Factor out the generation of the profile/channel directory

Make sure that all the code paths use the same one, and that the
backwards-compatibility measures are probably in place when needed

src/libexpr/eval.cc

@@ -8,6 +8,7 @@
 #include "eval-inline.hh"
 #include "filetransfer.hh"
 #include "function-trace.hh"
+#include "profiles.hh"
 
 #include <algorithm>
 #include <chrono>
@@ -2491,8 +2492,8 @@ Strings EvalSettings::getDefaultNixPath()
 
     if (!evalSettings.restrictEval && !evalSettings.pureEval) {
         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");
+        add(rootChannelsDir() + "/nixpkgs", "nixpkgs");
+        add(rootChannelsDir());
     }
 
     return res;

src/libstore/profiles.cc

@@ -282,28 +282,48 @@ std::string optimisticLockProfile(const Path & profile)
 
 Path profilesDir()
 {
-    auto profileRoot = createNixStateDir() + "/profiles";
+    auto profileRoot =
+        (getuid() == 0)
+        ? rootProfilesDir()
+        : createNixStateDir() + "/profiles";
     createDirs(profileRoot);
     return profileRoot;
 }
 
+Path rootProfilesDir()
+{
+    return settings.nixStateDir + "/profiles/per-user/root";
+}
+
 
 Path getDefaultProfile()
 {
     Path profileLink = settings.useXDGBaseDirectories ? createNixStateDir() + "/profile" : getHome() + "/.nix-profile";
     try {
-        auto profile =
-            getuid() == 0
-            ? settings.nixStateDir + "/profiles/default"
-            : profilesDir() + "/profile";
+        auto profile = profilesDir() + "/profile";
         if (!pathExists(profileLink)) {
             replaceSymlink(profile, profileLink);
         }
+        // Backwards compatibiliy measure: Make root's profile available as
+        // `.../default` as it's what NixOS and most of the init scripts expect
+        Path globalProfileLink = settings.nixStateDir + "/profiles/default";
+        if (getuid() == 0 && !pathExists(globalProfileLink)) {
+            replaceSymlink(profile, globalProfileLink);
+        }
         return absPath(readLink(profileLink), dirOf(profileLink));
     } catch (Error &) {
         return profileLink;
     }
 }
 
+Path defaultChannelsDir()
+{
+    return profilesDir() + "/channels";
+}
+
+Path rootChannelsDir()
+{
+    return rootProfilesDir() + "/channels";
+}
 
 }

src/libstore/profiles.hh

@@ -72,6 +72,15 @@ std::string optimisticLockProfile(const Path & profile);
    profiles. */
 Path profilesDir();
 
+/* Returns the path to the profile directory for root (but doesn't try creating it) */
+Path rootProfilesDir();
+
+/* Creates and returns the path to the file used for storing the users's channels */
+Path defaultChannelsDir();
+
+/* Returns the path to the channel directory for root (but doesn't try creating it) */
+Path rootChannelsDir();
+
 /* 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 */

src/nix-channel/nix-channel.cc

@@ -168,12 +168,7 @@ static int main_nix_channel(int argc, char ** argv)
         nixDefExpr = settings.useXDGBaseDirectories ? createNixStateDir() + "/defexpr" : home + "/.nix-defexpr";
 
         // Figure out the name of the channels profile.
-        // For backwards-compatibility, install the root channels under
-        // a custom location, as these are also used as "global" channeles, and
-        // their location is hardcoded in a number of places.
-        profile = getuid() == 0
-            ? settings.nixStateDir + "/profiles/per-user/root/channels"
-            : profilesDir() +  "/channels";
+        profile = profilesDir() +  "/channels";
         createDirs(dirOf(profile));
 
         enum {

src/nix-env/nix-env.cc

@@ -1403,11 +1403,11 @@ static int main_nix_env(int argc, char * * argv)
             try {
                 createDirs(globals.instSource.nixExprPath);
                 replaceSymlink(
-                    fmt("%s/profiles/per-user/%s/channels", settings.nixStateDir, getUserName()),
+                    defaultChannelsDir(),
                     globals.instSource.nixExprPath + "/channels");
                 if (getuid() != 0)
                     replaceSymlink(
-                        fmt("%s/profiles/per-user/root/channels", settings.nixStateDir),
+                        rootChannelsDir(),
                         globals.instSource.nixExprPath + "/channels_root");
             } catch (Error &) { }
         }