feat(nix/buildManPages): infra for generating man page dirs from nix

Very simple builder which builds (optionally) gzipped man pages from a
list of attrsets and links them into a common man directory with the
correct layout, so it should be installable immediately.

Additionally runs mandoc -T lint, but by default only for informational
purposes as it is very strict and some things are almost never true (for
example all Xrs being present in the respective directory).

buildManPages.single exposes the internal builder for a single,
optionally gzipped man page from a nix attrset.

Change-Id: I43fce011716f4a7cc80521f222800ca99ba54060
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2654
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

nix/buildManPages/OWNERS

@@ -0,0 +1,3 @@
+inherited: true
+owners:
+  - sterni

nix/buildManPages/default.nix

@@ -0,0 +1,77 @@
+{ depot, ... }:
+
+let
+  inherit (depot.third_party)
+    lib
+    gzip
+    mandoc
+    coreutils
+    ;
+
+  inherit (depot.nix)
+    runExecline
+    getBins
+    ;
+
+  bins = getBins mandoc [ "mandoc" ]
+      // getBins gzip   [ "gzip" ]
+      // getBins coreutils [ "mkdir" "ln" "cp" ]
+      ;
+
+  defaultGzip = true;
+
+  basename = gzip: { name, section, ... }:
+    "${name}.${toString section}${lib.optionalString gzip ".gz"}";
+
+  manDir = { section, ... }:
+    "\${out}/share/man/man${toString section}";
+
+  target = gzip: args:
+    "${manDir args}/${basename gzip args}";
+
+  buildManPage =
+    { requireLint ? false
+    , gzip ? defaultGzip
+    , ...
+    }:
+    { content
+    , ...
+    }@page: let
+      source = builtins.toFile (basename false page) content;
+    in runExecline (basename gzip page) {} ([
+      (if requireLint then "if" else "foreground") [
+        bins.mandoc "-mdoc" "-T" "lint" source
+      ]
+      "importas" "out" "out"
+    ] ++ (if gzip then [
+      "redirfd" "-w" "1" "$out"
+      bins.gzip "-c" source
+    ] else [
+      bins.cp "--reflink=auto" source "$out"
+    ]));
+
+  buildManPages =
+    name:
+    { derivationArgs ? {}
+    , gzip ? defaultGzip
+    , ...
+    }@args:
+    pages:
+    runExecline "${name}-man-pages" {
+      inherit derivationArgs;
+    } ([
+      "importas" "out" "out"
+    ] ++ lib.concatMap ({ name, section, content }@page: [
+      "if" [ bins.mkdir "-p" (manDir page) ]
+      "if" [
+        bins.ln "-s"
+        (buildManPage args page)
+        (target gzip page)
+      ]
+    ]) pages);
+
+in {
+  __functor = _: buildManPages;
+
+  single = buildManPage;
+}