If class A is derived from class B, then an instance of class A can be
converted to B via a static cast, so a slower QI is not needed.
Differential Revision: https://phabricator.services.mozilla.com/D6861
Currently VarCache prefs are setup in two parts:
- The vanilla pref part, installed via a data file such as all.js, or via an
API call.
- The VarCache variable part, setup by an Add*VarCache() call.
Both parts are needed for the pref to actually operate as a proper VarCache
pref. (There are various prefs for which we do one but not the other unless a
certain condition is met.)
This patch introduces a new way of doing things. There is a new file,
modules/libpref/init/StaticPrefList.h, which defines prefs like this:
> VARCACHE_PREF(
> "layout.accessiblecaret.width",
> layout_accessiblecaret_width,
> float, 34.0
> )
This replaces both the existing parts.
The preprocessor is used to generate multiple things from this single
definition:
- A global variable (the VarCache itself).
- A getter for that global variable.
- A call to an init function that unconditionally installs the pref in the
prefs hash table at startup.
C++ files can include the new StaticPrefs.h file to access the getter.
Rust code cannot use the getter, but can access the global variable directly
via structs.rs. This is similar to how things currently work for Rust code.
Non-VarCache prefs can also be declared in StaticPrefList.h, using PREF instead
of the VARCACHE_PREF.
The new approach has the following advantages.
+ It eliminates the duplication (in all.js and the Add*VarCache() call) of the
pref name and default value, preventing potential mismatches. (This is a real
problem in practice!)
+ There is now a single initialization point for these VarCache prefs.
+ This avoids need to find a place to insert the Add*VarCache() calls, which
are currently spread all over the place.
+ It also eliminates the common pattern whereby these calls are wrapped in a
execute-once block protected by a static boolean (see bug 1346224).
+ It's no longer possible to have a VarCache pref for which only one of the
pieces has been setup.
+ It encapsulates the VarCache global variable, so there is no need to declare
it separately.
+ VarCache reads are done via a getter (e.g. StaticPrefs::foo_bar_baz())
instead of a raw global variable read.
+ This makes it clearer that you're reading a pref value, and easier to
search for uses.
+ This prevents accidental writes to the global variable.
+ This prevents accidental mistyping of the pref name.
+ This provides a single chokepoint in the code for such accesses, which make
adding checking and instrumentation feasible.
+ It subsumes MediaPrefs, and will allow that class to be removed. (gfxPrefs is
a harder lift, unfortunately.)
+ Once all VarCache prefs are migrated to the new approach, the VarCache
mechanism will be better encapsulated, with fewer details publicly visible.
+ (Future work) This will allow the pref names to be stored statically, saving
memory in every process.
The main downside of the new approach is that all of these prefs are in a
single header that is included in quite a few places, so any changes to this
header will cause a fair amount of recompilation.
Another minor downside is that all VarCache prefs are defined and visible from
start-up. For test-only prefs like network.predictor.doing-tests, having them
show in about:config isn't particularly useful.
The patch also moves three network VarCache prefs to the new mechanism as a
basic demonstration. (And note the inconsistencies in the multiple initial
values that were provided for
network.auth.subresource-img-cross-origin-http-auth-allow!) There will be
numerous follow-up bugs to convert the remaining VarCache prefs.
MozReview-Commit-ID: 9ABNpOR16uW
* * *
[mq]: fixup
MozReview-Commit-ID: 6ToT9dQjIAq
These are all easy cases where an nsXPIDLCString local variable is set via
getter_Copies() and then is only used in ways that nsCStrings can also be used
(i.e. no null checks or implicit conversions to |char*|).
In every case the patch trivially replaces the nsXPIDLCString with an
nsCString. (Also, there are a couple of unused nsXPIDLCString variables that
the patch simply removes.)
This removes about 2/3 of the occurrences of nsXPIDLString in the tree. The
places where nsXPIDLStrings are null-checked are replaced with |rv| checks.
The patch also removes a couple of unused declarations from
nsIStringBundle.idl.
Note that nsStringBundle::GetStringFromNameHelper() was merged into
GetStringFromName(), because they both would have had the same signature.
Most of the names passed to nsIStringBundle::{Get,Format}StringFromUTF8Name
have one of the two following forms:
- a 16-bit C string literal, which is then converted to an 8-bit string in
order for the lookup to occur;
- an 8-bit C string literal converted to a 16-bit string, which is then
converted back to an 8-bit string in order for the lookup to occur.
This patch introduces and uses alternative methods that can take an 8-bit C
string literal, which requires changing some signatures in other methods and
functions. It replaces all C++ uses of the old methods.
The patch also changes the existing {Get,Format}StringFromName() methods so
they take an AUTF8String argument for the name instead of a wstring, because
that's nicer for JS code.
Even though there is a method for C++ code and a different one for JS code,
|binaryname| is used so that the existing method names can be used for the
common case in both languages.
The change reduces the number of NS_ConvertUTF8toUTF16 and
NS_ConvertUTF16toUTF8 conversions while running Speedometer v2 from ~270,000 to
~160,000. (Most of these conversions involved the string
"deprecatedReferrerDirective" in nsCSPParser.cpp.)
