[PATCH] D143418: [libclang] Add API to override preamble storage path

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added inline comments.


================
Comment at: clang/include/clang-c/Index.h:329
+ * CXIndexOptions Opts = { sizeof(CXIndexOptions),
+ *                         clang_getDefaultGlobalOptions() };
+ * \endcode
----------------
vedgy wrote:
> When I almost finished the requested changes, I remembered that the return value of `clang_getDefaultGlobalOptions()` depends on environment variables, and thus `0` is not necessarily the default. Adjusted the changes and updated this revision.
> 
> Does the extra requirement to non-zero initialize this second member sway your opinion on the usefulness of the helper function `inline CXIndexOptions clang_getDefaultIndexOptions()`? Note that there may be same (environment) or other important reasons why future new options couldn't be zeroes by default.
Thinking out loud a bit... (potentially bad idea incoming)

What if we dropped `clang_getDefaultGlobalOptions()` and instead made a change to `CXGlobalOptFlags`:
```
typedef enum {
  /**
   * Used to indicate that the default CXIndex options are used. By default, no
   * global options will be used. However, environment variables may change which
   * global options are in effect at runtime.
   */
  CXGlobalOpt_Default = 0x0,

  /**
   * Used to indicate that threads that libclang creates for indexing
   * purposes should use background priority.
   *
   * Affects #clang_indexSourceFile, #clang_indexTranslationUnit,
   * #clang_parseTranslationUnit, #clang_saveTranslationUnit.
   */
  CXGlobalOpt_ThreadBackgroundPriorityForIndexing = 0x1,

  /**
   * Used to indicate that threads that libclang creates for editing
   * purposes should use background priority.
   *
   * Affects #clang_reparseTranslationUnit, #clang_codeCompleteAt,
   * #clang_annotateTokens
   */
  CXGlobalOpt_ThreadBackgroundPriorityForEditing = 0x2,

  /**
   * Used to indicate that all threads that libclang creates should use
   * background priority.
   */
  CXGlobalOpt_ThreadBackgroundPriorityForAll =
      CXGlobalOpt_ThreadBackgroundPriorityForIndexing |
      CXGlobalOpt_ThreadBackgroundPriorityForEditing,

  /**
   * Used to indicate that no global options should be used, even
   * in the presence of environment variables.
   */
  CXGlobalOpt_None = 0xFFFFFFFF
} CXGlobalOptFlags;
```
so that when the user passes `0` they get the previous behavior.

`clang_CXIndex_setGlobalOptions()` would remain deprecated. `clang_CXIndex_getGlobalOptions()` would be interesting though -- would it return `CXGlobalOpt_None` or `CXGlobalOpt_Default` in the event the index was created without any global options? Hmmm.

Err, actually, I suppose this won't work too well because then code silently changes behavior if it does `clang_CXIndex_setGlobalOptions(CXGlobalOpt_None);` because that would change from "do what the environment says" to "ignore the environment". But I have to wonder whether anyone actually *does* that or not... my intuition is that folks would not call `clang_CXIndex_setGlobalOptions()` at all unless they were setting an option to a non-none value. We could rename `CXGlobalOpt_None` to `CXGlobalOpt_Nothing` (or something along those lines) to force a compilation error, but that's a bit of a nuclear option for what's supposed to be a stable API.

So I'm on the fence, I guess. I'd still prefer for zero to give sensible defaults and I don't think there's enough use of the global options + environment variables to matter. But I also don't like silently breaking code, so my idea above may be a nonstarter.

I suppose another possible idea is: deprecate the notion of global options enum and setter/getter entirely, add two new fields to `CXIndexOptions`:
```
typedef enum {
  CXChoice_Default = 0,
  CXChoice_Enabled = 1,
  CXChoice_Disabled = 2
} CXChoice;

...
unsigned ThreadPriorityBackgroundForIndexing;
unsigned ThreadPriorityBackgroundForEditing;
...
```
so that `0` gives the correct default behavior based on environment variable. There would be no global setter or getter for this information (and we'd eventually remove `clang_CXIndex_[gs]etGlobalOptions()`).


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D143418/new/

https://reviews.llvm.org/D143418