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>
This commit is contained in:
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134
web/bubblegum/examples/blog.nix
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134
web/bubblegum/examples/blog.nix
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{ depot, ... }:
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let
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inherit (depot)
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lib
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;
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inherit (depot.users.sterni.nix)
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url
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fun
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string
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;
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inherit (depot.web.bubblegum)
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pathInfo
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scriptName
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respond
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absolutePath
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;
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# substituted using substituteAll in default.nix
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blogdir = "@blogdir@";
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# blogdir = toString ./posts; # for local testing
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parseDate = post:
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let
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matched = builtins.match "/?([0-9]+)-([0-9]+)-([0-9]+)-.+" post;
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in
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if matched == null
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then [ 0 0 0 ]
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else builtins.map builtins.fromJSON matched;
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parseTitle = post:
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let
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matched = builtins.match "/?[0-9]+-[0-9]+-[0-9]+-(.+).html" post;
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in
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if matched == null
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then "no title"
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else builtins.head matched;
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dateAtLeast = a: b:
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builtins.all fun.id
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(lib.zipListsWith (partA: partB: partA >= partB) a b);
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byPostDate = a: b:
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dateAtLeast (parseDate a) (parseDate b);
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posts = builtins.sort byPostDate
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(builtins.attrNames
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(lib.filterAttrs (_: v: v == "regular")
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(builtins.readDir blogdir)));
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generic = { title, inner, ... }: ''
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<!doctype html>
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<html>
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<head>
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<meta charset="utf-8">
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<title>${title}</title>
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<style>a:link, a:visited { color: blue; }</style>
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</head>
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<body>
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${inner}
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</body>
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</html>
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'';
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index = posts: ''
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<main>
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<h1>blog posts</h1>
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<ul>
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'' + lib.concatMapStrings (post: ''
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<li>
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<a href="${absolutePath (url.encode {} post)}">${parseTitle post}</a>
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</li>
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'') posts + ''
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</ul>
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</main>
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'';
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formatDate =
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let
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# Assume we never deal with years < 1000
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formatDigit = d: string.fit {
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char = "0"; width = 2;
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} (toString d);
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in lib.concatMapStringsSep "-" formatDigit;
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post = title: post: ''
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<main>
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<h1>${title}</h1>
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<div id="content">
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${builtins.readFile (blogdir + "/" + post)}
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</div>
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</main>
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<footer>
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<p>Posted on ${formatDate (parseDate post)}</p>
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<nav><a href="${scriptName}">index</a></nav>
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</footer>
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'';
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validatePathInfo = pathInfo:
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let
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chars = string.toChars pathInfo;
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in builtins.length chars > 1
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&& !(builtins.elem "/" (builtins.tail chars));
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response =
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if pathInfo == "/"
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then {
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title = "blog";
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status = "OK";
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inner = index posts;
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}
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else if !(validatePathInfo pathInfo)
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then {
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title = "Bad Request";
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status = "Bad Request";
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inner = "No slashes in post names 😡";
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}
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# CGI should already url.decode for us
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else if builtins.pathExists (blogdir + "/" + pathInfo)
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then rec {
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title = parseTitle pathInfo;
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status = "OK";
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inner = post title pathInfo;
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} else {
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title = "Not Found";
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status = "Not Found";
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inner = "<h1>404 — not found</h1>";
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};
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in
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respond response.status {
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"Content-type" = "text/html";
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} (generic response)
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