Firefox: the desktop profile wrapper keeps prefs.js when rewriting it fails

A failed write of prefs.js.sfn (e.g. a full disk) emptied prefs.js; now
prefs.js and the user.js link stay for the next launch to retry.

modules/firefox/check.nix

@@ -1,8 +1,9 @@
 # Checks of the Firefox wrapper's desktop profile user.js (wrapper.nix)
 # against a fake flatpak: the link exists only while Firefox runs, a second
 # launch that hands over to the running Firefox doesn't remove it, the value
-# Firefox stored from it leaves prefs.js once the profile is unlocked, a
-# user.js of the user's own is never touched, the profile manager action and
+# Firefox stored from it leaves prefs.js once the profile is unlocked (not
+# lost if prefs.js can't be rewritten), a user.js of the user's own is never
+# touched, the profile manager action and
 # the Steam session get no --profile.
 { pkgs }:
 let
@@ -61,6 +62,17 @@ pkgs.runCommand "firefox-check" { nativeBuildInputs = [ pkgs.findutils pkgs.gnug
   grep -q ignore-widgets $p/prefs.js || fail "prefs.js changed with an own user.js"
   rm $p/user.js
 
+  # prefs.js can't be rewritten: it and the link stay; the next launch retries
+  echo 'user_pref("other", 1);' > $p/prefs.js
+  mkdir $p/prefs.js.sfn
+  ${launcher} || true
+  grep -q other $p/prefs.js && grep -q ignore-widgets $p/prefs.js || fail "prefs.js lost on a write error"
+  [ -L $p/user.js ] || fail "link removed on a write error"
+  rmdir $p/prefs.js.sfn
+  ${launcher} || true
+  [ "$(cat $p/prefs.js)" = 'user_pref("other", 1);' ] || fail "retry: $(cat $p/prefs.js)"
+  [ ! -L $p/user.js ] || fail "retry left the link"
+
   # profile manager action: no --profile, no link
   : > $FAKE/log
   ${launcher} --ProfileManager || true

modules/firefox/wrapper.nix

@@ -8,7 +8,8 @@
 # no longer in use (a second launch that only hands its URL to the running
 # Firefox returns at once) removes the link and the value Firefox stored from
 # it in prefs.js. A user.js of the user's own is never touched. After a crash
-# the next launch or steam-frame-nix-cleanup finishes the removal.
+# (or a failed rewrite of prefs.js) the next launch or steam-frame-nix-cleanup
+# finishes the removal.
 { lib, writeShellScript, coreutils, findutils, gnugrep
 , profileDir             # shell word, e.g. "$HOME/.var/app/…/desktop"
 , desktopFix ? false
@@ -33,9 +34,13 @@ writeShellScript "firefox-wrapper" (''
   sfn_unlink() {
     ours && ! inUse || return 0
     if [ -f "$prof/prefs.js" ] && grep -qF "$pats" "$prof/prefs.js"; then
-      grep -vF "$pats" "$prof/prefs.js" > "$prof/prefs.js.sfn" || true
-      cat "$prof/prefs.js.sfn" > "$prof/prefs.js"
+      # If that fails (grep status 2, a failed write, e.g. a full disk),
+      # prefs.js and the link stay for the next try.
+      { grep -vF "$pats" "$prof/prefs.js"; [ $? -le 1 ]; } > "$prof/prefs.js.sfn" &&
+        cat "$prof/prefs.js.sfn" > "$prof/prefs.js"
+      ok=$?
       rm -f "$prof/prefs.js.sfn"
+      [ $ok = 0 ] || return 0
     fi
     rm -f "$u"
   }