Add CallPDFiumWideStringBufferApiAndReturnOptional().

Add a variant of CallPDFiumWideStringBufferApi() that distinguishes
between API call failure and empty string return values.

Change-Id: I4ec2c1ed16f9109854493f184acd597c8b622196
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2322167Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792454}

pdf/pdfium/pdfium_api_string_buffer_adapter.h

@@ -12,6 +12,7 @@
 #include "base/callback.h"
 #include "base/macros.h"
 #include "base/numerics/safe_math.h"
+#include "base/optional.h"
 #include "base/strings/string16.h"
 
 namespace chrome_pdf {
@@ -103,19 +104,33 @@ template <class AdapterType,
           class StringType,
           typename BufferType,
           typename ReturnType>
-StringType CallPDFiumStringBufferApi(
+base::Optional<StringType> CallPDFiumStringBufferApiAndReturnOptional(
     base::RepeatingCallback<ReturnType(BufferType*, ReturnType)> api,
     bool check_expected_size) {
-  StringType str;
   ReturnType expected_size = api.Run(nullptr, 0);
-  if (expected_size > 0) {
+  if (expected_size == 0)
+    return base::nullopt;
+
+  StringType str;
   AdapterType api_string_adapter(&str, expected_size, check_expected_size);
   auto* data = reinterpret_cast<BufferType*>(api_string_adapter.GetData());
   api_string_adapter.Close(api.Run(data, expected_size));
-  }
   return str;
 }
 
+template <class AdapterType,
+          class StringType,
+          typename BufferType,
+          typename ReturnType>
+StringType CallPDFiumStringBufferApi(
+    base::RepeatingCallback<ReturnType(BufferType*, ReturnType)> api,
+    bool check_expected_size) {
+  base::Optional<StringType> result =
+      CallPDFiumStringBufferApiAndReturnOptional<AdapterType, StringType>(
+          api, check_expected_size);
+  return result.value_or(StringType());
+}
+
 }  // namespace internal
 
 // Helper function to call PDFium APIs where the output buffer is expected to
@@ -129,6 +144,18 @@ base::string16 CallPDFiumWideStringBufferApi(
       api, check_expected_size);
 }
 
+// Variant of CallPDFiumWideStringBufferApi() that distinguishes between API
+// call failures and empty string return values.
+template <typename BufferType>
+base::Optional<base::string16> CallPDFiumWideStringBufferApiAndReturnOptional(
+    base::RepeatingCallback<unsigned long(BufferType*, unsigned long)> api,
+    bool check_expected_size) {
+  using adapter_type = internal::PDFiumAPIStringBufferSizeInBytesAdapter;
+  return internal::CallPDFiumStringBufferApiAndReturnOptional<adapter_type,
+                                                              base::string16>(
+      api, check_expected_size);
+}
+
 // Helper function to call PDFium APIs where the output buffer is expected to
 // hold ASCII or UTF-8 data, and the buffer length is specified in bytes.
 template <typename BufferType, typename ReturnType>