[DRAFT] Guard temporary environment variable changes

aws-lc-sys/builder/cc_builder.rs

@@ -15,9 +15,10 @@ mod x86_64_unknown_linux_gnu;
 mod x86_64_unknown_linux_musl;
 
 use crate::{
-    cargo_env, effective_target, emit_warning, env_var_to_bool, execute_command, get_crate_cflags,
-    is_no_asm, optional_env_optional_crate_target, optional_env_target, out_dir, requested_c_std,
-    set_env_for_target, target, target_arch, target_env, target_os, CStdRequested, OutputLibType,
+    cargo_env, effective_target, emit_warning, env_name_for_target, env_var_to_bool,
+    execute_command, get_crate_cflags, is_no_asm, optional_env_optional_crate_target,
+    optional_env_target, out_dir, requested_c_std, set_env_for_target, target, target_arch,
+    target_env, target_os, CStdRequested, EnvGuard, OutputLibType,
 };
 use std::cell::Cell;
 use std::collections::HashMap;
@@ -373,14 +374,12 @@ impl CcBuilder {
             option.apply_cc(&mut je_builder);
         }
 
-        let compiler = if let Some(original_cflags) = optional_env_target("CFLAGS") {
-            let mut new_cflags = original_cflags.clone();
-            new_cflags.push_str(" -O0");
-            set_env_for_target("CFLAGS", &new_cflags);
+        let compiler = if let Some(mut cflags) = optional_env_target("CFLAGS") {
+            cflags.push_str(" -O0");
+            let _guard_cflags = EnvGuard::new(&env_name_for_target("CFLAGS"), &cflags);
             // cc-rs currently prioritizes flags provided by CFLAGS over the flags provided by the build script.
             // The environment variables used by the compiler are set when `get_compiler` is called.
             let compiler = je_builder.get_compiler();
-            set_env_for_target("CFLAGS", &original_cflags);
             compiler
         } else {
             je_builder.get_compiler()

aws-lc-sys/builder/main.rs

@@ -141,6 +141,12 @@ fn cargo_env<N: AsRef<str>>(name: N) -> String {
     env::var(name).unwrap_or_else(|_| panic!("missing env var {name:?}"))
 }
 
+fn env_name_for_target<K: AsRef<OsStr>>(env_var: K) -> String {
+    let target = target().to_lowercase();
+    let target = target.replace('-', "_");
+    format!("{}_{target}", env_var.as_ref().to_str().unwrap())
+}
+
 // "CFLAGS" =>
 // "AWS_LC_SYS_CFLAGS_aarch64_unknown_linux_gnu" OR "AWS_LC_SYS_CFLAGS"
 //    OR "CFLAGS_aarch64_unknown_linux_gnu" OR "CFLAGS"
@@ -153,16 +159,13 @@ fn optional_env_optional_crate_target<N: AsRef<str>>(name: N) -> Option<String>
 fn optional_env_crate_target<N: AsRef<str>>(name: N) -> Option<String> {
     let name = name.as_ref();
     let crate_name = crate_name().to_uppercase().replace('-', "_");
-    let target_name = target().to_lowercase().replace('-', "_");
     let name_for_crate = format!("{crate_name}_{name}");
-    let name_for_crate_target = format!("{crate_name}_{name}_{target_name}");
+    let name_for_crate_target = env_name_for_target(&name_for_crate);
     optional_env(name_for_crate_target).or(optional_env(name_for_crate))
 }
 
 fn optional_env_target<N: AsRef<str>>(name: N) -> Option<String> {
-    let name = name.as_ref();
-    let target_name = target().to_lowercase().replace('-', "_");
-    let name_for_target = format!("{}_{}", &name, target_name);
+    let name_for_target = env_name_for_target(name.as_ref());
     optional_env(name_for_target).or(optional_env(name))
 }
 
@@ -184,9 +187,7 @@ where
     K: AsRef<OsStr>,
     V: AsRef<OsStr>,
 {
-    let target = target().to_lowercase();
-    let target = target.replace('-', "_");
-    let env_var = format!("{}_{target}", env_var.as_ref().to_str().unwrap());
+    let env_var = env_name_for_target(env_var);
     env::set_var(&env_var, &value);
     emit_warning(&format!(
         "Setting {env_var}: {}",
@@ -912,6 +913,32 @@ const PRELUDE: &str = r"
 )]
 ";
 
+pub(crate) struct EnvGuard {
+    key: String,
+    original_value: Option<String>,
+}
+
+impl EnvGuard {
+    fn new<T: AsRef<OsStr>>(key: &str, new_value: T) -> Self {
+        let original_value = env::var(key).ok();
+        env::set_var(key, new_value);
+        Self {
+            key: key.to_string(),
+            original_value,
+        }
+    }
+}
+
+impl Drop for EnvGuard {
+    fn drop(&mut self) {
+        if let Some(ref value) = self.original_value {
+            env::set_var(&self.key, value);
+        } else {
+            env::remove_var(&self.key);
+        }
+    }
+}
+
 fn invoke_external_bindgen(
     manifest_dir: &Path,
     prefix: &Option<String>,
@@ -924,6 +951,8 @@ fn invoke_external_bindgen(
         effective_target()
     ));
 
+    let _guard_target = EnvGuard::new("TARGET", effective_target());
+
     let options = BindingOptions {
         // We collect the symbols w/o the prefix added
         build_prefix: None,

aws-lc-sys/builder/sys_bindgen.rs

@@ -1,7 +1,9 @@
 // Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0 OR ISC
 
-use crate::{get_rust_include_path, BindingOptions, COPYRIGHT, PRELUDE};
+use crate::{
+    effective_target, get_rust_include_path, BindingOptions, EnvGuard, COPYRIGHT, PRELUDE,
+};
 use bindgen::callbacks::{ItemInfo, ParseCallbacks};
 use std::fmt::Debug;
 use std::path::Path;
@@ -78,6 +80,7 @@ pub(crate) fn generate_bindings(
     manifest_dir: &Path,
     options: &BindingOptions,
 ) -> bindgen::Bindings {
+    let _guard_target = EnvGuard::new("TARGET", effective_target());
     prepare_bindings_builder(manifest_dir, options)
         .generate()
         .expect("Unable to generate bindings.")