new wrap-mode: forcefallback

This can be useful to make sure that a project builds when
its fallbacks are used on systems where external dependencies
satisfy the version requirements, or to easily hack on the sources
of a dependency for which a fallback exists.

data/shell-completions/zsh/_meson

@@ -31,7 +31,7 @@ local -i ret
 
 local __meson_backends="(ninja xcode ${(j. .)${:-vs{,2010,2015,2017}}})"
 local __meson_build_types="(plain debug debugoptimized minsize release)"
-local __meson_wrap_modes="(WrapMode.{default,nofallback,nodownload})"
+local __meson_wrap_modes="(WrapMode.{default,nofallback,nodownload,forcefallback})"
 
 local -a meson_commands=(
 'setup:set up a build directory'

docs/markdown/FAQ.md

@@ -288,3 +288,9 @@ has a option called `wrap-mode` which can be used to disable wrap
 downloads altogether with `--wrap-mode=nodownload`. You can also
 disable dependency fallbacks altogether with `--wrap-mode=nofallback`,
 which also implies the `nodownload` option.
+
+If on the other hand, you want meson to always use the fallback
+for dependencies, even when an external dependency exists and could
+satisfy the version requirements, for example in order to make
+sure your project builds when fallbacks are used, you can use
+`--wrap-mode=forcefallback`.

docs/markdown/snippets/new-wrap-mode.md

@@ -0,0 +1,3 @@
+A new wrap mode was added, `--wrap-mode=forcefallback`. When this is set,
+dependencies for which a fallback was provided will always use it, even
+if an external dependency exists and satisfies the version requirements.

mesonbuild/interpreter.py

@@ -2404,10 +2404,13 @@ to directly access options of other subprojects.''')
             dep = None
 
             # Search for it outside the project
+            if self.coredata.wrap_mode != WrapMode.forcefallback or 'fallback' not in kwargs:
                 try:
                     dep = dependencies.find_external_dependency(name, self.environment, kwargs)
                 except DependencyException as e:
                     exception = e
+            else:
+                exception = DependencyException("fallback for %s not found" % name)
 
             # Search inside the projects list
             if not dep or not dep.found():

mesonbuild/wrap/__init__.py

@@ -25,7 +25,12 @@ from enum import Enum
 # to use 'nofallback' so that any 'copylib' wraps will be
 # download as subprojects.
 #
+# --wrap-mode=forcefallback will ignore external dependencies,
+# even if they match the version requirements, and automatically
+# use the fallback if one was provided. This is useful for example
+# to make sure a project builds when using the fallbacks.
+#
 # Note that these options do not affect subprojects that
 # are git submodules since those are only usable in git
 # repositories, and you almost always want to download them.
-WrapMode = Enum('WrapMode', 'default nofallback nodownload')
+WrapMode = Enum('WrapMode', 'default nofallback nodownload forcefallback')