chore(nint): move from //users/sterni to //nix r/2833

Since //web/bubblegum depends on nint, we need to move it to a non user
directory to conform with the policy established via cl/3434.
Note that this likely doesn't mean greater stability (which isn't
really implied in depot anyways), since I still would like to use a more
elaborate calling convention to allow for additional useful features.

Change-Id: I616f905d8df13e3363674aab69a797b0d39fdd79
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3506
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

1 files changed, 85 insertions, 0 deletions

diff --git a/nix/nint/README.md b/nix/nint/README.md
new file mode 100644
index 0000000000..ddd8045f73
--- /dev/null
+++ b/nix/nint/README.md
@@ -0,0 +1,85 @@
+# nint — Nix INTerpreter
+
+`nint` is a shebang compatible interpreter for nix. It is currently
+implemented as a fairly trivial wrapper around `nix-instantiate --eval`.
+It allows to run nix expressions as command line tools if they conform
+to the following calling convention:
+
+* Every nix script needs to evaluate to a function which takes an
+  attribute set as its single argument. Ideally a set pattern with
+  an ellipsis should be used. By default `nint` passes the following
+  arguments:
+
+  * `currentDir`: the current working directory as a nix path
+  * `argv`: a list of arguments to the invokation including the
+    program name at `builtins.head argv`.
+  * Extra arguments can be manually passed as described below.
+
+* The return value should always be a string (throwing is also okay)
+  which is printed to stdout by `nint`.
+
+## Usage
+
+```
+nint [ --arg ARG VALUE … ] script.nix [ ARGS … ]
+```
+
+Instead of `--arg`, `--argstr` can also be used. They both work
+like the flags of the same name for `nix-instantiate` and may
+be specified any number of times as long as they are passed
+*before* the nix expression to run.
+
+Below is a shebang which also passes `depot` as an argument
+(note the usage of `env -S` to get around the shebang limitation
+to two arguments).
+
+```nix
+#!/usr/bin/env -S nint --arg depot /path/to/depot
+```
+
+## Limitations
+
+* No side effects except for writing to `stdout`.
+
+* Output is not streaming, i. e. even if the output is incrementally
+  calculated, nothing will be printed until the full output is available.
+  With plain nix strings we can't do better anyways.
+
+* Limited error handling for the script, no way to set the exit code etc.
+
+Some of these limitations may be possible to address in the future by using
+an alternative nix interpreter and a more elaborate calling convention.
+
+## Example
+
+Below is a (very simple) implementation of a `ls(1)`-like program in nix:
+
+```nix
+#!/usr/bin/env nint
+{ currentDir, argv, ... }:
+
+let
+  lib = import <nixpkgs/lib>;
+
+  dirs =
+    let
+      args = builtins.tail argv;
+    in
+      if args == []
+      then [ currentDir ]
+      else args;
+
+  makeAbsolute = p:
+    if builtins.isPath p
+    then p
+    else if builtins.match "^/.*" p != null
+    then p
+    else "${toString currentDir}/${p}";
+in
+
+  lib.concatStringsSep "\n"
+    (lib.flatten
+      (builtins.map
+        (d: (builtins.attrNames (builtins.readDir (makeAbsolute d))))
+        dirs)) + "\n"
+```