fix nix upgrade-nix on new-style profiles

nix3-profile automatically migrates any profile its used on to its style
of profile -- the ones with manifest.json instead of manifest.nix. On
non-NixOS systems, Nix is conventionally installed to the profile at
/nix/var/nix/profiles/default, so if a user passed that to `--profile`
of `nix profile`, then it would break upgrade-nix from ever working
again, without recreating the profile.

This commit fixes that, and allows upgrade-nix to work on either kind of
profile.

Fixes #16.

Change-Id: I4c49b1beba93bb50e8f8a107edc451affe08c3f7

doc/manual/rl-next/upgrade-nix-profile-compat.md

@@ -0,0 +1,8 @@
+---
+synopsis: using `nix profile` on `/nix/var/nix/profiles/default` no longer breaks `nix upgrade-nix`
+cls: 952
+---
+
+On non-NixOS, Nix is conventionally installed into a `nix-env` style profile at /nix/var/nix/profiles/default.
+Like any `nix-env` profile, using `nix profile` on it automatically migrates it to a `nix profile` style profile, which is incompatible with `nix-env`.
+`nix upgrade-nix` previously relied solely on `nix-env` to do the upgrade, but now will work fine with either kind of profile.

src/libcmd/cmd-profiles.cc

@@ -141,10 +141,7 @@ ProfileManifest::ProfileManifest(EvalState & state, const Path & profile)
             }
             elements.emplace_back(std::move(element));
         }
-    }
+    } else if (pathExists(profile + "/manifest.nix")) {
-
-    else if (pathExists(profile + "/manifest.nix"))
-    {
         // FIXME: needed because of pure mode; ugly.
         state.allowPath(state.store->followLinksToStore(profile));
         state.allowPath(state.store->followLinksToStore(profile + "/manifest.nix"));

src/nix/upgrade-nix.cc

@@ -1,5 +1,11 @@
+#include <algorithm>
+
+#include "cmd-profiles.hh"
 #include "command.hh"
 #include "common-args.hh"
+#include "local-fs-store.hh"
+#include "logging.hh"
+#include "profiles.hh"
 #include "store-api.hh"
 #include "filetransfer.hh"
 #include "eval.hh"
@@ -10,7 +16,7 @@
 
 using namespace nix;
 
-struct CmdUpgradeNix : MixDryRun, StoreCommand
+struct CmdUpgradeNix : MixDryRun, EvalCommand
 {
     Path profileDir;
     std::string storePathsUrl = "https://github.com/NixOS/nixpkgs/raw/master/nixos/modules/installer/tools/nix-fallback-paths.nix";
@@ -59,12 +65,15 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
     {
         evalSettings.pureEval = true;
 
-        if (profileDir == "")
+        if (profileDir == "") {
             profileDir = getProfileDir(store);
+        }
+
+        auto canonProfileDir = canonPath(profileDir, true);
 
         printInfo("upgrading Nix in profile '%s'", profileDir);
 
-        auto storePath = getLatestNix(store);
+        StorePath storePath = getLatestNix(store);
 
         auto version = DrvName(storePath.name()).version;
 
@@ -89,11 +98,31 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
 
         stopProgressBar();
 
-        {
+        auto const fullStorePath = store->printStorePath(storePath);
-            Activity act(*logger, lvlInfo, actUnknown,
+
-                fmt("installing '%s' into profile '%s'...", store->printStorePath(storePath), profileDir));
+        if (pathExists(canonProfileDir + "/manifest.nix")) {
-            runProgram(settings.nixBinDir + "/nix-env", false,
+
-                {"--profile", profileDir, "-i", store->printStorePath(storePath), "--no-sandbox"});
+            std::string nixEnvCmd = settings.nixBinDir + "/nix-env";
+            Strings upgradeArgs = {
+                "--profile",
+                this->profileDir,
+                "--install",
+                fullStorePath,
+                "--no-sandbox",
+            };
+
+            printTalkative("running %s %s", nixEnvCmd, concatStringsSep(" ", upgradeArgs));
+            runProgram(nixEnvCmd, false, upgradeArgs);
+        } else if (pathExists(canonProfileDir + "/manifest.json")) {
+            this->upgradeNewStyleProfile(store, storePath);
+        } else {
+            // No I will not use std::unreachable.
+            // That is undefined behavior if you're wrong.
+            // This will have a better error message and coredump.
+            assert(
+                false && "tried to upgrade unexpected kind of profile, "
+                "we can only handle `user-environment` and `profile`"
+            );
         }
 
         printInfo(ANSI_GREEN "upgrade to version %s done" ANSI_NORMAL, version);
@@ -121,23 +150,99 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
         Path profileDir = dirOf(where);
 
