[vm/bin] Improve fallback error message for process creation

Instead of just giving up try to give as much information as possible
including error code, error returned by Dart_StringFromUtf8 and
ascii portion of the OS error message.

We are making this change because we are getting reports of
ProcessException being thrown with "OS error message was a
not a utf8 string." message. There is no way to reproduce
these situations so instead we are going to add additional
information into the fallback message to aid the investigation.

TEST=tested manually by simulating fallback to the newly added code

Bug: b/178383611
Change-Id: Idf528dc762723694145a15aa07320220e8af9bfc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/181200
Commit-Queue: Vyacheslav Egorov <vegorov@google.com>
Reviewed-by: Daco Harkes <dacoharkes@google.com>

runtime/bin/dartutils.cc

@@ -758,6 +758,22 @@ Dart_Handle DartUtils::NewInternalError(const char* message) {
   return NewDartExceptionWithMessage(kCoreLibURL, "_InternalError", message);
 }
 
+Dart_Handle DartUtils::NewStringFormatted(const char* format, ...) {
+  va_list args;
+  va_start(args, format);
+  intptr_t len = vsnprintf(NULL, 0, format, args);
+  va_end(args);
+
+  char* buffer = reinterpret_cast<char*>(Dart_ScopeAllocate(len + 1));
+  MSAN_UNPOISON(buffer, (len + 1));
+  va_list args2;
+  va_start(args2, format);
+  vsnprintf(buffer, (len + 1), format, args2);
+  va_end(args2);
+
+  return NewString(buffer);
+}
+
 bool DartUtils::SetOriginalWorkingDirectory() {
   // If we happen to re-initialize the Dart VM multiple times, make sure to free
   // the old string (allocated by getcwd()) before setting a new one.

runtime/bin/dartutils.h

@@ -195,13 +195,16 @@ class DartUtils {
                                         const char* message,
                                         Dart_Handle os_error);
 
-  // Create a new Dart String object from a C String.
+  // Create a new Dart String object from a UTF8 encoded C String.
   static Dart_Handle NewString(const char* str) {
     ASSERT(str != NULL);
     return Dart_NewStringFromUTF8(reinterpret_cast<const uint8_t*>(str),
                                   strlen(str));
   }
 
+  // Create a new Dart String object from a formatted string.
+  static Dart_Handle NewStringFormatted(const char* format, ...);
+
   // Allocate length bytes for a C string with Dart_ScopeAllocate.
   static char* ScopedCString(intptr_t length) {
     char* result = NULL;

runtime/bin/process.cc

@@ -164,13 +164,29 @@ void FUNCTION_NAME(Process_Start)(Dart_NativeArguments args) {
     result =
         DartUtils::SetIntegerField(status_handle, "_errorCode", error_code);
     ThrowIfError(result);
-    Dart_Handle val = DartUtils::NewString(os_error_message != NULL
-                                               ? os_error_message
-                                               : "Cannot get error message");
+
+    const char* error_message = (os_error_message != NULL)
+                                    ? os_error_message
+                                    : "Failed to get error message";
+    Dart_Handle val = DartUtils::NewString(error_message);
     if (Dart_IsError(val)) {
-      // If conversion of the OS error message to a Dart string fails, fall back
-      // on a stock message.
-      val = DartUtils::NewString("OS error message was a not a utf8 string.");
+      // Try to clean the message from non-ASCII characters.
+      const intptr_t len = strlen(error_message);
+      char* ascii_message =
+          reinterpret_cast<char*>(Dart_ScopeAllocate(len + 1));
+      for (intptr_t i = 0; i < len; i++) {
+        if (static_cast<uint8_t>(error_message[i]) < 0x80) {
+          ascii_message[i] = error_message[i];
+        } else {
+          ascii_message[i] = '?';
+        }
+      }
+      ascii_message[len] = '\0';
+
+      val = DartUtils::NewStringFormatted(
+          "Failed to start %s. OS returned an error (code %d) which can't be "
+          "fully converted to Dart string (%s): %s",
+          path, error_code, Dart_GetError(val), ascii_message);
     }
     result = Dart_SetField(status_handle, DartUtils::NewString("_errorMessage"),
                            val);

runtime/bin/process_linux.cc

@@ -399,6 +399,8 @@ class ProcessStarter {
   }
 
  private:
+  static constexpr int kErrorBufferSize = 1024;
+
   int CreatePipes() {
     int result;
     result = TEMP_FAILURE_RETRY(pipe2(exec_control_, O_CLOEXEC));
@@ -705,9 +707,8 @@ class ProcessStarter {
   }
 
   void SetChildOsErrorMessage() {
-    const int kBufferSize = 1024;
-    char* error_message = DartUtils::ScopedCString(kBufferSize);
-    Utils::StrError(errno, error_message, kBufferSize);
+    char* error_message = DartUtils::ScopedCString(kErrorBufferSize);
+    Utils::StrError(errno, error_message, kErrorBufferSize);
     *os_error_message_ = error_message;
   }
 
@@ -715,9 +716,9 @@ class ProcessStarter {
     // In the case of failure in the child process write the errno and
     // the OS error message to the exec control pipe and exit.
     int child_errno = errno;
-    const int kBufferSize = 1024;
-    char error_buf[kBufferSize];
-    char* os_error_message = Utils::StrError(errno, error_buf, kBufferSize);
+    char error_buf[kErrorBufferSize];
+    char* os_error_message =
+        Utils::StrError(errno, error_buf, kErrorBufferSize);
     int bytes_written = FDUtils::WriteToBlocking(exec_control_[1], &child_errno,
                                                  sizeof(child_errno));
     if (bytes_written == sizeof(child_errno)) {
@@ -741,11 +742,10 @@ class ProcessStarter {
   }
 
   void ReadChildError() {
-    const int kMaxMessageSize = 256;
-    char* message = DartUtils::ScopedCString(kMaxMessageSize);
+    char* message = DartUtils::ScopedCString(kErrorBufferSize);
     if (message != NULL) {
-      FDUtils::ReadFromBlocking(exec_control_[0], message, kMaxMessageSize);
-      message[kMaxMessageSize - 1] = '\0';
+      FDUtils::ReadFromBlocking(exec_control_[0], message, kErrorBufferSize);
+      message[kErrorBufferSize - 1] = '\0';
       *os_error_message_ = message;
     } else {
       // Could not get error message. It will be NULL.