home-manager: handle missing per-user profiles directory

Specifically, if the global per-user profiles path do not exist and we
cannot create it during the activation, then place our profile in the
Home Manager data directory. We prefer to use the global location,
though, since it makes it visible to `nix-collect-garbage`.

This is intended to improve compatibility with Nix version 2.14 and
later, which no longer creates the per-user directories.

Also, use the Home Manager data directory to manage the gcroot for the
current generation. It does not have to sit in the global per-user
gcroots directory since it should never be eligible for GC.

modules/lib-bash/activation-init.sh

@@ -1,14 +1,60 @@
+# Moves the existing profile from /nix to ~ to match changed behavior in Nix
+# 2.14. See https://github.com/NixOS/nix/pull/5226.
+#
+# Note, this function is intentionally unused for now. There remains a few open
+# questions about backwards compatibility and support from
+# `nix-collect-garbage`.
+function migrateProfile() {
+    declare -r stateHome="${XDG_STATE_HOME:-$HOME/.local/state}"
+    declare -r hmStateDir="$stateHome/home-manager"
+    declare -r nixStateDir="${NIX_STATE_DIR:-/nix/var/nix}"
+
+    declare -r newProfilesDir="$hmStateDir/profiles"
+    declare -r oldProfilesDir="$nixStateDir/profiles/per-user/$USER"
+
+    if [[ ! -d $newProfilesDir ]]; then
+        _i 'Migrating profiles from %s to %s' "$oldProfilesDir" "$newProfilesDir"
+        mkdir -p "$newProfilesDir"
+        for p in "$oldProfilesDir"/home-manager-*; do
+            declare -r name="${p##*/}"
+            nix-store --realise "$p" --add-root "$newProfilesDir/$name" > /dev/null
+        done
+        cp -P "$oldProfilesDir/home-manager" "$newProfilesDir"
+    fi
+
+    rm "$oldProfilesDir"/home-manager-*
+}
+
 function setupVars() {
-    local nixStateDir="${NIX_STATE_DIR:-/nix/var/nix}"
-    local profilesPath="$nixStateDir/profiles/per-user/$USER"
-    local gcPath="$nixStateDir/gcroots/per-user/$USER"
+    declare -r nixStateDir="${NIX_STATE_DIR:-/nix/var/nix}"
+    declare -r globalProfilesDir="$nixStateDir/profiles/per-user/$USER"
+    declare -r globalGcrootsDir="$nixStateDir/gcroots/per-user/$USER"
 
-    declare -gr nixProfilePath="$profilesPath/profile"
-    declare -gr genProfilePath="$profilesPath/home-manager"
+    declare -r stateHome="${XDG_STATE_HOME:-$HOME/.local/state}"
+    declare -r hmStateDir="$stateHome/home-manager"
+    declare -r hmGcrootsDir="$hmStateDir/gcroots"
+
+    # If the global profiles path exists or we can create it, then place the HM
+    # profile there. Otherwise place it in the HM data directory. We prefer to
+    # use the global location since it makes it visible to
+    # `nix-collect-garbage`.
+    #
+    # In the future we may perform a one-shot migration to the new location.
+    #
+    # shellcheck disable=2174
+    if [[ -d "$globalProfilesDir" ]] || mkdir -m 0755 -p "$globalProfilesDir"; then
+        declare -r hmProfilesDir="$globalProfilesDir"
+    else
+        declare -r hmProfilesDir="$hmStateDir/profiles"
+        mkdir -m 0755 -p "$hmProfilesDir"
+    fi
+
+    declare -gr genProfilePath="$hmProfilesDir/home-manager"
     declare -gr newGenPath="@GENERATION_DIR@";
-    declare -gr newGenGcPath="$gcPath/current-home"
+    declare -gr newGenGcPath="$hmGcrootsDir/current-home"
+    declare -gr legacyGenGcPath="$globalGcrootsDir/current-home"
 
-    local greatestGenNum
+    declare greatestGenNum
     greatestGenNum=$( \
         nix-env --list-generations --profile "$genProfilePath" \
             | tail -1 \
@@ -21,9 +67,9 @@ function setupVars() {
         declare -gr newGenNum=1
     fi
 
-    if [[ -e $profilesPath/home-manager ]] ; then
-        oldGenPath="$(readlink -e "$profilesPath/home-manager")"
-        declare -gr oldGenPath
+    if [[ -e $genProfilePath ]] ; then
+        declare -g oldGenPath
+        oldGenPath="$(readlink -e "$genProfilePath")"
     fi
 
     $VERBOSE_RUN _i "Sanity checking oldGenNum and oldGenPath"
@@ -31,7 +77,7 @@ function setupVars() {
             || ! -v oldGenNum && -v oldGenPath ]]; then
         _i $'The previous generation number and path are in conflict! These\nmust be either both empty or both set but are now set to\n\n    \'%s\' and \'%s\'\n\nIf you don\'t mind losing previous profile generations then\nthe easiest solution is probably to run\n\n   rm %s/home-manager*\n   rm %s/current-home\n\nand trying home-manager switch again. Good luck!' \
            "${oldGenNum:-}" "${oldGenPath:-}" \
-           "$profilesPath" "$gcPath"
+           "$hmProfilesDir" "$hmGcrootsDir"
         exit 1
     fi
 }
@@ -58,9 +104,12 @@ setupVars
 if [[ -v DRY_RUN ]] ; then
     _i "This is a dry run"
     export DRY_RUN_CMD=echo
+    export DRY_RUN_NULL=/dev/stdout
 else
     $VERBOSE_RUN _i "This is a live run"
     export DRY_RUN_CMD=""
+    export DRY_RUN_NULL=/dev/null
+
 fi
 
 if [[ -v VERBOSE ]]; then
@@ -77,5 +126,6 @@ else
 fi
 $VERBOSE_ECHO "  newGenPath=$newGenPath"
 $VERBOSE_ECHO "  newGenNum=$newGenNum"
-$VERBOSE_ECHO "  newGenGcPath=$newGenGcPath"
 $VERBOSE_ECHO "  genProfilePath=$genProfilePath"
+$VERBOSE_ECHO "  newGenGcPath=$newGenGcPath"
+$VERBOSE_ECHO "  legacyGenGcPath=$legacyGenGcPath"