feat(users/Profpatsch): add nix-run, nix-run-bin, nix-eval

Change-Id: I76f6fa4866c5792c4b9babe2c3a16401162ad833
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11170
Autosubmit: Profpatsch <mail@profpatsch.de>
Reviewed-by: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI

users/Profpatsch/nix-tools.nix

@@ -0,0 +1,159 @@
+{ depot, pkgs, ... }:
+
+let
+  bins = depot.nix.getBins pkgs.nix [ "nix-build" "nix-instantiate" ];
+
+  # TODO: both of these don’t prevent `result` from being created. good? bad?
+
+  # Usage (execline syntax):
+  #    nix-run { -A foo <more_nix_options> } args...
+  #
+  # Takes an execline block of `nix-build` arguments, which should produce an executable store path.
+  # Then runs the store path with `prog...`.
+  nix-run = depot.nix.writeExecline "nix-run" { argMode = "env"; } [
+    "backtick"
+    "-iE"
+    "storepath"
+    [
+      runblock
+      "1"
+      bins.nix-build
+    ]
+    runblock
+    "-r"
+    "2"
+    "$storepath"
+  ];
+
+  # Usage (execline syntax):
+  #    nix-run-bin { -A foo <more_nix_options> } <foo_bin_name> args...
+  #
+  # Takes an execline block of `nix-build` arguments, which should produce a store path with a bin/ directory in it.
+  # Then runs the given command line with the given arguments. All executables in the built storepath’s bin directory are prepended to `PATH`.
+  nix-run-bin = depot.nix.writeExecline "nix-run-bin" { argMode = "env"; } [
+    "backtick"
+    "-iE"
+    "storepath"
+    [
+      runblock
+      "1"
+      bins.nix-build
+    ]
+    "importas"
+    "-ui"
+    "PATH"
+    "PATH"
+    "export"
+    "PATH"
+    "\${storepath}/bin:\${PATH}"
+    runblock
+    "-r"
+    "2"
+  ];
+
+  nix-eval = depot.nix.writeExecline "nix-eval" { } [
+    bins.nix-instantiate
+    "--read-write-mode"
+    "--eval"
+    "--strict"
+    "$@"
+  ];
+
+  # This is a rewrite of execline’s runblock.
+  # It adds the feature that instead of just
+  # executing the block it reads, it can also
+  # pass it as argv to given commands.
+  #
+  # This is going to be added to a future version
+  # of execline by skarnet, but for now it’s easier
+  # to just dirtily reimplement it in Python.
+  #
+  # TODO: this was added to recent execline versions,
+  # but it doesn’t seem to be a drop-in replacement,
+  # if I use execline’s runblock in nix-run-bin above,
+  # I get errors like
+  # > export: fatal: unable to exec runblock: Success
+  runblock = pkgs.writers.writePython3 "runblock"
+    {
+      flakeIgnore = [ "E501" "E226" ];
+    } ''
+    import sys
+    import os
+    from pathlib import Path
+
+    skip = False
+    one = sys.argv[1]
+    if one == "-r":
+        skip = True
+        block_number = int(sys.argv[2])
+        block_start = 3
+    elif one.startswith("-"):
+        print("runblock-python: only -r supported", file=sys.stderr)
+        sys.exit(100)
+    else:
+        block_number = int(one)
+        block_start = 2
+
+    execline_argv_no = int(os.getenvb(b"#"))
+    runblock_argv = [os.getenv(str(no)) for no in range(1, execline_argv_no + 1)]
+
+
+    def parse_block(args):
+        new_args = []
+        if args == []:
+            print(
+                "runblock-python: empty block",
+                file=sys.stderr
+            )
+            sys.exit(100)
+        for arg in args:
+            if arg == "":
+                break
+            elif arg.startswith(" "):
+                new_args.append(arg[1:])
+            else:
+                print(
+                    "runblock-python: unterminated block: {}".format(args),
+                    file=sys.stderr
+                )
+                sys.exit(100)
+        args_rest = args[len(new_args)+1:]
+        return (new_args, args_rest)
+
+
+    if skip:
+        rest = runblock_argv
+        for _ in range(0, block_number-1):
+            (_, rest) = parse_block(rest)
+        new_argv = rest
+    else:
+        new_argv = []
+        rest = runblock_argv
+        for _ in range(0, block_number):
+            (new_argv, rest) = parse_block(rest)
+
+    given_argv = sys.argv[block_start:]
+    run = given_argv + new_argv
+    if os.path.isabs(run[0]):
+        # TODO: ideally I’d check if it’s an executable here, but it was too hard to figure out and I couldn’t be bothered tbh
+        if not Path(run[0]).is_file():
+            print(
+                "runblock-python: Executable {} does not exist or is not a file.".format(run[0]),
+                file=sys.stderr
+            )
+            sys.exit(100)
+    os.execvp(
+        file=run[0],
+        args=run
+    )
+  '';
+
+
+in
+{
+  inherit
+    nix-run
+    nix-run-bin
+    nix-eval
+    ;
+}