allow bypassing the build directory lock

As bootstrap locks its entire build directory, parallel bootstrapping
for anything becomes impossible. This change enables developers to bypass
the locking mechanism when it is unnecessary for their specific use case.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

Author: onur-ozkan

src/bootstrap/src/bin/main.rs

@@ -23,35 +23,40 @@ fn main() {
     let mut build_lock;
     #[cfg(all(any(unix, windows), not(target_os = "solaris")))]
     let _build_lock_guard;
-    #[cfg(all(any(unix, windows), not(target_os = "solaris")))]
-    // Display PID of process holding the lock
-    // PID will be stored in a lock file
-    {
-        let path = config.out.join("lock");
-        let pid = match fs::read_to_string(&path) {
-            Ok(contents) => contents,
-            Err(_) => String::new(),
-        };
-
-        build_lock =
-            fd_lock::RwLock::new(t!(fs::OpenOptions::new().write(true).create(true).open(&path)));
-        _build_lock_guard = match build_lock.try_write() {
-            Ok(mut lock) => {
-                t!(lock.write(&process::id().to_string().as_ref()));
-                lock
-            }
-            err => {
-                drop(err);
-                println!("WARNING: build directory locked by process {pid}, waiting for lock");
-                let mut lock = t!(build_lock.write());
-                t!(lock.write(&process::id().to_string().as_ref()));
-                lock
-            }
-        };
-    }
 
-    #[cfg(any(not(any(unix, windows)), target_os = "solaris"))]
-    println!("WARNING: file locking not supported for target, not locking build directory");
+    if !config.bypass_bootstrap_lock {
+        // Display PID of process holding the lock
+        // PID will be stored in a lock file
+        #[cfg(all(any(unix, windows), not(target_os = "solaris")))]
+        {
+            let path = config.out.join("lock");
+            let pid = match fs::read_to_string(&path) {
+                Ok(contents) => contents,
+                Err(_) => String::new(),
+            };
+
+            build_lock = fd_lock::RwLock::new(t!(fs::OpenOptions::new()
+                .write(true)
+                .create(true)
+                .open(&path)));
+            _build_lock_guard = match build_lock.try_write() {
+                Ok(mut lock) => {
+                    t!(lock.write(&process::id().to_string().as_ref()));
+                    lock
+                }
+                err => {
+                    drop(err);
+                    println!("WARNING: build directory locked by process {pid}, waiting for lock");
+                    let mut lock = t!(build_lock.write());
+                    t!(lock.write(&process::id().to_string().as_ref()));
+                    lock
+                }
+            };
+        }
+
+        #[cfg(any(not(any(unix, windows)), target_os = "solaris"))]
+        println!("WARNING: file locking not supported for target, not locking build directory");
+    }
 
     // check_version warnings are not printed during setup
     let changelog_suggestion =

src/bootstrap/src/core/config/config.rs

@@ -117,6 +117,7 @@ impl Display for DebuginfoLevel {
 pub struct Config {
     pub changelog_seen: Option<usize>, // FIXME: Deprecated field. Remove it at 2024.
     pub change_id: Option<usize>,
+    pub bypass_bootstrap_lock: bool,
     pub ccache: Option<String>,
     /// Call Build::ninja() instead of this.
     pub ninja_in_file: bool,
@@ -1057,6 +1058,7 @@ define_config! {
 impl Config {
     pub fn default_opts() -> Config {
         let mut config = Config::default();
+        config.bypass_bootstrap_lock = false;
         config.llvm_optimize = true;
         config.ninja_in_file = true;
         config.llvm_static_stdcpp = false;
@@ -1135,6 +1137,7 @@ impl Config {
         config.llvm_profile_use = flags.llvm_profile_use;
         config.llvm_profile_generate = flags.llvm_profile_generate;
         config.enable_bolt_settings = flags.enable_bolt_settings;
+        config.bypass_bootstrap_lock = flags.bypass_bootstrap_lock;
 
         // Infer the rest of the configuration.
 

src/bootstrap/src/core/config/flags.rs

@@ -133,6 +133,13 @@ pub struct Flags {
     /// whether to use color in cargo and rustc output
     pub color: Color,
 
+    #[arg(global(true), long)]
+    /// Bootstrap uses this value to decide whether it should bypass locking the build process.
+    /// This is rarely needed (e.g., compiling the std library for different targets in parallel).
+    ///
+    /// Unless you know exactly what you are doing, you probably don't need this.
+    pub bypass_bootstrap_lock: bool,
+
     /// whether rebuilding llvm should be skipped, overriding `skip-rebuld` in config.toml
     #[arg(global(true), long, value_name = "VALUE")]
     pub llvm_skip_rebuild: Option<bool>,