Bug 187185 – [lldb-webkit] Non-empty strings may be pretty-printed as empty

• Component: Tools / Tests
• Status: RESOLVED FIXED

[Daniel Bates]

Today, I set a breakpoint at <https://trac.webkit.org/browser/trunk/Source/WebCore/platform/network/cocoa/ResourceRequestCocoa.mm?rev=233316#L91>. Using a debug built of WebKit, I started MiniBrowser and opened a new WK2 window. When the breakpoint was hit I ran `p m_url.string()`. LLDB emitted the following:

(lldb) p m_url.string()
(const WTF::String) $9 = { length = 0, contents = '' } {
  m_impl = {
    m_ptr = 0x000000011ce6fb18 { length = 0, is8bit = 0, contents = '' }
  }
}

But m_url.string() is not an empty string:

(lldb) p m_url.string().isEmpty()
(bool) $10 = false

I am using Xcode 10.0 (10L177m) on macOS Mojave (18A323).

[Daniel Bates]

$ Tools/Scripts/dump-class-layout -c Debug JavaScriptCore StringImpl
Found 1 types matching "StringImpl" in "/Volumes/.../WebKitBuild/Debug/JavaScriptCore.framework/JavaScriptCore" for x86_64
  +0 { 24} StringImpl
  +0 { 24}     WTF::StringImplShape
  +0 <  4>         unsigned int m_refCount;
  +4 <  4>         unsigned int m_length;
  +8 <  8>         WTF::StringImplShape::(anonymous union) None;
 +16 <  4>         unsigned int m_hashAndFlags;
 +20 <  4>         <PADDING>
 +20 <  4>     <PADDING>
Total byte size: 24
Total pad bytes: 8
Padding percentage: 33.33 %

[Daniel Bates]

dump-class-layout does not have an issue resolving that WTF::StringImpl is effectively a WTF::StringImplShape. The LLDB repl seems to have an issue with this.

[Daniel Bates]

Created attachment 343923 [details]
Patch

[Daniel Bates]

Committed r233375: <https://trac.webkit.org/changeset/233375>

[Radar WebKit Bug Importer]

<rdar://problem/41657759>