Use non-blocking lock on non-interactive terminals

I don't think this is shippable, but it may help to find out more about the issues we saw in self-hosted CI jobs.

Author: neonichu

Sources/CoreCommands/SwiftTool.swift

@@ -101,6 +101,16 @@ public protocol SwiftCommand: ParsableCommand, _SwiftCommand {
 extension SwiftCommand {
     public static var _errorLabel: String { "error" }
 
+    private func acquireScratchDirectoryLockIfNeeded<T>(swiftTool: SwiftTool, blocking: Bool = true, _ body: () throws -> T) throws {
+        let interactive = isatty(STDOUT_FILENO) == 1
+        _ = try swiftTool.fileSystem.withLock(
+            on: swiftTool.scratchDirectory,
+            type: .exclusive,
+            blocking: interactive ? blocking : false,
+            body
+        )
+    }
+
     public func run() throws {
         let swiftTool = try SwiftTool(
             options: globalOptions,
@@ -112,7 +122,7 @@ extension SwiftCommand {
 
         // Try a non-blocking lock first so that we can inform the user about an already running SwiftPM.
         do {
-            try swiftTool.fileSystem.withLock(on: swiftTool.scratchDirectory, type: .exclusive, blocking: false) {}
+            try acquireScratchDirectoryLockIfNeeded(swiftTool: swiftTool, 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)
@@ -121,7 +131,7 @@ extension SwiftCommand {
         }
 
         var toolError: Error? = .none
-        try swiftTool.fileSystem.withLock(on: swiftTool.scratchDirectory, type: .exclusive) {
+        try acquireScratchDirectoryLockIfNeeded(swiftTool: swiftTool) {
             do {
                 try self.run(swiftTool)
                 if swiftTool.observabilityScope.errorsReported || swiftTool.executionStatus == .failure {