[OE-core] question about order of processing SRC_URI items

[Robert P. J. Day]

  i'm currently building up a BSP layer which is going to involve
quite a bit of FILESEXTRAPATHS and OVERRIDES and conditionals in my
kernel bbappend files so i want to make sure i understand the
processing order (or what there is of it).

  first, if one simply appends items to SRC_URI, as in:

  SRC_URI += " \
	derp.scc \
	gorp.scc \
	...
  "

then regardless of how FILESOVERRIDES adjusts the search order in the
eventual FILESPATH, those items *must* be found somewhere, correct?
(this seems pretty obvious to me, but i've been burned by obvious
things before.)

  next, i realize there are two forms of appending to SRC_URI:

 * SRC_URI +=
 * SRC_URI_append =

first, is there an aesthetic preference for one over the other if the
end result is the same? (just curious about what people here prefer to
use.)

  more significantly, i realize the final order of SRC_URI items
depending on which one you use might be different, as "_append" leaves
the appending until the final stage of processing. so what this tells
me is that all of the items you list in SRC_URI should *not* be
order-dependent, is that a fair statement? or, IOW, if your layer
depends on the items in SRC_URI being processed in a specific order,
you probably need to rework your recipe file.

  (side note: i do realize that, in a single .scc file, the patches
listed will be applied in precisely that order, but that's a different
issue and not relevant here.)

  next, if there are truly conditional SRC_URI items, the only way to
identify those is:

  SRC_URI_append_<override> = " ... blah blah ..."

you can't do this, can you?

  SRC_URI_<override> += " more stuff "

i don't think i've ever seen an example of the latter, but maybe i
just didn't look closely enough.

  and, finally, given that you can have a combination of SRC_URI
assignments and appends in the same .bbappend file:

  SRC_URI += "..."
  SRC_URI_append = "..."
  SRC_URI_append_<override> = "..."

that tells me that you *really* don't want any of that to be
order-dependent, as i mentioned before.

  is that about it? am i missing anything about SRC_URI definition and
processing? thanks muchly.

rday

p.s.  oh, wait, two more curiosities. first, i still see the
occasional example of this in numerous layers, including oe-core:

    meta/recipes-support/attr/attr_2.4.47.bb:
        SRC_URI_append += "file://attr-Missing-configure.ac.patch \
        ^^^^^^^^^^^^^^^^^

pretty sure combining "SRC_URI_append" with "+=" is just as redundant
as ever, no? :-)

  also, if the list of items in SRC_URI should be order-independent,
then it would seem to make little sense to specifically use:

  SRC_URI_prepend = "..."

anywhere, right? IOW, there should be no situation in which
specifically *prepending* to SRC_URI is necessary. but i found one
example in oe-core:

  meta/recipes-devtools/qemu/qemu_2.5.0.bb:

SRC_URI += "file://configure-fix-Darwin-target-detection.patch \
            file://qemu-enlarge-env-entry-size.patch \
            file://Qemu-Arm-versatilepb-Add-memory-size-checking.patch \
            file://no-valgrind.patch \
            file://CVE-2016-1568.patch \
            file://CVE-2016-2197.patch \
            file://CVE-2016-2198.patch \
            file://pathlimit.patch \
           "
SRC_URI_prepend = "http://wiki.qemu-project.org/download/${BP}.tar.bz2"
SRC_URI[md5sum] = "f469f2330bbe76e3e39db10e9ac4f8db"
SRC_URI[sha256sum] = "3443887401619fe33bfa5d900a4f2d6a79425ae2b7e43d5b8c36eb7a683772d4"

  uh ... yeah, i get that the above does the right thing but, come on,
that just looks weird. is there a reason the qemu recipe deliberately
goes out of its way to do this?

  ok, back to work ...