Installing bash-completion makes bash startup slow on NixOS

[hedning]

Issue description

Opening up a new issue, as the last one, #22648, is closed.

bash-completion is designed to lazy load completions via __completion_loader. NixOS's generated /etc/bashrc however eagerly sources everything in $NIX_PROFILES/bash-completion/completions/* on startup (bash.nix).

This usually isn't a big problem when bash-completion isn't installed into a profile, since the big bulk of completion scripts bash-completion ships with will be correctly lazy loaded, leaving only a small amount of scripts being sourced on startup.

This however breaks when installing bash-completion as everything will be sourced on startup.

The fix

I believe the fix is simply not sourcing $NIX_PROFILES/bash-completion/completions/* via /etc/bashrc, leaving it fully up to bash-completion lazely load the scripts (it should look in its own nix store path and in ~/.nix-profile/share via $XDG_DATA_DIRS (ref).

Though I might've missed something of course so ccing @peti as he's the author of e627d30 and might know if there's any pitfalls in not sourcing on startup.

Steps to reproduce

• enable programs.bash.enableCompletion
• install bash-completion, either via. systemPackages or nix-env -iA nixos.bash-completion
• start a new interactive bash shell

Technical details

• system: "x86_64-linux"
• host os: Linux 4.9.65, NixOS, 18.03.git.4cbbecc (Impala)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 1.11.15
• channels(hed): ""
• channels(root): "nixos-17.09.1462.07ca7b64d2"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs

[hedning]

Tested the fix out now, and it does seem to work, (completion scripts are being picked up from /run/current-system/sw/share/bash-completion/completions and ~/.nix-profile/share/bash-completion/completions), will make PR.

diff --git a/nixos/modules/programs/bash/bash.nix b/nixos/modules/programs/bash/bash.nix
index ef1acdfe66e..52d3780e045 100644
--- a/nixos/modules/programs/bash/bash.nix
+++ b/nixos/modules/programs/bash/bash.nix
@@ -20,7 +20,7 @@ let
       nullglobStatus=$(shopt -p nullglob)
       shopt -s nullglob
       for p in $NIX_PROFILES; do
-        for m in "$p/etc/bash_completion.d/"* "$p/share/bash-completion/completions/"*; do
+        for m in "$p/etc/bash_completion.d/"*; do
           . $m
         done
       done