[bindings] Conditionally report module request source position

Calculating the source position for a module request is not free.
Even the information is only used by DevTools we currently always
calculate the line and column information.

V8 only lazily initializes line-end information. This behavior
currently causes needless memory and performance overhead.

To partially mitigate the issue we only report the detailed source
position if there is an active DevTools session.

Ideally we would only report the source offset and let all the clients
(in this case DevTools) resolve the offset to line and column.

Bug: 1162107
Change-Id: I9942cde3bc28e483f1aa2c2b03e8cef2e182d838
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622217
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844598}

third_party/blink/renderer/bindings/core/v8/module_record.cc

@@ -18,6 +18,8 @@
 #include "third_party/blink/renderer/platform/bindings/script_state.h"
 #include "third_party/blink/renderer/platform/loader/fetch/script_fetch_options.h"
 #include "third_party/blink/renderer/platform/weborigin/kurl.h"
+#include "third_party/blink/renderer/platform/wtf/text/text_position.h"
+#include "third_party/blink/renderer/platform/wtf/wtf.h"
 
 namespace blink {
 
@@ -135,17 +137,29 @@ Vector<ModuleRequest> ModuleRecord::ModuleRequests(
   int length = v8_module_requests->Length();
   Vector<ModuleRequest> requests;
   requests.ReserveInitialCapacity(length);
+  bool needs_text_position =
+      !WTF::IsMainThread() ||
+      probe::ToCoreProbeSink(ExecutionContext::From(script_state))
+          ->HasDevToolsSessions();
 
   for (int i = 0; i < length; ++i) {
     v8::Local<v8::ModuleRequest> v8_module_request =
         v8_module_requests->Get(script_state->GetContext(), i)
             .As<v8::ModuleRequest>();
     v8::Local<v8::String> v8_specifier = v8_module_request->GetSpecifier();
-    int source_offset = v8_module_request->GetSourceOffset();
-    v8::Location v8_loc = record->SourceOffsetToLocation(source_offset);
-    TextPosition position(
-        OrdinalNumber::FromZeroBasedInt(v8_loc.GetLineNumber()),
-        OrdinalNumber::FromZeroBasedInt(v8_loc.GetColumnNumber()));
+    TextPosition position = TextPosition::MinimumPosition();
+    if (needs_text_position) {
+      // The source position is only used by DevTools for module requests and
+      // only visible if devtools is open when the request is initiated.
+      // Calculating the source position is not free and V8 has to initialize
+      // the line end information for the complete module, thus we try to
+      // avoid this additional work here if DevTools is closed.
+      int source_offset = v8_module_request->GetSourceOffset();
+      v8::Location v8_loc = record->SourceOffsetToLocation(source_offset);
+      position = TextPosition(
+          OrdinalNumber::FromZeroBasedInt(v8_loc.GetLineNumber()),
+          OrdinalNumber::FromZeroBasedInt(v8_loc.GetColumnNumber()));
+    }
     Vector<ImportAssertion> import_assertions =
         ModuleRecord::ToBlinkImportAssertions(
             script_state->GetContext(), record,