bootstrap: respect modern jobserver

When bootstrapping Rust, the `-j N` flag was passed to CMake, which was
then forwarded to Ninja. This prevents the jobserver from being used,
and as a result leads to oversubscription when Rust is just one of the
many packages built as part of a larger software stack.

Since Cargo and the Rust compiler have long supported the jobserver, it
would be good if also bootstrapping Rust itself would participate in the
protocol, leading to composable parallelism.

This change allows bootstrapping to respect an existing FIFO based
jobserver. Old pipe based jobservers are not supported, because they are
brittle: currently the Python scripts in bootstrap do not inherit the
file descriptors, but do pass on `MAKEFLAGS`. Because Ninja only
supports FIFO based jobservers, it's better to focus on new jobservers
only.

In summary:

* Bootstrap Cargo passes `MAKEFLAGS` verbatim to subprocesses if it
  advertises a FIFO style jobserver, otherwise it unsets it.
* `llvm.rs` does not pass `-j` to `cmake` when a FIFO style jobserver is
  set in `MAKEFLAGS.
* Bootstrap Cargo no longer unsets `MKFLAGS`: from git blame, GNU Make
  considered it a historical artifact back in 1992, and it is never read
  by GNU Make, it's only set for backwards compatibility.

Signed-off-by: Harmen Stoppels <me@harmenstoppels.nl>

Author: haampie

src/bootstrap/src/core/build_steps/llvm.rs

@@ -773,7 +773,15 @@ fn configure_cmake(
         .define("CMAKE_CXX_COMPILER", sanitize_cc(&cxx))
         .define("CMAKE_ASM_COMPILER", sanitize_cc(&cc));
 
-    cfg.build_arg("-j").build_arg(builder.jobs().to_string());
+    // If we are running under a FIFO jobserver, we should not pass -j to CMake; otherwise it
+    // overrides the jobserver settings and can lead to oversubscription.
+    let has_modern_jobserver = env::var("MAKEFLAGS")
+        .map(|flags| flags.contains("--jobserver-auth=fifo:"))
+        .unwrap_or(false);
+
+    if !has_modern_jobserver {
+        cfg.build_arg("-j").build_arg(builder.jobs().to_string());
+    }
     let mut cflags = ccflags.cflags.clone();
     // FIXME(madsmtm): Allow `cmake-rs` to select flags by itself by passing
     // our flags via `.cflag`/`.cxxflag` instead.

src/bootstrap/src/core/builder/cargo.rs

@@ -547,9 +547,17 @@ impl Builder<'_> {
             assert_eq!(target, compiler.host);
         }
 
-        // Remove make-related flags to ensure Cargo can correctly set things up
-        cargo.env_remove("MAKEFLAGS");
-        cargo.env_remove("MFLAGS");
+        // Bootstrap only supports modern FIFO jobservers. Older pipe-based jobservers can run into
+        // "invalid file descriptor" errors, as the jobserver file descriptors are not inherited by
+        // scripts like bootstrap.py, while the environment variable is propagated. So, we pass
+        // MAKEFLAGS only if we detect a FIFO jobserver, otherwise we clear it.
+        let has_modern_jobserver = env::var("MAKEFLAGS")
+            .map(|flags| flags.contains("--jobserver-auth=fifo:"))
+            .unwrap_or(false);
+
+        if !has_modern_jobserver {
+            cargo.env_remove("MAKEFLAGS");
+        }
 
         cargo
     }