fix: unsound use of env::set_var, was breaking stdlib change to make unsafe

It is generally not safe to set env variables. The correct way to set a config
value that needs to be overridden is to hold a copy internal to the library and
only read from the environment.

Author: sftse

tokenizers/src/utils/parallelism.rs

@@ -6,6 +6,7 @@ use rayon::iter::IterBridge;
 use rayon::prelude::*;
 use rayon_cond::CondIterator;
 use std::sync::atomic::AtomicBool;
+use std::sync::atomic::AtomicU8;
 use std::sync::atomic::Ordering;
 
 // Re-export rayon current_num_threads
@@ -14,35 +15,54 @@ pub use rayon::current_num_threads;
 pub const ENV_VARIABLE: &str = "TOKENIZERS_PARALLELISM";
 
 static USED_PARALLELISM: AtomicBool = AtomicBool::new(false);
+static PARALLELISM: AtomicU8 = AtomicU8::new(0);
 
 /// Check if the TOKENIZERS_PARALLELISM env variable has been explicitly set
 pub fn is_parallelism_configured() -> bool {
-    std::env::var(ENV_VARIABLE).is_ok()
+    std::env::var(ENV_VARIABLE).is_ok() || get_override_parallelism().is_some()
 }
 
 /// Check if at some point we used a parallel iterator
 pub fn has_parallelism_been_used() -> bool {
     USED_PARALLELISM.load(Ordering::SeqCst)
 }
 
+/// Get internally set parallelism
+fn get_override_parallelism() -> Option<bool> {
+    match PARALLELISM.load(Ordering::SeqCst) {
+        0 => None,
+        1 => Some(false),
+        2 => Some(true),
+        _ => unreachable!(),
+    }
+}
+
 /// Get the currently set value for `TOKENIZERS_PARALLELISM` env variable
-pub fn get_parallelism() -> bool {
+fn get_env_parallelism() -> bool {
     match std::env::var(ENV_VARIABLE) {
         Ok(mut v) => {
             v.make_ascii_lowercase();
             !matches!(v.as_ref(), "" | "off" | "false" | "f" | "no" | "n" | "0")
         }
-        #[cfg(not(miri))]
         Err(_) => true, // If we couldn't get the variable, we use the default
-        // FIXME: for now turn parallelism off under miri, otherwise complains about crossbeam-epoch
-        #[cfg(miri)]
-        Err(_) => false,
+    }
+}
+
+pub fn get_parallelism() -> bool {
+    // FIXME: for now turn parallelism off under miri, otherwise complains about crossbeam-epoch
+    #[cfg(miri)]
+    return false;
+    #[cfg(not(miri))]
+    if let Some(parallel) = get_override_parallelism() {
+        parallel
+    } else {
+        get_env_parallelism()
     }
 }
 
 /// Set the value for `TOKENIZERS_PARALLELISM` for the current process
 pub fn set_parallelism(val: bool) {
-    std::env::set_var(ENV_VARIABLE, if val { "true" } else { "false" })
+    PARALLELISM.store(if val { 2 } else { 1 }, Ordering::SeqCst);
 }
 
 /// Allows to convert into an iterator that can be executed either parallelly or serially.