component build: static_library / source_set weirdness (linker errors)

[Erik Chen]

I have a CL that has linker errors (but only on windows, component build). Changing a single source_set to a static_library gets rid of the errors, but I don't understand why.

Example of linker error:

lld-link: error: undefined symbol: public: __cdecl ui::DialogModelSection::Builder::Builder(void)

I'm guessing this is related to previous thread/bug, but the symptoms are different. 

I reviewed the remove component build thread and summary, maybe this is a variant of "Putting exported symbols in static libraries", although I'm not quite sure how?

I estimate this cost me ~1-2 hours to find the workaround, and it's not clear that the workaround is correct. Prior to that I mashed "+1" 3 times because I thought this was a tip of tree bug. This is a CL (which in turns blocks a bunch of my other CLs) which should have landed today, but instead will likely land tomorrow due to the total delay (3 hours of mashing CQ and doing other stuff, 1-2 hours of active investigation).

[Greg Thompson]

Hi Erik.

The Builder class within DialogModelSection is missing its COMPONENT_EXPORT(UI_BASE). It seems that you're just the first dev unlucky enough to notice. This'll fix it:

diff --git a/ui/base/models/dialog_model_field.h b/ui/base/models/dialog_model_field.h
index bb3deb43d4162..428fe9c131ac6 100644
--- a/ui/base/models/dialog_model_field.h
+++ b/ui/base/models/dialog_model_field.h
@@ -527,7 +527,7 @@ class COMPONENT_EXPORT(UI_BASE) DialogModelCustomField
 class COMPONENT_EXPORT(UI_BASE) DialogModelSection final
     : public DialogModelField {
  public:
-  class Builder final {
+  class COMPONENT_EXPORT(UI_BASE) Builder final {
    public:
     Builder();
     Builder(const Builder&) = delete;

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[Nico Weber]

For "why does static_library help": A source_set always has all its files linked in to executables that depend on it. A static library only has the files inside it linked in to executables that depend on it that are referenced from other files.

As an example: If you have:

* a target t1 with files file1.cc and file2.cc

* file1.cc contains symbol symbol1()

* file2.cc contains symbol symbol2()

…and you link that target into an executable that has

* a call to symbol1()

* but nothing calls symbol2()

…then:

* if t1 is a source_set, both file1.o and file2.o will be linked into the executable

* but if t1 is a static_library(), only file1.o will be linked into the executable

If file2.o contains references to undefined symbols, you'll end up with linker errors in the first case, but not in the second.

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[Erik Chen]

Thank you all for the feedback!