A means to turn off all platform-specific features in `s2n-quic-platform`

[dongcarl]

The conversation arose out of: https://github.com/aws/s2n-quic/pull/1849

Basically, non-s2n_quic_platform_socket_mmsg and non-s2n_quic_platform_socket_msg (as in, falling back to s2n_quic_platform::message::simple) codepaths were not being compiled, leading to the issue that #1849 fixed.

@camshaft indicated that it would be good to have some way of disabling all platform-specific features to prevent issues like this.

I had patched build.rs to notice an env var, although a cfg var can be used too and is probably more appropriate.

[camshaft]

Thinking about it a bit, I think the best approach is to use a environment variable rather than a cfg, since that would require passing that cfg to all of the dependencies and force a recompilation if changed. For the env var, I think something like S2N_QUIC_PLATFORM_FEATURES_OVERRIDE=msg,pktinfo,gso would be the most flexible for testing weird combinations in CI. If you want to disable all of the features, then you can simply pass an empty env value.

[dongcarl]

Here's a patch, I won't open the PR since I have no idea how the CI works:

diff --git a/quic/s2n-quic-platform/build.rs b/quic/s2n-quic-platform/build.rs
index 0777e6cf..ca2012ce 100644
--- a/quic/s2n-quic-platform/build.rs
+++ b/quic/s2n-quic-platform/build.rs
@@ -1,9 +1,22 @@
 // Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0

-use std::{fs::read_dir, io::Error, path::Path, process::Command};
+use std::{error, fs::read_dir, io, path::Path, process::Command};
+
+fn main() -> Result<(), Box<dyn error::Error>> {
+    const FEATURES_OVERRIDE_ENV_VAR: &str = "S2N_QUIC_PLATFORM_FEATURES_OVERRIDE";
+    println!("cargo:rerun-if-env-changed={}", FEATURES_OVERRIDE_ENV_VAR);
+    match std::env::var(FEATURES_OVERRIDE_ENV_VAR) {
+        Ok(val) => {
+            for feature in val.split(",") {
+                supports(feature);
+            }
+            return Ok(());
+        }
+        Err(std::env::VarError::NotPresent) => {},
+        Err(e) => return Err(e.into()),
+    }

-fn main() -> Result<(), Error> {
     let env = Env::new();

     for feature in read_dir("features")? {
@@ -77,7 +90,7 @@ impl Env {
     }

     // Tries to compile the program and returns if it was successful
-    fn check(&self, path: &Path) -> Result<bool, Error> {
+    fn check(&self, path: &Path) -> Result<bool, io::Error> {
         let mut command = Command::new(&self.rustc);

         command