Merge #1185

1185: Provide clearenv() r=asomers a=jgallagher

Maybe closes #481.

"Maybe" for a couple reasons that are discussed on that issue:

* The issue mentions adding the other environment-variable-related functions (`setenv`, `getenv`, etc.), but I omitted those because they're already covered by `std::env` AFAICT, plus they're wildly unthreadsafe. It looks like `std::env` avoids the thread unsafety via a library-level lock and a warning to be careful when using ffi functions that might modify the environment concurrently.
* I opted to return `Error::UnsupportedOperation` in the (impossible?) case of `libc::clearenv()` returning non-zero instead of having the function return a `Result<(), ()>`. The latter seems like it would be onerous (since every other possibly-failing function in nix returns a `nix::Result`), but if you'd prefer a different error return I'm not going to put up any argument.

Co-authored-by: John Gallagher <jgallagher@bignerdranch.com>

CHANGELOG.md

@@ -5,6 +5,10 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased] - ReleaseDate
 ### Added
+- Added `env::clearenv()`: calls `libc::clearenv` on platforms
+  where it's available, and clears the environment of all variables
+  via `std::env::vars` and `std::env::remove_var` on others.
+  (#[1185](https://github.com/nix-rust/nix/pull/1185))
 ### Changed
 ### Fixed
 ### Removed

Cargo.toml

@@ -18,7 +18,7 @@ exclude     = [
 [dependencies]
 libc = { version = "0.2.60", features = [ "extra_traits" ] }
 bitflags = "1.1"
-cfg-if = "0.1.2"
+cfg-if = "0.1.10"
 void = "1.0.2"
 
 [target.'cfg(target_os = "dragonfly")'.build-dependencies]
@@ -44,6 +44,10 @@ path = "test/test.rs"
 name = "test-aio-drop"
 path = "test/sys/test_aio_drop.rs"
 
+[[test]]
+name = "test-clearenv"
+path = "test/test_clearenv.rs"
+
 [[test]]
 name = "test-lio-listio-resubmit"
 path = "test/sys/test_lio_listio_resubmit.rs"

src/env.rs

@@ -0,0 +1,52 @@
+use {Error, Result};
+
+/// Clear the environment of all name-value pairs.
+///
+/// On platforms where libc provides `clearenv()`, it will be used. libc's
+/// `clearenv()` is documented to return an error code but not set errno; if the
+/// return value indicates a failure, this function will return
+/// `Error::UnsupportedOperation`.
+///
+/// On platforms where libc does not provide `clearenv()`, a fallback
+/// implementation will be used that iterates over all environment variables and
+/// removes them one-by-one.
+///
+/// # Safety
+///
+/// This function is not threadsafe and can cause undefined behavior in
+/// combination with `std::env` or other program components that access the
+/// environment. See, for example, the discussion on `std::env::remove_var`; this
+/// function is a case of an "inherently unsafe non-threadsafe API" dealing with
+/// the environment.
+///
+///  The caller must ensure no other threads access the process environment while
+///  this function executes and that no raw pointers to an element of libc's
+///  `environ` is currently held. The latter is not an issue if the only other
+///  environment access in the program is via `std::env`, but the requirement on
+///  thread safety must still be upheld.
+pub unsafe fn clearenv() -> Result<()> {
+    let ret;
+    cfg_if! {
+        if #[cfg(any(target_os = "fuchsia",
+                     target_os = "wasi",
+                     target_env = "wasi",
+                     target_env = "uclibc",
+                     target_os = "linux",
+                     target_os = "android",
+                     target_os = "emscripten"))] {
+            ret = libc::clearenv();
+        } else {
+            use std::env;
+            for (name, _) in env::vars_os() {
+                env::remove_var(name);
+            }
+            ret = 0;
+        }
+    }
+
+    if ret == 0 {
+        Ok(())
+    } else {
+        Err(Error::UnsupportedOperation)
+    }
+}

src/lib.rs

@@ -30,6 +30,7 @@ pub extern crate libc;
 
 // Public crates
 pub mod dir;
+pub mod env;
 pub mod errno;
 #[deny(missing_docs)]
 pub mod features;

test/test_clearenv.rs

@@ -0,0 +1,11 @@
+extern crate nix;
+
+use std::env;
+
+#[test]
+fn clearenv() {
+    env::set_var("FOO", "BAR");
+    unsafe { nix::env::clearenv() }.unwrap();
+    assert_eq!(env::var("FOO").unwrap_err(), env::VarError::NotPresent);
+    assert_eq!(env::vars().count(), 0);
+}