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Makefile: fix build when $(O) ends in _defconfig

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Commit e6195c5304 (Makefile: fix use of many br2-external trees) fixed
a slowdown with many br2-external trees. In doing so, it changed the
type of the %_defconfig rule: the stem is no longer present in the
prerequisites, so it changes from a pattern rule to an implicit pattern
rule [0].

It is not unusual to name the build directory after the defconfig that
is being built, so we may end up with a build directory named
meh_defconfig. Before e6195c5304, the pattern rule would not match
[1], but now it does, which causes somewhat-cryptic build failures:

    Makefile:1015: *** "Can't find /some/path/meh_defconfig".  Stop.

The issue is that we have this set of rules and assignments (elided and
reordered for legibility):

    all: world
    world: target-post-image
    target-post-image: staging-finalize
    staging-finalize: $(STAGING_DIR_SYMLINK)
    $(STAGING_DIR_SYMLINK): | $(BASE_DIR)
    BASE_DIR := $(CANONICAL_O)
    CANONICAL_O := $(shell mkdir -p $(O) >/dev/null 2>&1)$(realpath $(O))

So, there is a rule that (eventually) has a dependency on $(O), but we
have no rule that provides it explicitly, so the %_defconfig rule kicks
in, with the stem as "/some/path/meh". When the loop searches all the
".../configs/" directories for a file named ".../configs/%_defconfig",
it actually looks for a file named ".../configs//some/path/meh_defconfig"
and that indeed never matches anything.

The solution is to provide an actual rule for $(BASE_DIR), so that the
implicit rule does not kick in.

[0] Terminology and behaviour in make is hard, so the terms we used here
    may be wrong or incorrectly used, and/or the explanations for the
    behaviour be wrong or incomplete... Still, the reasoning stands, and
    the root cause is the removal of the stem in the RHS of the rule
    (adding one back does fix the issue).

[1] not sure how the prerequisite was solved  before e6195c5304,
    though...

Fixes: e6195c5304

Reported-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Nevo Hed <nhed+buildroot@starry.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Tested-by: Sebastian Weyer <sebastian.weyer@smile.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit 5bd2cafc32)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
This commit is contained in:
Yann E. MORIN 2023-01-22 10:09:47 +01:00 committed by Peter Korsgaard
parent a3fd22e80c
commit 2759b8a308
1 changed files with 1 additions and 1 deletions
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Makefile
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@ -1041,7 +1041,7 @@ savedefconfig: $(BUILD_DIR)/buildroot-config/conf outputmakefile
# staging and target directories do NOT list these as
# dependencies anywhere else
$(BUILD_DIR) $(BASE_TARGET_DIR) $(HOST_DIR) $(BINARIES_DIR) $(LEGAL_INFO_DIR) $(REDIST_SOURCES_DIR_TARGET) $(REDIST_SOURCES_DIR_HOST) $(PER_PACKAGE_DIR):
$(BASE_DIR) $(BUILD_DIR) $(BASE_TARGET_DIR) $(HOST_DIR) $(BINARIES_DIR) $(LEGAL_INFO_DIR) $(REDIST_SOURCES_DIR_TARGET) $(REDIST_SOURCES_DIR_HOST) $(PER_PACKAGE_DIR):
@mkdir -p $@
# outputmakefile generates a Makefile in the output directory, if using a