nix3-upgrade-nix: always use the /new/ nix-env to perform the installation

Fixes #411.

Change-Id: I8d87c0e9295deea26ff33234e15ee33cc68ab303

src/nix/upgrade-nix.cc

@@ -97,12 +97,19 @@ struct CmdUpgradeNix : MixDryRun, EvalCommand
             store->ensurePath(storePath);
         }
 
+        // {profileDir}/bin/nix-env is a symlink to {profileDir}/bin/nix, which *then*
+        // is a symlink to /nix/store/meow-nix/bin/nix.
+        // We want /nix/store/meow-nix/bin/nix-env.
+        Path const oldNixInStore = realPath(canonProfileDir + "/bin/nix");
+        Path const oldNixEnv = dirOf(oldNixInStore) + "/nix-env";
+
+        Path const newNixEnv = store->printStorePath(storePath) + "/bin/nix-env";
+
         {
             Activity act(*logger, lvlInfo, actUnknown, fmt("verifying that '%s' works...", store->printStorePath(storePath)));
-            auto program = store->printStorePath(storePath) + "/bin/nix-env";
+            auto s = runProgram(newNixEnv, false, {"--version"});
-            auto s = runProgram(program, false, {"--version"});
             if (s.find("Nix") == std::string::npos)
-                throw Error("could not verify that '%s' works", program);
+                throw Error("could not verify that '%s' works", newNixEnv);
         }
 
         logger->pause();
@@ -110,23 +117,17 @@ struct CmdUpgradeNix : MixDryRun, EvalCommand
         auto const fullStorePath = store->printStorePath(storePath);
 
         if (pathExists(canonProfileDir + "/manifest.nix")) {
-
+            // First remove the existing Nix, then use the *new* Nix by absolute path to
-            // {settings.nixBinDir}/nix-env is a symlink to a {settings.nixBinDir}/nix, which *then*
-            // is a symlink to /nix/store/meow-nix/bin/nix. We want /nix/store/meow-nix/bin/nix-env.
-            Path const nixInStore = canonPath(settings.nixBinDir + "/nix-env", true);
-            Path const nixEnvCmd = dirOf(nixInStore) + "/nix-env";
-
-            // First remove the existing Nix, then use that Nix by absolute path to
             // install the new one, in case the new and old versions aren't considered
             // to be "the same package" by nix-env's logic (e.g., if their pnames differ).
             Strings removeArgs = {
                 "--uninstall",
-                nixEnvCmd,
+                oldNixEnv,
                 "--profile",
                 this->profileDir,
             };
-            printTalkative("running %s %s", nixEnvCmd, concatStringsSep(" ", removeArgs));
+            printTalkative("running %s %s", newNixEnv, concatStringsSep(" ", removeArgs));
-            runProgram(nixEnvCmd, false, removeArgs);
+            runProgram(newNixEnv, false, removeArgs);
 
             Strings upgradeArgs = {
                 "--profile",
@@ -136,8 +137,8 @@ struct CmdUpgradeNix : MixDryRun, EvalCommand
                 "--no-sandbox",
             };
 
-            printTalkative("running %s %s", nixEnvCmd, concatStringsSep(" ", upgradeArgs));
+            printTalkative("running %s %s", newNixEnv, concatStringsSep(" ", upgradeArgs));
-            runProgram(nixEnvCmd, false, upgradeArgs);
+            runProgram(newNixEnv, false, upgradeArgs);
         } else if (pathExists(canonProfileDir + "/manifest.json")) {
             this->upgradeNewStyleProfile(store, storePath);
         } else {

tests/nixos/nix-upgrade-nix.nix

@@ -35,9 +35,8 @@ in {
 
     machine.succeed("nix --version >&2")
 
-    # Install Lix into the default profile, overriding /run/current-system/sw/bin/nix,
+    # Use Lix to install CppNix into the default profile, overriding /run/current-system/sw/bin/nix
-    # and thus making Lix think we're not on NixOS.
+    machine.succeed("nix-env --install '${lib.getBin newNix}' --profile /nix/var/nix/profiles/default")
-    machine.succeed("nix-env --install '${lib.getBin lix}' --profile /nix/var/nix/profiles/default >&2")
 
     # Make sure that correctly got inserted into our PATH.
     default_profile_nix_path = machine.succeed("command -v nix")
@@ -45,16 +44,15 @@ in {
     assert default_profile_nix_path.strip() == "/nix/var/nix/profiles/default/bin/nix", \
       f"{default_profile_nix_path.strip()=} != /nix/var/nix/profiles/default/bin/nix"
 
-    # And that it's the Nix we specified.
+    # And that it's the Nix we specified
     default_profile_version = machine.succeed("nix --version")
-    assert "${lixVersion}" in default_profile_version, f"${lixVersion} not in {default_profile_version}"
+    assert "${newNixVersion}" in default_profile_version, f"${newNixVersion} not in {default_profile_version}"
 
-    # Upgrade to a different version of Nix, and make sure that also worked.
+    # Now upgrade to Lix, and make sure that worked.
-
+    machine.succeed("${lib.getExe lix} upgrade-nix --debug --store-path ${lix} 2>&1")
-    machine.succeed("nix upgrade-nix --store-path ${newNix} >&2")
     default_profile_version = machine.succeed("nix --version")
     print(default_profile_version)
-    assert "${newNixVersion}" in default_profile_version, f"${newNixVersion} not in {default_profile_version}"
+    assert "${lixVersion}" in default_profile_version, f"${lixVersion} not in {default_profile_version}"
 
     # Now 'break' this profile -- use nix profile on it so nix-env will no longer work on it.
     machine.succeed(