disable jobserver on unix, if file descriptors are negative

src/error.rs

@@ -18,6 +18,10 @@ pub enum FromEnvErrorKind {
     /// There is no jobserver in the environment variable.
     /// Variables associated with Make can be used for passing data other than jobserver info.
     NoJobserver,
+    /// The jobserver style is 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,
     /// Cannot parse jobserver environment variable value, incorrect format.
     CannotParse,
     /// Cannot open path or name from the jobserver environment variable value.
@@ -36,6 +40,7 @@ impl FromEnvError {
         match self.inner {
             FromEnvErrorInner::NoEnvVar => FromEnvErrorKind::NoEnvVar,
             FromEnvErrorInner::NoJobserver => FromEnvErrorKind::NoJobserver,
+            FromEnvErrorInner::NegativeFd(..) => FromEnvErrorKind::NegativeFd,
             FromEnvErrorInner::CannotParse(_) => FromEnvErrorKind::CannotParse,
             FromEnvErrorInner::CannotOpenPath(..) => FromEnvErrorKind::CannotOpenPath,
             FromEnvErrorInner::CannotOpenFd(..) => FromEnvErrorKind::CannotOpenFd,
@@ -50,6 +55,7 @@ impl std::fmt::Display for FromEnvError {
         match &self.inner {
             FromEnvErrorInner::NoEnvVar => write!(f, "there is no environment variable that describes jobserver to inherit"),
             FromEnvErrorInner::NoJobserver => write!(f, "there is no `--jobserver-fds=` or `--jobserver-auth=` in the environment variable"),
+            FromEnvErrorInner::NegativeFd(read, write) => write!(f, "either or both passed file descriptors are negative: {read}, {write}"),
             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}"),
@@ -76,6 +82,7 @@ impl std::error::Error for FromEnvError {
 pub(crate) enum FromEnvErrorInner {
     NoEnvVar,
     NoJobserver,
+    NegativeFd(RawFd, RawFd),
     CannotParse(String),
     CannotOpenPath(String, std::io::Error),
     CannotOpenFd(RawFd, std::io::Error),

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::NegativeFd(read, 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