feat(mutagen): pair a host the first time we ssh into it

nix/home/mutagen-sync.nix

@@ -20,8 +20,17 @@
 # far side. That is fine for machines we own (homelab, macmini) and not fine for an external
 # company server, so those go through mutagen over the SSH connection we already have.
 #
+# Which hosts get a session: whichever ones we actually ssh into. The zsh wrapper below pairs
+# a host the first time we reach it, so the list maintains itself instead of being curated here.
+# The peers list stays for the one host that needs a hand-written entry (see sshHost).
+#
 # Why not restic-backed up: the data exists on both sides by definition, so restic keeps
-# covering ~/Sync/syncthing only (see home/restic-backup.nix).
+# covering ~/Sync/syncthing only (see home/restic-backup.nix). Nothing here is the only copy
+# of anything — that is the price of not backing it up, and it is the rule for what we drop in.
+#
+# Why no failure notification, unlike restic: a stalled sync is noticed the moment a file does
+# not show up on the other side, which is the only time this matters. `mutagen sync list` is the
+# check. (If that stops being true, the pattern to copy is restic-backup.nix's ntfy notify().)
 #
 # Not for source code. Repositories are cloned on both sides with ghq and moved with git;
 # .git carries absolute paths (git worktrees) and an index that must stay consistent with the
@@ -100,9 +109,95 @@ let
       fi
       exit 0
     '';
+
+  # Pair whatever host we just reached over ssh. Called by the zsh wrapper with the very argv
+  # the user typed, so the destination has to be dug out of it the way ssh itself would.
+  ensureScript = pkgs.writeShellScriptBin "mutagen-sync-ensure" ''
+    set -uo pipefail
+    export PATH=${mutagenBin}:$PATH
+
+    stateDir="${home}/.local/state/mutagen-sync"
+    mkdir -p "$stateDir" "${logDir}"
+    # Nothing is ever printed to the caller: this runs behind an interactive ssh.
+    exec >>"${logDir}/auto.log" 2>&1
+
+    dest=""
+    while [ $# -gt 0 ]; do
+      case "$1" in
+        # the short options that take a separate value
+        -[bcDEeFIiJLlmOopQRSWw]) shift 2 || exit 0 ;;
+        -*) shift ;;
+        *) dest="$1"; break ;;
+      esac
+    done
+    [ -n "$dest" ] || exit 0
+    host="''${dest#*@}"
+
+    # Only named entries pair. An address is a one-off, and it would name the sync directory
+    # after something that changes; *-sync is our own alias for a host the peers list owns.
+    case "$host" in
+      *.* | *:* | */*) exit 0 ;;
+      *-sync) exit 0 ;;
+    esac
+    case " github localhost " in *" $host "*) exit 0 ;; esac
+
+    # Alias-proof identity: ssh -G resolves locally, so rpi4 / rpi / raspberrypi collapse into
+    # one key and cannot end up as three sessions pointed at the same directory.
+    key=$(/usr/bin/ssh -G "$host" 2>/dev/null | awk '
+      $1=="hostname"{h=$2} $1=="user"{u=$2} $1=="port"{p=$2}
+      END{if(h==""){exit 1}; s=u"_"h"_"p; gsub(/[^A-Za-z0-9_.-]/,"_",s); print s}')
+    [ -n "$key" ] || exit 0
+    marker="$stateDir/$key"
+    if [ -f "$marker" ] && mutagen sync list "$(cat "$marker")" >/dev/null 2>&1; then
+      exit 0
+    fi
+    if mutagen sync list "$host" >/dev/null 2>&1; then
+      printf '%s' "$host" >"$marker"
+      exit 0
+    fi
+
+    localDir="${home}/Sync/$host"
+    remoteDir="Sync/$(scutil --get LocalHostName 2>/dev/null || hostname -s)"
+
+    # Reachability and the parent directory in one round trip. BatchMode so a host that wants a
+    # password fails here instead of waiting on a prompt that nobody can see.
+    if ! /usr/bin/ssh -o BatchMode=yes -o ConnectTimeout=10 "$host" "mkdir -p '$remoteDir'"; then
+      echo "$(date '+%F %T') skip $host: no unattended login"
+      exit 0
+    fi
+
+    mkdir -p "$localDir"
+    echo "$(date '+%F %T') creating session $host"
+    if mutagen sync create \
+      --name="$host" \
+      --ignore-vcs \
+      --ignore=.DS_Store \
+      "$localDir" "$host:$remoteDir"; then
+      printf '%s' "$host" >"$marker"
+    else
+      # leave no empty ~/Sync/<host> behind to suggest a pairing that does not exist
+      rmdir "$localDir" 2>/dev/null || true
+    fi
+    exit 0
+  '';
 in
 {
-  home.packages = [ pkgs.mutagen ];
+  home.packages = [
+    pkgs.mutagen
+    ensureScript
+  ];
+
+  # Pair a host the first time we ssh into it. Deliberately after the session ends: during it a
+  # second connection would ask the Bitwarden agent to approve again, and on a host carrying port
+  # forwards it would fail outright. The wrapper never touches ssh's own behaviour or exit code.
+  programs.zsh.initContent = lib.mkAfter ''
+    function ssh() {
+      command ssh "$@"
+      local rc=$?
+      (mutagen-sync-ensure "$@" &) >/dev/null 2>&1
+      return $rc
+    }
+  '';
 
   # One agent per peer. The agent only ensures the session exists; the synchronization itself is
   # carried by mutagen's own daemon, which the CLI starts on demand and which stays resident.