meson: use absolute cross-compiler paths

Among the files generated by meson is compile_commands.json. It is not
used by bitbake during the build. However, if the devtool workspace is
opened inside an IDE, that IDE can use compile_commands.json to
configure linting and code completion. This is notably relied on by the
new devtool ide-sdk command.

The problem is that the IDE using compile_commands.json does not know
the $PATH set-up by bitbake, so it won't find the compiler. This results
in linting errors, like missing headers. We can fix this by expliciting
the absolute compiler paths in meson.cross.

The compile_commands.json specification expressly states:
"All paths specified in the command or file fields must be either
absolute or relative to this directory."
Link: https://clang.llvm.org/docs/JSONCompilationDatabase.html

An alternative way to implement this is to directly change CXX inside
bitbake.conf to make all recipes use absolute compiler paths.Since this
would affect all recipes, so I would like to have the maintainers'
opinion on this. It could make sense to use absolute compiler paths for
all toolchain binaries, we already do so for the sysroot
TOOLCHAIN_OPTIONS.

Discussions have been opened with meson/ninja maintainers to implement
this at their level:
 - https://github.com/ninja-build/ninja/issues/2383
 - https://github.com/mesonbuild/meson/issues/12834
These tools have even less information on the environment so it makes
sense for Yocto to provide the absolute paths.

(From OE-Core rev: b4e00248049c2627b05eafa9313a48cf253623fa)

Signed-off-by: Enguerrand de Ribaucourt <enguerrand.de-ribaucourt@savoirfairelinux.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes-recipe/meson-routines.bbclass

@@ -10,6 +10,12 @@ def meson_array(var, d):
     items = d.getVar(var).split()
     return repr(items[0] if len(items) == 1 else items)
 
+def meson_array_abspath(var, d):
+    import shutil
+    items = d.getVar(var).split()
+    items[0] = shutil.which(items[0]) or items[0]
+    return repr(items[0] if len(items) == 1 else items)
+
 # Map our ARCH values to what Meson expects:
 # http://mesonbuild.com/Reference-tables.html#cpu-families
 def meson_cpu_family(var, d):

meta/classes-recipe/meson.bbclass

@@ -64,10 +64,13 @@ addtask write_config before do_configure
 do_write_config[vardeps] += "CC CXX AR NM STRIP READELF OBJCOPY CFLAGS CXXFLAGS LDFLAGS RUSTC RUSTFLAGS EXEWRAPPER_ENABLED"
 do_write_config() {
     # This needs to be Py to split the args into single-element lists
+    # The generated compile_commands.json file can be used by external IDEs
+    # which do not know the $PATH set-up by bitbake. They need the absolute
+    # compiler paths.
     cat >${WORKDIR}/meson.cross <<EOF
 [binaries]
-c = ${@meson_array('CC', d)}
+c = ${@meson_array_abspath('CC', d)}
-cpp = ${@meson_array('CXX', d)}
+cpp = ${@meson_array_abspath('CXX', d)}
 cython = 'cython3'
 ar = ${@meson_array('AR', d)}
 nm = ${@meson_array('NM', d)}