feat(web/bubblegum): nix CGI programming framework

So here is what has been keeping me up at night: At some point I
realized that nix actually made a somewhat passable language for CGI
programming:

* That `builtins.getEnv` exists as one of the impurities of Nix is
  perfect as environment variables are the main way of communication
  from the web server to the CGI application.

* We can actually read from the filesystem via builtins.readDir and
  builtins.readFile with bearable overhead if we avoid importing the
  used paths into the nix store.

* Templating and routing are convenient to implement via indented strings
  and attribute sets respectively.

Of course there are obvious limitation:

* The overhead of derivations is probably much to great for them to be
  useful via IfD.

* Even without derivations, nix evaluation is very slow to the point
  were a trivial application takes between 100ms and 400ms to produce a
  response.

* We can't really cause effects other than producing a response which
  makes it not viable for a lot of applications. There are some ways
  around this:

  * With a custom interpreter we could have streaming and multiplexed
    I/O (using lazy lists emulated via attrsets) to cause such effects,
    but it would probably perform terribly.

  * We can use builtins.fetchurl to call other HTTP-based microservices,
    but only in very limited constraints, i. e. only GET, no headers,
    and only if the tarball ttl is set to 0 in the global nix.conf.

* Terrible error handling capabilities because builtins.tryEval actually
  doesn't catch a lot of errors.

To prove that it actually works, there are some demo applications,
which I invite you to run and potentially break horribly:

    nix-build -A web.bubblegum.examples && ./result
    # navigate to http://localhost:9000

The setup uses thttpd and executes the nix CGI scripts using
users.sterni.nint which automatically passed `depot`, so they can
import the cgi library.

Change-Id: I3a22a749612211627e5f8301c31ec2e7a872812c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2746
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

web/bubblegum/examples/default.nix

@@ -0,0 +1,61 @@
+{ depot, pkgs, lib, ... }:
+
+let
+
+  scripts = [
+    ./hello.nix
+    ./derivation-svg.nix
+    (substituteAll {
+      src = ./blog.nix;
+      # by making this a plain string this
+      # can be something outside the nix store!
+      blogdir = ./posts;
+    })
+  ];
+
+  inherit (depot.nix)
+    writeExecline
+    runExecline
+    getBins
+    ;
+
+  inherit (depot.web.bubblegum)
+    writeCGI
+    ;
+
+  inherit (pkgs)
+    runCommandLocal
+    substituteAll
+    ;
+
+  bins = (getBins pkgs.thttpd [ "thttpd" ])
+      // (getBins pkgs.coreutils [ "printf" "cp" "mkdir" ]);
+
+  webRoot =
+    let
+      copyScripts = lib.concatMap
+        (path: let
+          cgi = writeCGI {
+            # assume we are on NixOS since thttpd doesn't set PATH.
+            # using third_party.nix is tricky because not everyone
+            # has a tvix daemon running.
+            binPath = "/run/current-system/sw/bin";
+          } path;
+        in [
+          "if" [ bins.cp cgi "\${out}/${cgi.name}" ]
+        ]) scripts;
+    in runExecline.local "webroot" {} ([
+      "importas" "out" "out"
+      "if" [ bins.mkdir "-p" "$out" ]
+    ] ++ copyScripts);
+
+  port = 9000;
+
+in
+  writeExecline "serve-examples" {} [
+    "foreground" [
+      bins.printf "%s\n" "Running on http://localhost:${toString port}"
+    ]
+    "${bins.thttpd}" "-D" "-p" (toString port) "-l" "/dev/stderr"
+                     "-c" "*.nix" "-d" webRoot
+  ]