Revert "Revert "Disfavor current working directory when looking up executables""

Author: stevapple

Sources/SwiftDriver/Toolchains/Toolchain.swift

@@ -211,13 +211,14 @@ extension Toolchain {
   /// looks in the `executableDir`, `xcrunFind` or in the `searchPaths`.
   /// - Parameter executable: executable to look for [i.e. `swift`].
   func lookup(executable: String) throws -> AbsolutePath {
-    if let overrideString = envVar(forExecutable: executableName(executable)) {
-      return try AbsolutePath(validating: overrideString)
+    if let overrideString = envVar(forExecutable: executableName(executable)),
+       let path = try? AbsolutePath(validating: overrideString) {
+      return path
     } else if let toolDir = toolDirectory,
-              let path = lookupExecutablePath(filename: executableName(executable), searchPaths: [toolDir]) {
+              let path = lookupExecutablePath(filename: executableName(executable), currentWorkingDirectory: nil, searchPaths: [toolDir]) {
       // Looking for tools from the tools directory.
       return path
-    } else if let path = lookupExecutablePath(filename: executableName(executable), searchPaths: [executableDir]) {
+    } else if let path = lookupExecutablePath(filename: executableName(executable), currentWorkingDirectory: nil, searchPaths: [executableDir]) {
       return path
     } else if let path = try? xcrunFind(executable: executableName(executable)) {
       return path

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -6609,6 +6609,58 @@ final class SwiftDriverTests: XCTestCase {
 #endif
 #endif
   }
+
+  func testToolSearching() throws {
+#if os(Windows)
+    let PATH = "Path"
+#else
+    let PATH = "PATH"
+#endif
+    let SWIFT_FRONTEND_EXEC = "SWIFT_DRIVER_SWIFT_FRONTEND_EXEC"
+
+    // We need to override SWIFT_DRIVER_SWIFT_FRONTEND_EXEC to prevent it from interfering the tests.
+    var driver = try Driver(args: ["swiftc", "-print-target-info"], env: [PATH: ProcessEnv.path!])
+    let jobs = try driver.planBuild()
+    XCTAssertEqual(jobs.count, 1)
+    let defaultSwiftFrontend = jobs.first!.tool.absolutePath!
+
+    try withTemporaryDirectory { toolsDirectory in
+      let customSwiftFrontend = toolsDirectory.appending(component: executableName("swift-frontend"))
+      try localFileSystem.createSymbolicLink(customSwiftFrontend, pointingAt: defaultSwiftFrontend, relative: false)
+
+      try withTemporaryDirectory { tempDirectory in 
+        try localFileSystem.changeCurrentWorkingDirectory(to: tempDirectory)
+        let anotherSwiftFrontend = localFileSystem.currentWorkingDirectory!.appending(component: executableName("swift-frontend"))
+        try localFileSystem.createSymbolicLink(anotherSwiftFrontend, pointingAt: defaultSwiftFrontend, relative: false)
+
+        // test if SWIFT_DRIVER_TOOLNAME_EXEC is respected
+        do {
+          var driver = try Driver(args: ["swiftc", "-print-target-info"],
+                                  env: [PATH: ProcessEnv.path!, SWIFT_FRONTEND_EXEC: customSwiftFrontend.pathString])
+          let jobs = try driver.planBuild()
+          XCTAssertEqual(jobs.count, 1)
+          XCTAssertEqual(jobs.first!.tool.name, customSwiftFrontend.pathString)
+        }
+
+        // test if tools directory is respected
+        do {
+          var driver = try Driver(args: ["swiftc", "-print-target-info", "-tools-directory", toolsDirectory.pathString],
+                                  env: [PATH: ProcessEnv.path!])
+          let jobs = try driver.planBuild()
+          XCTAssertEqual(jobs.count, 1)
+          XCTAssertEqual(jobs.first!.tool.name, customSwiftFrontend.pathString)
+        }
+
+        // test if current working directory is searched before PATH
+        do {
+          var driver = try Driver(args: ["swiftc", "-print-target-info"], env: [PATH: toolsDirectory.pathString])
+          let jobs = try driver.planBuild()
+          XCTAssertEqual(jobs.count, 1)
+          XCTAssertEqual(jobs.first!.tool.name, anotherSwiftFrontend.pathString)
+        }
+      }
+    }  
+  }
 }
 
 func assertString(