Provide missing command suggestions, Ubuntu style

If the user tries to run a program that doesn't exist from Bash, the
program name is looked up in a database that maps to Nix package
names.  If it is found, we print out a message like:

  $ pdflatex
  The program ‘pdflatex’ is currently not installed. It is provided by
  several packages. You can install it by typing one of the following:
    nix-env -i tetex
    nix-env -i texlive-core

If the environment variable $NIX_AUTO_INSTALL is set, the command is
installed and executed automatically:

  $ hello --version
  The program ‘hello’ is currently not installed. It is provided by
  the package ‘hello’, which I will now install for you.
  installing `hello-2.8'
  hello (GNU hello) 2.8
  Copyright (C) 2011 Free Software Foundation, Inc. ...

To use this, you must currently manually put the SQLite programs
database in /var/lib/nixos/programs.sqlite.  In the future, this file
should be provided as part of the NixOS channel so it gets updated
automatically.  To get a test version:

  $ curl http://nixos.org/~eelco/programs.sqlite.xz | xz -d > /var/lib/nixos/programs.sqlite

modules/module-list.nix

@@ -36,6 +36,7 @@
   ./misc/passthru.nix
   ./misc/version.nix
   ./programs/bash/bash.nix
+  ./programs/bash/command-not-found.nix
   ./programs/blcr.nix
   ./programs/info.nix
   ./programs/shadow.nix

modules/programs/bash/bash.nix

@@ -33,7 +33,7 @@ let
 
   options = {
 
-    environment.promptInit =  mkOption {
+    environment.promptInit = mkOption {
       default = ''
         # Provide a nice prompt.
         PROMPT_COLOR="1;31m"

modules/programs/bash/command-not-found.nix

@@ -0,0 +1,48 @@
+# This module provides suggestions of packages to install if the user
+# tries to run a missing command in Bash.  This is implemented using a
+# SQLite database that maps program names to Nix package names (e.g.,
+# "pdflatex" is mapped to "tetex").
+
+{ config, pkgs, ... }:
+
+with pkgs.lib;
+
+let
+
+  commandNotFound = pkgs.substituteAll {
+    name = "command-not-found";
+    dir = "bin";
+    src = ./command-not-found.pl;
+    isExecutable = true;
+    inherit (pkgs) perl;
+    perlFlags = concatStrings (map (path: "-I ${path}/lib/perl5/site_perl ")
+      [ pkgs.perlPackages.DBI pkgs.perlPackages.DBDSQLite ]);
+  };
+
+in
+
+{
+
+  environment.interactiveShellInit =
+    ''
+      # This function is called whenever a command is not found.
+      command_not_found_handle() {
+        local p=/run/current-system/sw/bin/command-not-found
+        if [ -x $p -a -f /var/lib/nixos/programs.sqlite ]; then
+          # Run the helper program.
+          $p "$1"
+          # Retry the command if we just installed it.
+          if [ $? = 126 ]; then
+            "$@"
+          fi
+        else
+          echo "$1: command not found" >&2
+        fi
+      }
+    '';
+
+  environment.systemPackages = [ commandNotFound ];
+
+  # TODO: tab completion for uninstalled commands! :-)
+
+}

modules/programs/bash/command-not-found.pl

@@ -0,0 +1,48 @@
+#! @perl@/bin/perl -w @perlFlags@
+
+use strict;
+use DBI;
+use DBD::SQLite;
+use Config;
+
+my $program = $ARGV[0];
+
+my $dbPath = "/var/lib/nixos/programs.sqlite";
+
+my $dbh = DBI->connect("dbi:SQLite:dbname=$dbPath", "", "")
+    or die "cannot open database `$dbPath'";
+$dbh->{RaiseError} = 0;
+$dbh->{PrintError} = 0;
+
+my $system = $ENV{"NIX_SYSTEM"} // $Config{myarchname};
+
+my $res = $dbh->selectall_arrayref(
+    "select package from Programs where system = ? and name = ?",
+    { Slice => {} }, $system, $program);
+
+if (!defined $res || scalar @$res == 0) {
+    print STDERR "$program: command not found\n";
+} elsif (scalar @$res == 1) {
+    my $package = @$res[0]->{package};
+    if ($ENV{"NIX_AUTO_INSTALL"} // "") {
+        print STDERR <<EOF;
+The program ‘$program’ is currently not installed. It is provided by
+the package ‘$package’, which I will now install for you.
+EOF
+        ;
+        exit 126 if system("nix-env", "-i", $package) == 0;
+    } else {
+        print STDERR <<EOF;
+The program ‘$program’ is currently not installed. You can install it by typing:
+  nix-env -i $package
+EOF
+    }
+} else {
+    print STDERR <<EOF;
+The program ‘$program’ is currently not installed. It is provided by
+several packages. You can install it by typing one of the following:
+EOF
+    print STDERR "  nix-env -i $_->{package}\n" foreach @$res;
+}
+
+exit 127;