Bring back "Lock scratch directory during tool execution"

This reverts commit e92df2d and brings back the changes from #7269.

resolves #6643
rdar://113964179

Author: neonichu

Sources/Basics/FileSystem/FileSystem+Extensions.swift

@@ -196,8 +196,8 @@ extension FileSystem {
     }
 
     /// Execute the given block while holding the lock.
-    public func withLock<T>(on path: AbsolutePath, type: FileLock.LockType, _ body: () throws -> T) throws -> T {
-        try self.withLock(on: path.underlying, type: type, body)
+    public func withLock<T>(on path: AbsolutePath, type: FileLock.LockType, blocking: Bool = true, _ body: () throws -> T) throws -> T {
+        try self.withLock(on: path.underlying, type: type, blocking: blocking, body)
     }
 
     /// Returns any known item replacement directories for a given path. These may be used by platform-specific
@@ -616,3 +616,14 @@ extension FileSystem {
         try self.removeFileTree(tempDirectory)
     }
 }
+
+// MARK: - Locking
+
+extension FileLock {
+    public static func prepareLock(
+        fileToLock: AbsolutePath,
+        at lockFilesDirectory: AbsolutePath? = nil
+    ) throws -> FileLock {
+        return try Self.prepareLock(fileToLock: fileToLock.underlying, at: lockFilesDirectory?.underlying)
+    }
+}

Sources/CoreCommands/SwiftTool.swift

@@ -39,9 +39,11 @@ import Musl
 #endif
 
 import func TSCBasic.exec
+import class TSCBasic.FileLock
 import protocol TSCBasic.OutputByteStream
 import class TSCBasic.Process
 import enum TSCBasic.ProcessEnv
+import enum TSCBasic.ProcessLockError
 import var TSCBasic.stderrStream
 import class TSCBasic.TerminalController
 import class TSCBasic.ThreadSafeOutputByteStream
@@ -118,6 +120,7 @@ extension SwiftCommand {
         } catch {
             toolError = error
         }
+        try swiftTool.releaseLockIfNeeded()
 
         // wait for all observability items to process
         swiftTool.waitForObservabilityEvents(timeout: .now() + 5)
@@ -396,6 +399,9 @@ public final class SwiftTool {
             return workspace
         }
 
+        // Before creating the workspace, we need to acquire a lock on the build directory.
+        try self.acquireLockIfNeeded()
+
         if options.resolver.skipDependencyUpdate {
             self.observabilityScope.emit(warning: "'--skip-update' option is deprecated and will be removed in a future release")
         }
@@ -866,6 +872,36 @@ public final class SwiftTool {
         case success
         case failure
     }
+
+    // MARK: - Locking
+
+    private var workspaceLock: FileLock?
+
+    fileprivate func acquireLockIfNeeded() throws {
+        guard workspaceLock == nil else {
+            throw InternalError("acquireLockIfNeeded() called multiple times")
+        }
+        let workspaceLock = try FileLock.prepareLock(fileToLock: self.scratchDirectory)
+
+        // Try a non-blocking lock first so that we can inform the user about an already running SwiftPM.
+        do {
+            try workspaceLock.lock(type: .exclusive, blocking: false)
+        } catch let ProcessLockError.unableToAquireLock(errno) {
+            if errno == EWOULDBLOCK {
+                self.outputStream.write("Another instance of SwiftPM is already running using '\(self.scratchDirectory)', waiting until that process has finished execution...".utf8)
+                self.outputStream.flush()
+
+                // Only if we fail because there's an existing lock we need to acquire again as blocking.
+                try workspaceLock.lock(type: .exclusive, blocking: true)
+            }
+        }
+
+        self.workspaceLock = workspaceLock
+    }
+
+    fileprivate func releaseLockIfNeeded() {
+        workspaceLock?.unlock()
+    }
 }
 
 /// Returns path of the nearest directory containing the manifest file w.r.t