         // Resolve profile to /nix/var/nix/profiles/<name> link.
-        while (canonPath(profileDir).find("/profiles/") == std::string::npos && isLink(profileDir))
+        while (canonPath(profileDir).find("/profiles/") == std::string::npos && isLink(profileDir)) {
             profileDir = readLink(profileDir);
+        }
 
         printInfo("found profile '%s'", profileDir);
 
         Path userEnv = canonPath(profileDir, true);
 
-        if (baseNameOf(where) != "bin" ||
+        if (baseNameOf(where) != "bin") {
-            !userEnv.ends_with("user-environment"))
+            throw Error("directory '%s' does not appear to be part of a Nix profile (no /bin dir?)", where);
-            throw Error("directory '%s' does not appear to be part of a Nix profile", where);
+        }
 
-        if (!store->isValidPath(store->parseStorePath(userEnv)))
+        if (!pathExists(userEnv + "/manifest.nix") && !pathExists(userEnv + "/manifest.json")) {
+            throw Error(
+                "directory '%s' does not have a compatible profile manifest; was it created by Nix?",
+                where
+            );
+        }
+
+        if (!store->isValidPath(store->parseStorePath(userEnv))) {
             throw Error("directory '%s' is not in the Nix store", userEnv);
+        }
 
         return profileDir;
     }
 
+    // TODO: Is there like, any good naming scheme that distinguishes
+    // "profiles which nix-env can use" and "profiles which nix profile can use"?
+    // You can't just say the manifest version since v2 and v3 are both the latter.
+    void upgradeNewStyleProfile(ref<Store> & store, StorePath const & newNix)
+    {
+        auto fsStore = store.dynamic_pointer_cast<LocalFSStore>();
+        // TODO(Qyriad): this check is here because we need to cast to a LocalFSStore,
+        // to pass to createGeneration(), ...but like, there's no way a remote store
+        // would work with the nix-env based upgrade either right?
+        if (!fsStore) {
+            throw Error("nix upgrade-nix cannot be used on a remote store");
+        }
+
+        // nb: nothing actually gets evaluated here.
+        // The ProfileManifest constructor only evaluates anything for manifest.nix
+        // profiles, which this is not.
+        auto evalState = this->getEvalState();
+
+        ProfileManifest manifest(*evalState, profileDir);
+
+        // Find which profile element has Nix in it.
+        // It should be impossible to *not* have Nix, since we grabbed this
+        // store path by looking for things with bin/nix-env in them anyway.
+        auto findNix = [&](ProfileElement const & elem) -> bool {
+            for (auto const & ePath : elem.storePaths) {
+                auto const nixEnv = store->printStorePath(ePath) + "/bin/nix-env";
+                if (pathExists(nixEnv)) {
+                    return true;
+                }
+            }
+            // We checked each store path in this element. No nixes here boss!
+            return false;
+        };
+        auto elemWithNix = std::find_if(
+            manifest.elements.begin(),
+            manifest.elements.end(),
+            findNix
+        );
+        // *Should* be impossible...
+        assert(elemWithNix != std::end(manifest.elements));
+
+        // Now create a new profile element for the new Nix version...
+        ProfileElement elemForNewNix = {
+            .storePaths = {newNix},
+        };
+
+        // ...and splork it into the manifest where the old profile element was.
+        // (Remember, elemWithNix is an iterator)
+        *elemWithNix = elemForNewNix;
+
+        // Build the new profile, and switch to it.
+        StorePath const newProfile = manifest.build(store);
+        printTalkative("built new profile '%s'", store->printStorePath(newProfile));
+        auto const newGeneration = createGeneration(*fsStore, this->profileDir, newProfile);
+        printTalkative(
+            "switching '%s' to newly created generation '%s'",
+            this->profileDir,
+            newGeneration
+        );
+        // TODO(Qyriad): use switchGeneration?
+        // switchLink's docstring seems to indicate that's preferred, but it's
+        // not used for any other `nix profile`-style profile code except for
+        // rollback, and it assumes you already have a generation number, which
+        // we don't.
+        switchLink(profileDir, newGeneration);
+    }
+
     /* Return the store path of the latest stable Nix. */
     StorePath getLatestNix(ref<Store> store)
     {