Merge changes I8d87c0e9,I25937702 into main

* changes:
  nix3-upgrade-nix: always use the /new/ nix-env to perform the installation
  libutil: implement a realPath() utility

src/libutil/file-system.cc

@@ -1,4 +1,5 @@
 #include <sys/time.h>
+#include <cstdlib>
 #include <filesystem>
 #include <atomic>
 
@@ -106,6 +107,24 @@ Path canonPath(PathView path, bool resolveSymlinks)
     return s.empty() ? "/" : std::move(s);
 }
 
+Path realPath(Path const & path)
+{
+    // With nullptr, realpath() malloc's and returns a new c-string.
+    char * resolved = realpath(path.c_str(), nullptr);
+    int saved = errno;
+    if (resolved == nullptr) {
+        throw SysError(saved, "cannot get realpath for '%s'", path);
+    }
+
+    Finally const _free([&] { free(resolved); });
+
+    // There's not really a from_raw_parts() for std::string.
+    // The copy is not a big deal.
+    Path ret(resolved);
+
+    return ret;
+}
+
 void chmodPath(const Path & path, mode_t mode)
 {
     if (chmod(path.c_str(), mode) == -1)

src/libutil/file-system.hh

@@ -46,6 +46,22 @@ Path absPath(Path path,
  */
 Path canonPath(PathView path, bool resolveSymlinks = false);
 
+/**
+ * Resolves a file path to a fully absolute path with no symbolic links.
+ *
+ * @param path The path to resolve. If it is relative, it will be resolved relative
+ * to the process's current directory.
+ *
+ * @note This is not a pure function; it performs this resolution by querying
+ * the filesystem.
+ *
+ * @note @ref path sadly must be (a reference to) an owned string, as std::string_view
+ * are not valid C strings...
+ *
+ * @return The fully resolved path.
+ */
+Path realPath(Path const & path);
+
 /**
  * Change the permissions of a path
  * Not called `chmod` as it shadows and could be confused with

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(