why _checkouts is not fetching all the contents of the application

[phani35]

I see that _checkouts is creating softlinks to src,include and priv folders from _build/default/checkouts/application.
if I have extra folders they are not projected to _build/default/checkouts/.

[_build/default/checkouts]$ ll test_app/
total 4
drwxr-xr-x 3 phani root 56 Oct 25 12:02 ./
drwxr-xr-x 4 phani root 29 Oct 25 12:02 ../
drwxr-xr-x 2 phani root 4096 Oct 25 13:41 ebin/
lrwxrwxrwx 1 phani root 34 Oct 25 12:02 include -> ../../../../_checkouts/test_app/include/
lrwxrwxrwx 1 phani root 31 Oct 25 12:02 priv -> ../../../../_checkouts/test_app/priv
lrwxrwxrwx 1 phani root 30 Oct 25 12:02 src -> ../../../../_checkouts/test_app/src/

[/_checkouts/test_app]$ ll -d */
drwxr-xr-x  5 phani root   40 Oct 25 10:23 doc/
drwxr-xr-x  2 phani root   24 Oct 25 10:23 priv/
drwxr-xr-x  2 phani root   24 Oct 25 10:23 include/
drwxr-xr-x  2 phani root 4096 Oct 25 13:40 src/
drwxr-xr-x 12 phani root 4096 Oct 25 13:40 sample2/

So when I use this function include_lib(test_app/sample2/include/sam2.hrl)
it couldn't find the file since sample2 folder is not projected to _build/default/checkouts

is there any way we can solve this , sample2 cannot be moved to include because this piece of code needs to be seperated from test_app so I created this extra folder.

[ferd]

I believe the code doing this is

rebar3/src/rebar_prv_compile.erl

Lines 457 to 465 in eabbdd8

	%% link to src_dirs to be adjacent to ebin is needed for R15 use of cover/xref
	%% priv/ and include/ are symlinked unconditionally to allow hooks
	%% to write to them _after_ compilation has taken place when the
	%% initial directory did not, and still work
	[symlink_or_copy(OldAppDir, AppDir, Dir) || Dir <- ["priv", "include"]],
	[symlink_or_copy_existing(OldAppDir, AppDir, Dir) || Dir <- SrcDirs],
	%% copy all extra_src_dirs as they build into themselves and linking means they
	%% are shared across profiles
	[copy(OldAppDir, AppDir, Dir) || Dir <- ExtraDirs];

We basically symlink-or-copy source directories and extra source directories, along with priv/ and ebin/. Ebin/ is copied because it can contain pre-built code or .app files we want to keep around but not clobber. Source directories are symlinked because we want the source files available in _build for operations requiring them (e.g. releases, code coverage) without paying the cost of a copy. The priv/ and include/ is because hooks and plugins may write to these directories as part of compilation but have the artifacts visible and trackable, as the comment shows.

If you declare {extra_src_dirs, ["sample2"]}, I believe it will start being tracked as a visible include and be copied, but not symlinked. If you added it to the src_dirs config it should be symlinked.

As to why none of this is really obvious nor documented, it's because this is likely the first time we encounter this in the wild.