nix3-upgrade-nix: allow manually specifying the store path for the new nix

Should this specify the profile? The derivation output that gets linked
into that profile? The latter is probably better.

Change-Id: Ib71711ffb466febf4a6892e3fdbda644e053770d

doc/manual/rl-next/upgrade-nix-override.md

@@ -0,0 +1,6 @@
+---
+synopsis: add --store-path argument to `nix upgrade-nix`, to manually specify the Nix to upgrade to
+cls: 953
+---
+
+`nix upgrade-nix` by default downloads a manifest to find the new Nix version to upgrade to, but now you can specify `--store-path` to upgrade Nix to an arbitrary version from the Nix store.

src/nix/upgrade-nix.cc

@@ -21,6 +21,8 @@ struct CmdUpgradeNix : MixDryRun, EvalCommand
     Path profileDir;
     std::string storePathsUrl = "https://github.com/NixOS/nixpkgs/raw/master/nixos/modules/installer/tools/nix-fallback-paths.nix";
 
+    std::optional<Path> overrideStorePath;
+
     CmdUpgradeNix()
     {
         addFlag({
@@ -31,6 +33,13 @@ struct CmdUpgradeNix : MixDryRun, EvalCommand
             .handler = {&profileDir}
         });
 
+        addFlag({
+            .longName = "store-path",
+            .description = "A specific store path to upgrade Nix to",
+            .labels = {"store-path"},
+            .handler = {&overrideStorePath},
+        });
+
         addFlag({
             .longName = "nix-store-paths-url",
             .description = "The URL of the file that contains the store paths of the latest Nix release.",
@@ -238,6 +247,14 @@ struct CmdUpgradeNix : MixDryRun, EvalCommand
     /* Return the store path of the latest stable Nix. */
     StorePath getLatestNix(ref<Store> store)
     {
+        if (this->overrideStorePath) {
+            printTalkative(
+                "skipping Nix version query and using '%s' as latest Nix",
+                *this->overrideStorePath
+            );
+            return store->parseStorePath(*this->overrideStorePath);
+        }
+
         Activity act(*logger, lvlInfo, actUnknown, "querying latest Nix version");
 
         // FIXME: use nixos.org?