA)</title>
+
+<para>This release has the following new features:</para>
+
+<itemizedlist>
+
+  <listitem>
+    <para>It is no longer necessary to set the
+    <envar>NIX_REMOTE</envar> environment variable if you need to use
+    the Nix daemon. Nix will use the daemon automatically if you don’t
+    have write access to the Nix database.</para>
+  </listitem>
+
+</itemizedlist>
+
+<para>This release has contributions from TBD.</para>
+
+</section>

src/libstore/local-store.cc

@@ -397,9 +397,15 @@ int LocalStore::getSchema()
 }
 
 
+bool LocalStore::haveWriteAccess()
+{
+    return access(settings.nixDBPath.c_str(), R_OK | W_OK) == 0;
+}
+
+
 void LocalStore::openDB(bool create)
 {
-    if (access(settings.nixDBPath.c_str(), R_OK | W_OK))
+    if (!haveWriteAccess())
         throw SysError(format("Nix database directory ‘%1%’ is not writable") % settings.nixDBPath);
 
     /* Open the Nix database. */

src/libstore/local-store.hh

@@ -253,6 +253,12 @@ private:
 
     int getSchema();
 
+public:
+
+    static bool haveWriteAccess();
+
+private:
+
     void openDB(bool create);
 
     void makeStoreWritable();

src/libstore/remote-store.cc

@@ -50,14 +50,8 @@ void RemoteStore::openConnection(bool reserveSpace)
     if (initialised) return;
     initialised = true;
 
-    string remoteMode = getEnv("NIX_REMOTE");
-
-    if (remoteMode == "daemon")
-        /* Connect to a daemon that does the privileged work for
-           us. */
+    /* Connect to a daemon that does the privileged work for us. */
     connectToDaemon();
-    else
-        throw Error(format("invalid setting for NIX_REMOTE, ‘%1%’") % remoteMode);
 
     from.fd = fdSocket;
     to.fd = fdSocket;

src/libstore/store-api.cc

@@ -311,10 +311,22 @@ std::shared_ptr<StoreAPI> store;
 
 std::shared_ptr<StoreAPI> openStore(bool reserveSpace)
 {
-    if (getEnv("NIX_REMOTE") == "")
-        return std::shared_ptr<StoreAPI>(new LocalStore(reserveSpace));
+    enum { mDaemon, mLocal, mAuto } mode;
+
+    mode = getEnv("NIX_REMOTE") == "daemon" ? mDaemon : mAuto;
+
+    if (mode == mAuto) {
+        if (LocalStore::haveWriteAccess())
+            mode = mLocal;
+        else if (pathExists(settings.nixDaemonSocketFile))
+            mode = mDaemon;
         else
-        return std::shared_ptr<StoreAPI>(new RemoteStore());
+            mode = mLocal;
+    }
+
+    return mode == mDaemon
+        ? (std::shared_ptr<StoreAPI>) std::make_shared<RemoteStore>()
+        : std::make_shared<LocalStore>(reserveSpace);
 }
 
 

tests/dump-db.sh

@@ -9,6 +9,7 @@ deps="$(nix-store -qR $TEST_ROOT/result)"
 nix-store --dump-db > $TEST_ROOT/dump
 
 rm -rf $NIX_DB_DIR
+mkdir $NIX_DB_DIR
 
 nix-store --load-db < $TEST_ROOT/dump