disable jobserver on unix, if file descriptors are negative

src/error.rs

@@ -15,7 +15,7 @@ pub struct FromEnvError {
 pub enum FromEnvErrorKind {
     /// There is no environment variable that describes jobserver to inherit.
     NoEnvVar,
-    /// There is no jobserver in the environment variable.
+    /// There is no jobserver in the environment variable, or it was disabled.
     /// Variables associated with Make can be used for passing data other than jobserver info.
     NoJobserver,
     /// Cannot parse jobserver environment variable value, incorrect format.

src/unix.rs

@@ -130,6 +130,12 @@ impl Client {
             .parse()
             .map_err(|e| FromEnvErrorInner::CannotParse(format!("cannot parse `write` fd: {e}")))?;
 
+        // If either or both of these file descriptors are negative,
+        // it means the jobserver is disabled for this process.
+        if read < 0 || write < 0 {
+            return Err(FromEnvErrorInner::NoJobserver);
+        }
+
         // Ok so we've got two integers that look like file descriptors, but
         // for extra sanity checking let's see if they actually look like
         // valid files and instances of a pipe if feature enabled before we