Migrate the old profiles to the new location

Make sure that we don’t just create the new profiles directory, but that
we also migrate every existing profile to it.

src/libstore/profiles.cc

@@ -293,17 +293,29 @@ Path getDefaultProfile()
     Path profileLink = getHome() + "/.nix-profile";
     try {
         // 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)
+        // If the link exists and points to the old location, then:
+        // - Rewrite it to point to the new location
+        // - For every generation of the old default profile, create a symlink
+        //   from the new directory to it (so that all the previous profiles
+        //   and generations are still available).
         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);
+        auto newProfile = profilesDir() + "/profile";
+        if (!pathExists(profileLink)
+            || (isLink(profileLink)
+                && readLink(profileLink) == legacyProfile)) {
+            warn("Migrating the default profile");
+            replaceSymlink(newProfile, profileLink);
+            replaceSymlink(legacyProfile, newProfile);
+            if (pathExists(legacyProfile)) {
+                for (auto & oldGen : findGenerations(legacyProfile).first) {
+                    replaceSymlink(
+                        oldGen.path,
+                        dirOf(newProfile) + "/"
+                            + std::string(baseNameOf(oldGen.path)));
+                }
+            }
         }
         return absPath(readLink(profileLink), dirOf(profileLink));
     } catch (Error &) {