[Driver] Handle failure-to-exec somewhat more politely, rdar://37865437.

Author: graydon

lib/Driver/Compilation.cpp

@@ -810,15 +810,29 @@ namespace driver {
       do {
         using namespace std::placeholders;
         // Ask the TaskQueue to execute.
-        TQ->execute(std::bind(&PerformJobsState::taskBegan, this,
-                              _1, _2),
-                    std::bind(&PerformJobsState::taskFinished, this,
-                              _1, _2, _3, _4, _5),
-                    std::bind(&PerformJobsState::taskSignalled, this,
-                              _1, _2, _3, _4, _5, _6));
-
-        // Returning from TaskQueue::execute should mean either an empty
-        // TaskQueue or a failed subprocess.
+        if (TQ->execute(std::bind(&PerformJobsState::taskBegan, this,
+                                  _1, _2),
+                        std::bind(&PerformJobsState::taskFinished, this,
+                                  _1, _2, _3, _4, _5),
+                        std::bind(&PerformJobsState::taskSignalled, this,
+                                  _1, _2, _3, _4, _5, _6))) {
+          if (Result == EXIT_SUCCESS) {
+            // FIXME: Error from task queue while Result == EXIT_SUCCESS most
+            // likely means some fork/exec or posix_spawn failed; TaskQueue saw
+            // "an error" at some stage before even calling us with a process
+            // exit / signal (or else a poll failed); unfortunately the task
+            // causing it was dropped on the floor and we have no way to recover
+            // it here, so we report a very poor, generic error.
+            Comp.Diags.diagnose(SourceLoc(), diag::error_unable_to_execute_command,
+                                "<unknown>");
+            Result = -2;
+            AnyAbnormalExit = true;
+            return;
+          }
+        }
+
+        // Returning without error from TaskQueue::execute should mean either an
+        // empty TaskQueue or a failed subprocess.
         assert(!(Result == 0 && TQ->hasRemainingTasks()));
 
         // Task-exit callbacks from TaskQueue::execute may have unblocked jobs,

test/Driver/bad_exec.swift

@@ -0,0 +1,7 @@
+// REQUIRES: OS=macosx
+// RUN: %empty-directory(%t)
+// RUN: not %swiftc_driver -driver-use-frontend-path /always/searching/never/finding %s 2>&1 | %FileCheck %s
+// CHECK: unable to execute command
+func thing() {
+    print(1)
+}