SERVER-116038 Investigate adding WASI SDK to be used in Bazel (#47863)

GitOrigin-RevId: aad4d9d1777e97293bfb52f064aadfa3145cfe8b

bazel/docs/toolchain.md

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+# About
+
+This documents some useful tools, concepts, and debugging strategies for bazel toolchains.
+This information was gathered while developing the WASI SDK toolchain.
+
+# Concepts
+
+[Toolchain](https://bazel.build/extending/toolchains#debugging-toolchains) and [Platform](https://bazel.build/extending/platforms) are the core relevant concepts.
+Toolchains define the tools used to compile, and the platform defines either the execution platform (for the compilation/compiler tools) and target platform (for the binary).
+Bazel tries to search for a toolchain based on these constraints.
+
+We also made use of [transitions](https://bazel.build/rules/lib/builtins/transition) which allow bazel to reconfigure itself before building a target to avoid passing irrelevant or incorrect compiler flags (e.g. WASI SDK doesn't support shared objects).
+Similarly, we used [actions](https://bazel.build/docs/cc-toolchain-config-reference#using-action-config) instead of the tool paths attribute because of, [possibly historical, lack of support for remote resources in tool paths](https://stackoverflow.com/questions/73504780/bazel-reference-binaries-from-packages-in-custom-toolchain-definition/73505313#73505313).
+
+# Debugging tools
+
+## Toolchain Debugging
+
+```bash
+bazel ... --toolchain_resolution_debug=.* ...
+```
+
+The above flag can be used to debug toolchain resolution as bazel tries to automatically satisfy constraints.
+
+## Debugging Remote Resources
+
+Toolchains may be remotely fetched, but the directory structure of the build environment after these remote resources are fetched may not be clear.
+`bazel info` can be used to find the bazel directory and inspect it `bazel info output_base`.
+Note: this may be different depending on your configuration and level of sandboxing.
+
+This is particularly useful when used in combination with the `find` command as shown below.
+
+```bash
+find $(bazel info output_base) -name THING
+```
+
+Note: this command is directory dependent because output_base is per bazel instance.
+
+## Debugging Bazel Compilation Actions
+
+```bash
+bazel ... -s ...
+```
+
+This will show verbose output such as cd actions and compiler/linker invocations.
+Note: bazel may recast paths relative to the exec directory.
+
+## Debugging on Engflow
+
+Engflow has a lot of helpful views showing remote execution stats and the remote file structure.
+We don't intent to duplicate their documentation but be careful as some of their data (particularly remotely executed actions) may not be accurate immediately after execution.

bazel/toolchains/cc/mongo_wasm/README.md

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+# Usage
+
+To use the WASI SDK apply the `wasi_compatible` with a select statement:
+
+```python
+    select({
+        "//bazel/toolchains/cc/wasm:wasi_compatible": [
+            "//bazel/toolchains/cc/wasm/sample:dist_hello_world",
+        ],
+        "//conditions:default": [],
+    })
+```
+
+If your target is defined in terms of a traditional bazel C/C++ target you can use the WASI transition in order to ensure the bazel options are WASI compatible.
+
+```python
+load("//bazel/toolchains/cc/wasm/toolchain:with_wasi_config.bzl", "with_wasi_config")
+with_wasi_config(
+    name = "dist_hello_world",
+    srcs = [
+        ":hello_world",
+    ],
+)
+```