disable jobserver on unix, if file descriptors are negative

src/error.rs

@@ -24,6 +24,10 @@ pub enum FromEnvErrorKind {
     CannotOpenPath,
     /// Cannot open file descriptor from the jobserver environment variable value.
     CannotOpenFd,
+    /// The jobserver style is a simple pipe, but at least one of the file descriptors
+    /// is negative, which means it is disabled for this process
+    /// ([GNU `make` manual: POSIX Jobserver Interaction](https://www.gnu.org/software/make/manual/make.html#POSIX-Jobserver)).
+    NegativeFd,
     /// File descriptor from the jobserver environment variable value is not a pipe.
     NotAPipe,
     /// Jobserver inheritance is not supported on this platform.
@@ -39,6 +43,7 @@ impl FromEnvError {
             FromEnvErrorInner::CannotParse(_) => FromEnvErrorKind::CannotParse,
             FromEnvErrorInner::CannotOpenPath(..) => FromEnvErrorKind::CannotOpenPath,
             FromEnvErrorInner::CannotOpenFd(..) => FromEnvErrorKind::CannotOpenFd,
+            FromEnvErrorInner::NegativeFd(..) => FromEnvErrorKind::NegativeFd,
             FromEnvErrorInner::NotAPipe(..) => FromEnvErrorKind::NotAPipe,
             FromEnvErrorInner::Unsupported => FromEnvErrorKind::Unsupported,
         }
@@ -53,6 +58,7 @@ impl std::fmt::Display for FromEnvError {
             FromEnvErrorInner::CannotParse(s) => write!(f, "cannot parse jobserver environment variable value: {s}"),
             FromEnvErrorInner::CannotOpenPath(s, err) => write!(f, "cannot open path or name {s} from the jobserver environment variable value: {err}"),
             FromEnvErrorInner::CannotOpenFd(fd, err) => write!(f, "cannot open file descriptor {fd} from the jobserver environment variable value: {err}"),
+            FromEnvErrorInner::NegativeFd(fd) => write!(f, "file descriptor {fd} from the jobserver environment variable value is negative"),
             FromEnvErrorInner::NotAPipe(fd, None) => write!(f, "file descriptor {fd} from the jobserver environment variable value is not a pipe"),
             FromEnvErrorInner::NotAPipe(fd, Some(err)) => write!(f, "file descriptor {fd} from the jobserver environment variable value is not a pipe: {err}"),
             FromEnvErrorInner::Unsupported => write!(f, "jobserver inheritance is not supported on this platform"),
@@ -79,6 +85,7 @@ pub(crate) enum FromEnvErrorInner {
     CannotParse(String),
     CannotOpenPath(String, std::io::Error),
     CannotOpenFd(RawFd, std::io::Error),
+    NegativeFd(RawFd),
     NotAPipe(RawFd, Option<std::io::Error>),
     Unsupported,
 }

src/unix.rs

@@ -130,6 +130,15 @@ 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 {
+            return Err(FromEnvErrorInner::NegativeFd(read));
+        }
+        if write < 0 {
+            return Err(FromEnvErrorInner::NegativeFd(write));
+        }
+
         // 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