[vm] Handle short writes in Builtin_PrintString.

Also avoid polluting the repository working directory when fuzzing.

TEST=lib/mirrors/invocation_fuzz_test
Bug: #60721
Change-Id: Ibe2f2d35479bbf969cff74c954acd606c1fc0679
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/428381
Commit-Queue: Ryan Macnak <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>

Author: rmacnak-google

runtime/bin/builtin_natives.cc

@@ -99,8 +99,14 @@ void FUNCTION_NAME(Builtin_PrintString)(Dart_NativeArguments args) {
   chars[length] = '\n';
 
   // Uses fwrite to support printing NUL bytes.
-  intptr_t res = fwrite(chars, 1, new_length, stdout);
-  ASSERT(res == new_length);
+  const uint8_t* cursor = chars;
+  intptr_t remaining = new_length;
+  while (remaining > 0) {
+    intptr_t written = fwrite(cursor, 1, remaining, stdout);
+    if (written == 0) break;  // EOF or error: ignore
+    cursor += written;
+    remaining -= written;
+  }
   fflush(stdout);
   if (ShouldCaptureStdout()) {
     // For now we report print output on the Stdout stream.

tests/lib/mirrors/invocation_fuzz_test.dart

@@ -162,10 +162,17 @@ doOneTask() {
 var fuzzArgument;
 
 main() {
+  // Avoid polluting the repository. Use a fixed path instead dynamically
+  // creating one because this test won't reliably cleanup on exit and we'd
+  // like to avoid accumulating temp directories.
+  var d = Directory(Directory.systemTemp.path + "/invocation_fuzz_test");
+  d.createSync();
+  Directory.current = d;
+
   fuzzArgument = null;
   fuzzArgument = 1; // //# smi: ok
   fuzzArgument = false; // //# false: ok
-  fuzzArgument = 'string'; // //# string: ok
+  fuzzArgument = 'fuzzstring'; // //# string: ok
   fuzzArgument = new List.filled(0, null); // //# emptyarray: ok
 
   print('Fuzzing with $fuzzArgument');