In the Salsa CI for ncurses a rather strange failure happens in the
reprotest job[1]:

,----
| dh_missing -i --fail-missing
| dh_missing: warning: usr/lib32/libncurses.so exists in debian/tmp but is not installed to anywhere (related file: "debian/tmp/usr/lib/x86_64-linux-gnu/libncurses.so")
| dh_missing: warning: usr/lib32/libncursesw.so exists in debian/tmp but is not installed to anywhere (related file: "debian/tmp/usr/lib/x86_64-linux-gnu/libncursesw.so")
| dh_missing: error: missing files, aborting
|
| 	While detecting missing files, dh_missing noted some files with a similar name to those
| 	that were missing.  This error /might/ be resolved by replacing references to the
| 	missing files with the similarly named ones that dh_missing found - assuming the content
| 	is identical.
|
| 	As an example, you might want to replace:
| 	 * debian/tmp/usr/lib/x86_64-linux-gnu/libncurses.so
| 	with:
| 	 * usr/lib32/libncurses.so
| 	in a file in debian/ or as argument to one of the dh_* tools called from debian/rules.
| 	(Note it is possible the paths are not used verbatim but instead directories
| 	containing or globs matching them are used instead)
|
| 	Alternatively, add the missing file to debian/not-installed if it cannot and should not
| 	be used.
|
| 	The following debhelper tools have reported what they installed (with files per package)
| 	 * dh_install: lib32ncurses-dev (21), lib32ncurses6 (8), lib32ncursesw6 (8), lib32tinfo6 (4), lib64ncurses-dev (1), lib64ncurses6 (0), lib64ncursesw6 (0), lib64tinfo6 (0), libncurses-dev (29), libncurses6 (8), libncursesw6 (8), libncursesw6-udeb (2), libtinfo6 (4), libtinfo6-udeb (2), ncurses-base (46), ncurses-bin (4), ncurses-doc (1), ncurses-examples (1), ncurses-term (1)
| 	 * dh_installdocs: lib32ncurses-dev (0), lib32ncurses6 (0), lib32ncursesw6 (0), lib32tinfo6 (0), lib64ncurses-dev (0), lib64ncurses6 (0), lib64ncursesw6 (0), lib64tinfo6 (0), libncurses-dev (0), libncurses6 (0), libncursesw6 (0), libtinfo6 (0), ncurses-base (1), ncurses-bin (0), ncurses-doc (3), ncurses-examples (0), ncurses-term (0)
| 	If the missing files are installed by another tool, please file a bug against it.
| 	When filing the report, if the tool is not part of debhelper itself, please reference the
| 	"Logging helpers and dh_missing" section from the "PROGRAMMING" guide for debhelper (10.6.3+).
| 	  (in the debhelper package: /usr/share/doc/debhelper/PROGRAMMING.gz)
| 	Be sure to test with dpkg-buildpackage -A/-B as the results may vary when only a subset is built
| 	If the omission is intentional or no other helper can take care of this consider adding the
| 	paths to debian/not-installed.
| make: *** [debian/rules:489: binary-indep] Error 25
| make: *** Waiting for unfinished jobs....
`----

The offending two files are created in the install-arch target in
debian/rules, installed in the first two lines of the
lib32ncurses-dev.install file, and since "make" even finished the
binary-arch target I could download the artifacts[2] and convince myself
that they landed in the lib32ncurses-dev package as intended.

I recall that I had seen this error occasionally in my local builds, but
was (and still am) not able to reproduce it :-(.  Maybe there is also an
error in the ncurses package that I do not see.  Any help is appreciated.


1. https://salsa.debian.org/debian/ncurses/-/jobs/4378348
2. https://salsa.debian.org/debian/ncurses/-/jobs/4378348/artifacts/download

Control: tags -1 + unreproducible
rescheduled the last one (for commit 1c832772[1]) on 2023-06-30 and it
succeeded.  Commit 78b0c63b[2] should sidestep this problem in the
future.

Clearly there is some kind of race condition involved here, but I have
not been able to figure out what it is, and will probably not spend more
time on it unless somebody gives me a hand.


1. https://salsa.debian.org/debian/ncurses/-/commit/1c832772028
2. https://salsa.debian.org/debian/ncurses/-/commit/78b0c63b5a8

Control: tags -1 - unreproducible

That was successful, but after some changes to ncurses' debian/rules
today I got another dh_missing failure in the reprotest job[1].

I think I am beginning to understand where the race condition is.  In
full builds of ncurses it is possible for 'make' to process the
install-arch and binary-indep targets in parallel, which can cause new
files to appear in debian/tmp after "dh_missing -i" has processed the
debian/.debhelper/generated/*/installed-by-* files, but before it
actually reads the contents of debian/tmp.  Those files will then be
falsely reported as not installed.

Probably such a situation is relatively unusual, and I guess dh_missing
has not been designed to handle it.  Will see how best to work around it
in ncurses.

Sven


1. https://salsa.debian.org/debian/ncurses/-/jobs/4400129

Sven Joachim:

Hi Sven

Thanks for the report and diagnosing the problem.

Indeed, debhelper was not never built for arbitrary parallelization of
dh commands or interactions between the commands.  The `dh_missing` tool
is a lot more susceptible to the problem as it tries to do a "global
view" analysis


A proper fix in your case is to split the binary-X targets, inject a
synchronization barrier and run dh_missing on the other side of it. I
have no clue how to do that reliably in Make while keeping the target
dependencies correct.


Unfortunately, there is not much I can do to fix this in debhelper as
debhelper is 80% married to d/rules being a Makefile with arbitrary code
execution injected "anywhere" and then 20% of features that need this
not to be the case.
   Indeed, it is a bug/misfeature, but I do not see a way to solve it.

Best regards,
Niels