A setting to follow XDG Base Directory standard

XDG Base Directory is a standard for locations for storing various
files. Nix has a few files which seem to fit in the standard, but
currently use a custom location directly in the user's ~, polluting
it:

- ~/.nix-profile
- ~/.nix-defexpr
- ~/.nix-channels

This commit adds a config option (use-xdg-base-directories) to follow
the XDG spec and instead use the following locations:

- $XDG_STATE_HOME/nix/profile
- $XDG_STATE_HOME/nix/defexpr
- $XDG_STATE_HOME/nix/channels

If $XDG_STATE_HOME is not set, it is assumed to be ~/.local/state.

Co-authored-by: Théophane Hufschmitt <7226587+thufschmitt@users.noreply.github.com>
Co-authored-by: Tim Fenney <kodekata@gmail.com>
Co-authored-by: pasqui23 <pasqui23@users.noreply.github.com>
Co-authored-by: Artturin <Artturin@artturin.com>
Co-authored-by: John Ericson <Ericson2314@Yahoo.com>

8 files changed, 53 insertions, 9 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index a48968656..3e37c7f60 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -2496,7 +2496,7 @@ Strings EvalSettings::getDefaultNixPath()
                 res.push_back(s ? *s + "=" + p : p);
         };
 
-        add(getHome() + "/.nix-defexpr/channels");
+        add(settings.useXDGBaseDirectories ? getStateDir() + "/nix/defexpr/channels" : getHome() + "/.nix-defexpr/channels");
         add(settings.nixStateDir + "/profiles/per-user/root/channels/nixpkgs", "nixpkgs");
         add(settings.nixStateDir + "/profiles/per-user/root/channels");
 
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index c3ccb5e11..40f10fde3 100644
--- a/src/libstore/globals.hh
+++ b/src/libstore/globals.hh
@@ -975,6 +975,27 @@ public:
           resolves to a different location from that of the build machine. You
           can enable this setting if you are sure you're not going to do that.
         )"};
+
+    Setting<bool> useXDGBaseDirectories{
+        this, false, "use-xdg-base-directories",
+        R"(
+          If set to `true`, Nix will conform to the [XDG Base Directory Specification] for files in `$HOME`.
+          The environment variables used to implement this are documented in the [Environment Variables section](@docroot@/installation/env-variables.md).
+
+          [XDG Base Directory Specification]: https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html
+
+          > **Warning**
+          > This changes the location of some well-known symlinks that Nix creates, which might break tools that rely on the old, non-XDG-conformant locations.
+
+          In particular, the following locations change:
+
+          | Old               | New                            |
+          |-------------------|--------------------------------|
+          | `~/.nix-profile`  | `$XDG_STATE_HOME/nix/profile`  |
+          | `~/.nix-defexpr`  | `$XDG_STATE_HOME/nix/defexpr`  |
+          | `~/.nix-channels` | `$XDG_STATE_HOME/nix/channels` |
+        )"
+    };
 };
 
 
diff --git a/src/libstore/profiles.cc b/src/libstore/profiles.cc
index b202351ce..c551c5f3e 100644
--- a/src/libstore/profiles.cc
+++ b/src/libstore/profiles.cc
@@ -282,7 +282,7 @@ std::string optimisticLockProfile(const Path & profile)
 
 Path profilesDir()
 {
-    auto profileRoot = getDataDir() + "/nix/profiles";
+    auto profileRoot = createNixStateDir() + "/profiles";
     createDirs(profileRoot);
     return profileRoot;
 }
@@ -290,7 +290,7 @@ Path profilesDir()
 
 Path getDefaultProfile()
 {
-    Path profileLink = getHome() + "/.nix-profile";
+    Path profileLink = settings.useXDGBaseDirectories ? createNixStateDir() + "/profile" : getHome() + "/.nix-profile";
     try {
         auto profile =
             getuid() == 0
diff --git a/src/libstore/profiles.hh b/src/libstore/profiles.hh
index 73667a798..fbf95b850 100644
--- a/src/libstore/profiles.hh
+++ b/src/libstore/profiles.hh
@@ -72,8 +72,9 @@ std::string optimisticLockProfile(const Path & profile);
    profiles. */
 Path profilesDir();
 
-/* Resolve ~/.nix-profile. If ~/.nix-profile doesn't exist yet, create
-   it. */
+/* Resolve the default profile (~/.nix-profile by default, $XDG_STATE_HOME/
+   nix/profile if XDG Base Directory Support is enabled), and create if doesn't
+   exist */
 Path getDefaultProfile();
 
 }
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 40faa4bf2..40a54b010 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -608,6 +608,19 @@ Path getDataDir()
     return dataDir ? *dataDir : getHome() + "/.local/share";
 }
 
+Path getStateDir()
+{
+    auto stateDir = getEnv("XDG_STATE_HOME");
+    return stateDir ? *stateDir : getHome() + "/.local/state";
+}
+
+Path createNixStateDir()
+{
+    Path dir = getStateDir() + "/nix";
+    createDirs(dir);
+    return dir;
+}
+
 
 std::optional<Path> getSelfExe()
 {
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index 266da0ae3..4fadafaf2 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -158,6 +158,12 @@ Path getDataDir();
 /* Return the path of the current executable. */
 std::optional<Path> getSelfExe();
 
+/* Return $XDG_STATE_HOME or $HOME/.local/state. */
+Path getStateDir();
+
+/* Create the Nix state directory and return the path to it. */
+Path createNixStateDir();
+
 /* Create a directory and all its parents, if necessary.  Returns the
    list of created directories, in order of creation. */
 Paths createDirs(const Path & path);
diff --git a/src/nix-channel/nix-channel.cc b/src/nix-channel/nix-channel.cc
index 263d85eea..338a7d18e 100755
--- a/src/nix-channel/nix-channel.cc
+++ b/src/nix-channel/nix-channel.cc
@@ -164,8 +164,8 @@ static int main_nix_channel(int argc, char ** argv)
     {
         // Figure out the name of the `.nix-channels' file to use
         auto home = getHome();
-        channelsList = home + "/.nix-channels";
-        nixDefExpr = home + "/.nix-defexpr";
+        channelsList = settings.useXDGBaseDirectories ? createNixStateDir() + "/channels" : home + "/.nix-channels";
+        nixDefExpr = settings.useXDGBaseDirectories ? createNixStateDir() + "/defexpr" : home + "/.nix-defexpr";
 
         // Figure out the name of the channels profile.
         profile = profilesDir() + "/channels";
diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index 406e548c0..0daf374de 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -1289,7 +1289,7 @@ static void opSwitchProfile(Globals & globals, Strings opFlags, Strings opArgs)
         throw UsageError("exactly one argument expected");
 
     Path profile = absPath(opArgs.front());
-    Path profileLink = getHome() + "/.nix-profile";
+    Path profileLink = settings.useXDGBaseDirectories ? createNixStateDir() + "/profile" : getHome() + "/.nix-profile";
 
     switchLink(profileLink, profile);
 }
@@ -1393,7 +1393,10 @@ static int main_nix_env(int argc, char * * argv)
         Globals globals;
 
         globals.instSource.type = srcUnknown;
-        globals.instSource.nixExprPath = getHome() + "/.nix-defexpr";
+        {
+            Path nixExprPath = settings.useXDGBaseDirectories ? createNixStateDir() + "/defexpr" : getHome() + "/.nix-defexpr";
+            globals.instSource.nixExprPath = nixExprPath;
+        }
         globals.instSource.systemFilter = "*";
 
         if (!pathExists(globals.instSource.nixExprPath)) {