Add README, LICENSE

.gitignore

+_build/
+_opam/
+*~
+.#*
+\#*#
+.merlin
+opam-custom-install

README.md

+# opam-custom-install - Experimental plugin for custom package installations in opam
+
+This is an attempt to help with workflows where you work on a package source,
+and currently have the choice to:
+
+1. either properly run `opam install .` or other proper pin/install commands
+2. or go brute-force with _e.g._ `make install` or `dune install` to install to
+   the opam prefix
+
+The first option keeps a clean and synchronised opam state, but it can be
+annoying because it will sync your source and recompile everything from scratch
+everytime, something you want to avoid on large projects like the compiler.
+Besides, it's not great for error reporting, so you probably build the package
+by hand already, and with git pinning you need to remember to commit beforehand.
+There are workarounds like `--inplace-build` or `--assume-built`, but they are
+not convenient and don't work very well in practice.
+
+The second option just skips opam entirely, and desynchronises the actual switch
+prefix state from what opam knows about it. It can be OK for simple quick tests,
+but can lead to all sort of problems downstream, with stale package files left
+lying around, confused reinstallation not corresponding to the actual state of
+packages, etc. It also won't allow you to use opam to easily test packages
+depending on the one you are editing.
+
+
+## Installing `opam-custom-install`
+
+The easiest way is to pin it directly, since it's unreleased yet:
+```
+opam pin https://gitlab.ocamlpro.com/louis/opam-custom-install.git
+```
+
+Since it's registered as a plugin, it should make the `opam custom-install`
+command available whatever switch you are in.
+
+If you prefer to compile it manually;
+```
+dune build @install
+ln -sf _build/default/opam-custom-install.exe opam-custom-install
+```
+
+## Usage
+
+```
+opam custom-install [--no-recompilations] PACKAGE[.VERSION] -- COMMAND [ARG]...
+```
+
+Typically, when working on package `foo`:
+```bash
+[.../foo]$ ./configure --prefix=$(opam var prefix)
+...
+[.../foo]$ make all
+...
+[.../foo]$ opam custom-install foo -- make install
+```
+
+Opam will run the supplied command (here `make install`), but with a few extra
+precautions:
+- the configured opam sandbox is used, making sure you install in the correct
+  opam prefix
+- the previously installed version of `foo`, if any, is first removed from the
+  prefix
+- unless you specified `-n`/`--no-recompilations`, any installed package
+  depending on `foo` that was installed is first removed, and reinstalled
+  afterwards
+- opam will track all files installed by the command, and register them as
+  belonging to this package, ensuring correct uninstallation later on
+- the usual locks are taken on the switch to prevent concurrent opam operations
+
+## Current status
+
+This is a proof of concept at this stage, but should be fairly usable. Feel free
+to report any issues or discuss it on this project tracker, ocamllabs.slack.com,
+or by email.
+
+Usage with the `-n`/`--no-recompilations` option may be more stable than usage
+without at this point.
+
+## Next steps
+
+If this seems useful, and once it's proven stable enough, this will be released
+on opam-repository as an optional opam plugin.
+
+If we decide this really provides workflow improvements, it'll be merged as a
+built-in command in opam (it's written as a commandliner subcommand already, so
+that should be just a copy-paste).
+
+Going even beyond that, we might want to deprecate opam flags such as
+`--assume-built`.
+
+## Copyright and license (same as opam)
+
+Copyright 2020 OCamlPro
+
+All rights reserved. this is distributed under the terms of the GNU Lesser
+General Public License version 2.1, with the special exception on linking
+described in the file LICENSE.
+
+opam-custom-install is distributed in the hope that it will be useful, but
+WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for
+more details.

opam-custom-install.ml

+(**************************************************************************)
+(*                                                                        *)
+(*    Copyright 2020 OCamlPro                                             *)
+(*                                                                        *)
+(*  All rights reserved. This file is distributed under the terms of the  *)
+(*  GNU Lesser General Public License version 2.1, with the special       *)
+(*  exception on linking described in the file LICENSE.                   *)
+(*                                                                        *)
+(**************************************************************************)
+
 open Cmdliner
 open OpamTypes
 open OpamPackage.Set.Op