Move the default profiles to the user’s home

Rather than using `/nix/var/nix/{profiles,gcroots}/per-user/`, put the user
profiles and gcroots under `$XDG_DATA_DIR/nix/{profiles,gcroots}`.

This means that the daemon no longer needs to manage these paths itself
(they are fully handled client-side). In particular, it doesn’t have to
`chown` them anymore (removing one need for root).

This does change the layout of the gc-roots created by nix-env, and is
likely to break some stuff, so I’m not sure how to properly handle that.

src/libstore/build/local-derivation-goal.cc

@@ -1517,7 +1517,7 @@ void LocalDerivationGoal::startDaemon()
                 try {
                     daemon::processConnection(store, from, to,
                         daemon::NotTrusted, daemon::Recursive,
-                        [&](Store & store) { store.createUser("nobody", 65535); });
+                        [&](Store & store) {});
                     debug("terminated daemon connection");
                 } catch (SysError &) {
                     ignoreException();

src/libstore/local-store.cc

@@ -201,8 +201,6 @@ LocalStore::LocalStore(const Params & params)
             throw SysError("could not set permissions on '%s' to 755", perUserDir);
     }
 
-    createUser(getUserName(), getuid());
-
     /* Optionally, create directories and set permissions for a
        multi-user install. */
     if (getuid() == 0 && settings.buildUsersGroup != "") {
@@ -1824,20 +1822,6 @@ void LocalStore::signPathInfo(ValidPathInfo & info)
 }
 
 
-void LocalStore::createUser(const std::string & userName, uid_t userId)
-{
-    for (auto & dir : {
-        fmt("%s/profiles/per-user/%s", stateDir, userName),
-        fmt("%s/gcroots/per-user/%s", stateDir, userName)
-    }) {
-        createDirs(dir);
-        if (chmod(dir.c_str(), 0755) == -1)
-            throw SysError("changing permissions of directory '%s'", dir);
-        if (chown(dir.c_str(), userId, getgid()) == -1)
-            throw SysError("changing owner of directory '%s'", dir);
-    }
-}
-
 std::optional<std::pair<int64_t, Realisation>> LocalStore::queryRealisationCore_(
         LocalStore::State & state,
         const DrvOutput & id)

src/libstore/local-store.hh

@@ -281,8 +281,6 @@ private:
     void signPathInfo(ValidPathInfo & info);
     void signRealisation(Realisation &);
 
-    void createUser(const std::string & userName, uid_t userId) override;
-
     // XXX: Make a generic `Store` method
     FixedOutputHash hashCAPath(
         const FileIngestionMethod & method,

src/libstore/profiles.cc

@@ -280,16 +280,30 @@ std::string optimisticLockProfile(const Path & profile)
 }
 
 
+Path profilesDir()
+{
+    auto profileRoot = getDataDir() + "/nix/profiles";
+    createDirs(profileRoot);
+    return profileRoot;
+}
+
+
 Path getDefaultProfile()
 {
     Path profileLink = getHome() + "/.nix-profile";
     try {
-        if (!pathExists(profileLink)) {
-            replaceSymlink(
-                getuid() == 0
-                ? settings.nixStateDir + "/profiles/default"
-                : fmt("%s/profiles/per-user/%s/profile", settings.nixStateDir, getUserName()),
-                profileLink);
+        // Migrate from the “old-style” profiles stored under `/nix/var`:
+        // If the link exists and points to the old location, rewrite it to the
+        // new one (otherwise keep-it as-it-is as it might have been
+        // intentionnally changed, in which case we shouldn’t touch it)
+        auto legacyProfile = getuid() == 0
+            ? settings.nixStateDir + "/profiles/default"
+            : fmt("%s/profiles/per-user/%s/profile", settings.nixStateDir, getUserName());
+        if (!pathExists(profileLink) ||
+            (isLink(profileLink) &&
+            readLink(profileLink) == legacyProfile)
+            ) {
+            replaceSymlink(profilesDir() + "/profile", profileLink);
         }
         return absPath(readLink(profileLink), dirOf(profileLink));
     } catch (Error &) {

src/libstore/profiles.hh

@@ -68,6 +68,10 @@ void lockProfile(PathLocks & lock, const Path & profile);
    rebuilt. */
 std::string optimisticLockProfile(const Path & profile);
 
+/* Creates and returns the path to a directory suitable for storing the user’s
+   profiles. */
+Path profilesDir();
+
 /* Resolve ~/.nix-profile. If ~/.nix-profile doesn't exist yet, create
    it. */
 Path getDefaultProfile();

src/libstore/store-api.hh

@@ -657,9 +657,6 @@ public:
         return toRealPath(printStorePath(storePath));
     }
 
-    virtual void createUser(const std::string & userName, uid_t userId)
-    { }
-
     /*
      * Synchronises the options of the client with those of the daemon
      * (a no-op when there’s no daemon)