# Emergency patching of OTP modules

The purpose of this directory is to hold patches for OTP modules.\
The patch files are generated using `git diff`, e.g.

## Usage

```
git diff OTP-20.1 -- lib/mnesia/src/mnesia_index.erl > $OTP_PATCHES/20.1/mnesia_index.erl.patch
```

Patch files are stored in version-specific subdirectories. In each such\
subdirectory, a `versions.eterm` file contains patterns identifying which\
OTP application versions are compatible with the patches.

The `versions.eterm` file is read with `file:consult/1`, and shall\
contain `{AppName, BaseNameRegexp}` tuples. For example:

```erlang
{mnesia, "mnesia-4\\.15\\.[123]"}.
```

which matches mnesia versions `"4.15.1"`, `"4.15.2"` and `"4.15.3"`.

The compile script will test each subdirectory, and pick the first one\
for which all application patterns match. It will then copy the relevant\
source files from the current OTP to the `otp_patches` directory, apply\
the patches and compile the result into `otp_patches/ebin`. This directory\
will then be copied into the release and pre-pended to the code path of\
the system.

## rebar3 hook

In the `rebar.config` file, the patch compilation can be specified as\
a `pre_hook`, e.g.

```
{pre_hooks, [{compile, "escript ./otp_patches/compile.escript"},
             ...]}.
```

## Compile options

Currently, files are compiled with `debug_info` and include paths to the`src/` and `include/` directories of the source file's application.\
More compile options, and support for specifying options per-application\
or per-module may have to be added later.


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