dependency: support boolean argument "allow_fallback"

Sometimes, distros want to configure a project so that it does not
use any bundled library.  In this case, meson.build might want
to do something like this, where slirp is a combo option
with values auto/system/internal:

  slirp = dependency('', required: false)
  if get_option('slirp') != 'internal'
    slirp = dependency('slirp',
                       required: get_option('slirp') == 'system')
  endif
  if not slirp.found()
    slirp = subproject('libslirp', ...) .variable('...')
  endif

and we cannot use "fallback" because the "system" value should never
look for a subproject.

This worked until 0.54.x, but in 0.55.x this breaks because of the
automatic subproject search.  Note that the desired effect here is
backwards compared to the policy of doing an automatic search on
"required: true"; we only want to do the search if "required" is false!

It would be possible to look for the dependency with  `required: false`
and issue the error manually, but it's ugly and it may produce an error
message that looks "different" from Meson's.

Instead, with this change it is possible to achieve this effect in an
even simpler way:

  slirp = dependency('slirp',
                     required: get_option('slirp') != 'auto',
                     allow_fallback: get_option('slirp') == 'system' ? false : ['slirp', 'libslirp_dep'])

The patch also adds support for "allow_fallback: true", which is
simple and enables automatic fallback to a wrap even for non-required
dependencies.

1 files changed, 4 insertions, 2 deletions

diff --git a/mesonbuild/dependencies/base.py b/mesonbuild/dependencies/base.py
index 7c64b9c..95202fe 100644
--- a/mesonbuild/dependencies/base.py
+++ b/mesonbuild/dependencies/base.py
@@ -2320,9 +2320,11 @@ def get_dep_identifier(name, kwargs) -> T.Tuple:
         # 'version' is irrelevant for caching; the caller must check version matches
         # 'native' is handled above with `for_machine`
         # 'required' is irrelevant for caching; the caller handles it separately
-        # 'fallback' subprojects cannot be cached -- they must be initialized
+        # 'fallback' and 'allow_fallback' is not part of the cache because,
+        #     once a dependency has been found through a fallback, it should
+        #     be used for the rest of the Meson run.
         # 'default_options' is only used in fallback case
-        if key in ('version', 'native', 'required', 'fallback', 'default_options'):
+        if key in ('version', 'native', 'required', 'fallback', 'allow_fallback', 'default_options'):
             continue
         # All keyword arguments are strings, ints, or lists (or lists of lists)
         if isinstance(value, list):