local=True marks a repository rule to unconditionally re-execute anytime
bazel re-evaluates the workspace (almost every invocation) It works well
for processes that execute in <10ms, not large ones like the embuilder
cache generation.
I have tested this locally and it seems to work fine, but I'll be honest
that I don't understand the nuances of repository rules that well, nor
do I know much of anything about emscripten or the circumstances in
which the cache ought to be rebuilt.
I checked the `external/@emscripten_cache.marker` file that Bazel uses
to store the cache key for if it needs to re-evaluate, and changing any
of the options passed into register_emscripten_toolchains resulted in
the cache key changing, and it correctly resulted in the cache being
regenerated.
The big question I don't have the context to answer: are there files on
disk other than these two that need to be watched for changes, so the
repository rule should change if they change:
```
FILE:@@emscripten_bin_linux//BUILD.bazel
FILE:@@emsdk//emscripten_toolchain/default_config
```
This is a working solution for generating a separate Emscripten cache.
Note that this requires an additional entry in the workspace as follows:
```starlark
load("@emsdk//:emscripten_cache.bzl", emsdk_emscripten_cache = "emscripten_cache")
emsdk_emscripten_cache()
```
When used like this, the default Emscripten cache will be used. However,
if the entry is as follows:
```starlark
load("@emsdk//:emscripten_cache.bzl", emsdk_emscripten_cache = "emscripten_cache")
emsdk_emscripten_cache(flags = ["--lto"])
```
Then embuilder will be called to build all system libraries and ports
(i.e., the `ALL` option to embuilder) with the LTO option enabled. This
can take awhile, so I have also made possible to specify which libraries
you want to build explicitly:
```starlark
load("@emsdk//:emscripten_cache.bzl", emsdk_emscripten_cache = "emscripten_cache")
emsdk_emscripten_cache(
flags = ["--lto"],
libraries = [
"crtbegin",
"libprintf_long_double-debug",
"libstubs-debug",
"libnoexit",
"libc-debug",
"libdlmalloc",
"libcompiler_rt",
"libc++-noexcept",
"libc++abi-debug-noexcept",
"libsockets"
]
)
```
Resolves#807, resolves#971, resolves#1099, resolves#1362, resolves
#1401
