Lock scratch directory during tool execution

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

Sources/CoreCommands/SwiftTool.swift

@@ -41,6 +41,7 @@ import func TSCBasic.exec
 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
@@ -108,14 +109,27 @@ extension SwiftCommand {
             workspaceLoaderProvider: self.workspaceLoaderProvider
         )
         swiftTool.buildSystemProvider = try buildSystemProvider(swiftTool)
-        var toolError: Error? = .none
+
+        // Try a non-blocking lock first so that we can inform the user about an already running SwiftPM.
         do {
-            try self.run(swiftTool)
-            if swiftTool.observabilityScope.errorsReported || swiftTool.executionStatus == .failure {
-                throw ExitCode.failure
+            try swiftTool.fileSystem.withLock(on: swiftTool.scratchDirectory, type: .exclusive, blocking: false) {}
+        } catch let ProcessLockError.unableToAquireLock(errno) {
+            if errno == EWOULDBLOCK {
+                swiftTool.outputStream.write("Another instance of SwiftPM is already running using '\(swiftTool.scratchDirectory)', waiting until that process has finished execution...".utf8)
+                swiftTool.outputStream.flush()
+            }
+        }
+
+        var toolError: Error? = .none
+        try swiftTool.fileSystem.withLock(on: swiftTool.scratchDirectory, type: .exclusive) {
+            do {
+                try self.run(swiftTool)
+                if swiftTool.observabilityScope.errorsReported || swiftTool.executionStatus == .failure {
+                    throw ExitCode.failure
+                }
+            } catch {
+                toolError = error
             }
-        } catch {
-            toolError = error
         }
 
         // wait for all observability items to process