[VM/Runtime] Ensure errno is saved to ensure it is not clobbered by a subsequent operation.

This should fix the issue raised in #50129

TEST=ci (requires a massive file which is over 2 GB to be created)

Change-Id: Ib6163e015ff8506bd6de6b65dc433398857379d6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/376820
Reviewed-by: Ben Konyi <[email protected]>
Commit-Queue: Siva Annamalai <[email protected]>

Author: a-siva

runtime/bin/file.cc

@@ -252,6 +252,7 @@ void FUNCTION_NAME(File_ReadInto)(Dart_NativeArguments args) {
   }
 
   int64_t bytes_read = file->Read(reinterpret_cast<void*>(buffer), length);
+  OSError os_error;  // capture error if any
   if (is_byte_data) {
     ThrowIfError(Dart_TypedDataReleaseData(buffer_obj));
   }
@@ -261,7 +262,7 @@ void FUNCTION_NAME(File_ReadInto)(Dart_NativeArguments args) {
     }
     Dart_SetIntegerReturnValue(args, bytes_read);
   } else {
-    Dart_SetReturnValue(args, DartUtils::NewDartOSError());
+    Dart_SetReturnValue(args, DartUtils::NewDartOSError(&os_error));
   }
 }
 
@@ -1145,8 +1146,9 @@ CObject* File::ReadRequest(const CObjectArray& request) {
   uint8_t* data = io_buffer->value.as_external_typed_data.data;
   const int64_t bytes_read = file->Read(data, length);
   if (bytes_read < 0) {
+    CObject* error = CObject::NewOSError();
     CObject::FreeIOBufferData(io_buffer);
-    return CObject::NewOSError();
+    return error;
   }
 
   // Possibly shrink the used malloc() storage if the actual number of bytes is
@@ -1180,8 +1182,9 @@ CObject* File::ReadIntoRequest(const CObjectArray& request) {
   uint8_t* data = io_buffer->value.as_external_typed_data.data;
   const int64_t bytes_read = file->Read(data, length);
   if (bytes_read < 0) {
+    CObject* error = CObject::NewOSError();
     CObject::FreeIOBufferData(io_buffer);
-    return CObject::NewOSError();
+    return error;
   }
 
   // Possibly shrink the used malloc() storage if the actual number of bytes is