chore(develop): prevent multiple develop

shell.nix

@@ -16,7 +16,19 @@ let
   '';
 
   develop = pkgs.writeScriptBin "develop" ''
-    #!${pkgs.zsh}/bin/zsh
+    #!${pkgs.zsh}/bin/zsh -e
+
+    basePath=$(pwd)
+    exec 4<>''${basePath}/.develop.env
+
+    flockRes=
+    set +e
+    ${pkgs.util-linux}/bin/flock -en 4; flockRes=$?
+    set -e
+    if [[ ''${flockRes} -ne 0 ]]; then
+      echo "Could not take exclusive lock; is another develop running?" >&2
+      exit ''${flockRes}
+    fi
 
     cleanup() {
       set +e -x
@@ -32,10 +44,6 @@ let
 
     trap cleanup EXIT
 
-    basePath=$(pwd)
-
-    echo "" > ''${basePath}/.develop.env
-
     export PORT_OFFSET=$(((16#$(whoami | sha256sum | head -c 16)) % 1000))
 
     if [[ -z "$PGHOST" ]]; then
@@ -140,7 +148,7 @@ let
 
     set -xe
 
-    cat >> ''${basePath}/.develop.env <<EOF
+    cat >&4 <<EOF
     PORT_OFFSET=''${PORT_OFFSET}
 
     PGHOST=''${pgSockDir}