tree cc466dc0881423da6bb8fe2e9b43697338d3f433
parent 1b948ef42679afb0727eded271ca2f883f1ec638
author Willy Tarreau <w@1wt.eu> 1651496223 +0200
committer Willy Tarreau <w@1wt.eu> 1651853615 +0200
encoding latin1

MINOR: applet: reserve some generic storage in the applet's context

Instead of using existing fields and having to put keyword-specific
contexts in the applet definition, let's have the appctx provide a
generic storage area that's currently large enough for existing CLI
commands and small applets, and a function to allocate that storage.

The function will be responsible for verifying that the requested size
fits in the area so that the caller doesn't need to add specific checks;
it is validated during development as this size is static and will
not change at runtime. In addition the caller doesn't even need to
free() the area since it's part of an existing context. For the
caller's convenience, a context pointer "svcctx" for the command is
also provided so that the allocated area can be placed there (or
possibly any other one in case a larger area is needed).

The struct's layout has been temporarily complicated by adding one
level of anonymous union on top of the "ctx" one. This will allow us
to preserve "ctx" during 2.6 for compatibility with possible external
code and get rid of it in 2.7. This explains why the diff extends to
the whole "ctx" union, but a "git show -b" shows that only one extra
layer was added. In order to make both the svcctx pointer and its
storage accessible without further enlarging the appctx structure,
both svcctx and the storage share the same storage as the ctx part.
This is done by having them placed in the union with a protected
overlapping area for svcctx, for which a shadow member is also
present in the storage area:

    union {
       void* svcctx;         // variable accessed by services
       struct {
           void *shadow;     // shadow of svcctx;
           char storage[];   // where most services store their data
       };
       union {               // older commands store here and ignore svcctx
          ...
       } ctx;
    };

I.e. new applications will use appctx->svcctx while older ones will be
able to continue to use appctx->ctx.*

The whole area (including the pointer's context) is zeroed before any
applet is initialized, and before CLI keyword processor's first invocation,
as it is an important part of the existing keyword processors, which makes
CLI keywords effectively behave like applets.