Fix Fuchsia implementation of base::RandBytes() to allow empty buffer.

base::RandBytes() may be called with output_length=0 and all
implementations supported this case, except Fuchsia. On Fuchsia the
function was crashing due to a failed CHECK(). This was breaking
MemoryDataSourceTest.EmptySource test.

Bug: 737802
Change-Id: Ib9ec20a82dd6966a146f0c66936e232c061466a5
Reviewed-on: https://chromium-review.googlesource.com/595012
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491086}

base/rand_util_fuchsia.cc

@@ -21,7 +21,7 @@ uint64_t RandUint64() {
 void RandBytes(void* output, size_t output_length) {
   size_t remaining = output_length;
   unsigned char* cur = reinterpret_cast<unsigned char*>(output);
-  do {
+  while (remaining > 0) {
     // The syscall has a maximum number of bytes that can be read at once.
     size_t read_len =
         std::min(remaining, static_cast<size_t>(MX_CPRNG_DRAW_MAX_LEN));
@@ -33,7 +33,7 @@ void RandBytes(void* output, size_t output_length) {
     CHECK(remaining >= actual);
     remaining -= actual;
     cur += actual;
-  } while (remaining > 0);
+  }
 }
 
 }  // namespace base

base/rand_util_unittest.cc

@@ -52,6 +52,11 @@ TEST(RandUtilTest, RandBytes) {
   EXPECT_GT(std::unique(buffer, buffer + buffer_size) - buffer, 25);
 }
 
+// Verify that calling base::RandBytes with an empty buffer doesn't fail.
+TEST(RandUtilTest, RandBytes0) {
+  base::RandBytes(nullptr, 0);
+}
+
 TEST(RandUtilTest, RandBytesAsString) {
   std::string random_string = base::RandBytesAsString(1);
   EXPECT_EQ(1U, random_string.size());