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authorsterni <sternenseemann@systemli.org>2021-02-21T11·57+0100
committersterni <sternenseemann@systemli.org>2021-04-01T18·50+0000
commit93a746aaaa092ffc3e7eb37e1df30bfd3a28435f (patch)
tree4197f3d5c7b5e2c4cdcb4be3dd1a17ed3ecc2ad7 /web/bubblegum/examples/blog.nix
parent68f3ac64c4a2ae50dcb125f067692536f647e370 (diff)
feat(web/bubblegum): nix CGI programming framework r/2394
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>
Diffstat (limited to 'web/bubblegum/examples/blog.nix')
-rw-r--r--web/bubblegum/examples/blog.nix134
1 files changed, 134 insertions, 0 deletions
diff --git a/web/bubblegum/examples/blog.nix b/web/bubblegum/examples/blog.nix
new file mode 100644
index 000000000000..f79ab0627e88
--- /dev/null
+++ b/web/bubblegum/examples/blog.nix
@@ -0,0 +1,134 @@
+{ depot, ... }:
+
+let
+  inherit (depot)
+    lib
+    ;
+
+  inherit (depot.users.sterni.nix)
+    url
+    fun
+    string
+    ;
+
+  inherit (depot.web.bubblegum)
+    pathInfo
+    scriptName
+    respond
+    absolutePath
+    ;
+
+  # substituted using substituteAll in default.nix
+  blogdir = "@blogdir@";
+  # blogdir = toString ./posts; # for local testing
+
+  parseDate = post:
+    let
+      matched = builtins.match "/?([0-9]+)-([0-9]+)-([0-9]+)-.+" post;
+    in
+      if matched == null
+      then [ 0 0 0 ]
+      else builtins.map builtins.fromJSON matched;
+
+  parseTitle = post:
+    let
+      matched = builtins.match "/?[0-9]+-[0-9]+-[0-9]+-(.+).html" post;
+    in
+      if matched == null
+      then "no title"
+      else builtins.head matched;
+
+  dateAtLeast = a: b:
+    builtins.all fun.id
+      (lib.zipListsWith (partA: partB: partA >= partB) a b);
+
+  byPostDate = a: b:
+    dateAtLeast (parseDate a) (parseDate b);
+
+  posts = builtins.sort byPostDate
+    (builtins.attrNames
+      (lib.filterAttrs (_: v: v == "regular")
+        (builtins.readDir blogdir)));
+
+  generic = { title, inner, ... }: ''
+    <!doctype html>
+    <html>
+      <head>
+        <meta charset="utf-8">
+        <title>${title}</title>
+        <style>a:link, a:visited { color: blue; }</style>
+      </head>
+      <body>
+      ${inner}
+      </body>
+    </html>
+  '';
+
+  index = posts: ''
+    <main>
+      <h1>blog posts</h1>
+      <ul>
+  '' + lib.concatMapStrings (post: ''
+      <li>
+        <a href="${absolutePath (url.encode {} post)}">${parseTitle post}</a>
+      </li>
+  '') posts + ''
+      </ul>
+    </main>
+  '';
+
+  formatDate =
+    let
+      # Assume we never deal with years < 1000
+      formatDigit = d: string.fit {
+        char = "0"; width = 2;
+      } (toString d);
+    in lib.concatMapStringsSep "-" formatDigit;
+
+  post = title: post: ''
+    <main>
+      <h1>${title}</h1>
+      <div id="content">
+        ${builtins.readFile (blogdir + "/" + post)}
+      </div>
+    </main>
+    <footer>
+      <p>Posted on ${formatDate (parseDate post)}</p>
+      <nav><a href="${scriptName}">index</a></nav>
+    </footer>
+  '';
+
+  validatePathInfo = pathInfo:
+    let
+      chars = string.toChars pathInfo;
+    in builtins.length chars > 1
+      && !(builtins.elem "/" (builtins.tail chars));
+
+  response =
+    if pathInfo == "/"
+    then {
+      title = "blog";
+      status = "OK";
+      inner = index posts;
+    }
+    else if !(validatePathInfo pathInfo)
+    then {
+      title = "Bad Request";
+      status = "Bad Request";
+      inner = "No slashes in post names 😡";
+    }
+    # CGI should already url.decode for us
+    else if builtins.pathExists (blogdir + "/" + pathInfo)
+    then rec {
+      title = parseTitle pathInfo;
+      status = "OK";
+      inner = post title pathInfo;
+    } else {
+      title = "Not Found";
+      status = "Not Found";
+      inner = "<h1>404 — not found</h1>";
+    };
+in
+  respond response.status {
+    "Content-type" = "text/html";
+  } (generic response)