Report a better error message when retrieving frontend target info fails

Author: owenv

Sources/SwiftDriver/Driver/Driver.swift

@@ -25,6 +25,8 @@ public struct Driver {
     case integratedReplRemoved
     case conflictingOptions(Option, Option)
     case unableToLoadOutputFileMap(String)
+    case unableToDecodeFrontendTargetInfo
+    case failedToRetrieveFrontendTargetInfo
     // Explicit Module Build Failures
     case malformedModuleDependency(String, String)
     case missingPCMArguments(String)
@@ -49,6 +51,10 @@ public struct Driver {
         return "Compiler-internal integrated REPL has been removed; use the LLDB-enhanced REPL instead."
       case .conflictingOptions(let one, let two):
         return "conflicting options '\(one.spelling)' and '\(two.spelling)'"
+      case .unableToDecodeFrontendTargetInfo:
+        return "could not decode frontend target info; compiler driver and frontend executables may be incompatible"
+      case .failedToRetrieveFrontendTargetInfo:
+        return "failed to retrieve frontend target info"
       // Explicit Module Build Failures
       case .malformedModuleDependency(let moduleName, let errorDescription):
         return "Malformed Module Dependency: \(moduleName), \(errorDescription)"
@@ -1639,7 +1645,8 @@ extension Driver {
       diagnosticsEngine: diagnosticsEngine, env: env)
 
     // Query the frontend to for target information.
-    var info = try executor.execute(
+    do {
+      var info = try executor.execute(
         job: toolchain.printTargetInfoJob(
           target: explicitTarget, targetVariant: explicitTargetVariant,
           sdkPath: sdkPath, resourceDirPath: resourceDirPath,
@@ -1650,21 +1657,27 @@ extension Driver {
         forceResponseFiles: false,
         recordedInputModificationDates: [:])
 
-    // Parse the runtime compatibility version. If present, it will override
-    // what is reported by the frontend.
-    if let versionString =
-        parsedOptions.getLastArgument(.runtimeCompatibilityVersion)?.asSingle {
-      if let version = SwiftVersion(string: versionString) {
-        info.target.swiftRuntimeCompatibilityVersion = version
-        info.targetVariant?.swiftRuntimeCompatibilityVersion = version
-      } else {
-        diagnosticsEngine.emit(
-          .error_invalid_arg_value(
-            arg: .runtimeCompatibilityVersion, value: versionString))
+
+      // Parse the runtime compatibility version. If present, it will override
+      // what is reported by the frontend.
+      if let versionString =
+          parsedOptions.getLastArgument(.runtimeCompatibilityVersion)?.asSingle {
+        if let version = SwiftVersion(string: versionString) {
+          info.target.swiftRuntimeCompatibilityVersion = version
+          info.targetVariant?.swiftRuntimeCompatibilityVersion = version
+        } else {
+          diagnosticsEngine.emit(
+            .error_invalid_arg_value(
+              arg: .runtimeCompatibilityVersion, value: versionString))
+        }
       }
-    }
 
-    return (toolchain, info, swiftCompilerPrefixArgs)
+      return (toolchain, info, swiftCompilerPrefixArgs)
+    } catch is DecodingError {
+      throw Error.unableToDecodeFrontendTargetInfo
+    } catch {
+      throw Error.failedToRetrieveFrontendTargetInfo
+    }
   }
 }
 

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -1747,6 +1747,37 @@ final class SwiftDriverTests: XCTestCase {
       XCTAssertTrue(job.commandLine.contains(.flag("-resource-dir")))
     }
 
+    do {
+      struct MockExecutor: DriverExecutor {
+        func execute(job: Job, forceResponseFiles: Bool, recordedInputModificationDates: [TypedVirtualPath : Date]) throws -> ProcessResult {
+          return ProcessResult(arguments: [], environment: [:], exitStatus: .terminated(code: 0), output: .success(Array("bad JSON".utf8)), stderrOutput: .success([]))
+        }
+        func execute(jobs: [Job], delegate: JobExecutionDelegate, numParallelJobs: Int, forceResponseFiles: Bool, recordedInputModificationDates: [TypedVirtualPath : Date]) throws {
+          fatalError()
+        }
+        func checkNonZeroExit(args: String..., environment: [String : String]) throws -> String {
+          return try Process.checkNonZeroExit(arguments: args, environment: environment)
+        }
+        func description(of job: Job, forceResponseFiles: Bool) throws -> String {
+          fatalError()
+        }
+      }
+
+      XCTAssertThrowsError(try Driver(args: ["swift", "-print-target-info"],
+                                      executor: MockExecutor())) {
+        error in
+        XCTAssertEqual(error as? Driver.Error, .unableToDecodeFrontendTargetInfo)
+      }
+    }
+
+    do {
+      XCTAssertThrowsError(try Driver(args: ["swift", "-print-target-info"],
+                                      env: ["SWIFT_DRIVER_SWIFT_FRONTEND_EXEC": "/bad/path/to/swift-frontend"])) {
+        error in
+        XCTAssertEqual(error as? Driver.Error, .failedToRetrieveFrontendTargetInfo)
+      }
+    }
+
     do {
       var driver = try Driver(args: ["swift", "-print-target-info", "-target", "x86_64-apple-ios13.0-macabi", "-target-variant", "x86_64-apple-macosx10.14", "-sdk", "bar", "-resource-dir", "baz"])
       let plannedJobs = try driver.planBuild()