scanner: don't skip directory entries with type `DT_UNKNOWN`

According to the readdir(2) man-page, not all file-systems support returning the entry's file-type in d_type. For example, the reprotest reproducibility tool, uses the disorderfs FUSE file-system to shuffle the order in which directory entries are returned, and this does not set d_type. Therefore, in addition to entries with type DT_DIR and DT_LNK, also process entries with type DT_UNKNOWN.

For example, install disorderfs:

# apt-get install disorderfs

Mount mu over it:

$ mkdir mu-disordered
$ disorderfs mu mu-disordered
disorderfs: reversing directory entries

Build mu:

$ cd mu-disordered/
$ meson setup build .
The Meson build system
Version: 1.4.0
Source dir: /space/azazel/work/git/repos/github/a3a3el/mu-disordered
Build dir: /space/azazel/work/git/repos/github/a3a3el/mu-disordered/build
Build type: native build
Project name: mu
Project version: 1.12.4
[snip]
$ ninja -C build
ninja: Entering directory `build'
[snip]

Then run the test-suite:

$ ninja -C build test
ninja: Entering directory `build'
[0/1] Running all tests.
 1/46 test-sexp                     OK              0.11s
 2/46 test-regex                    OK              0.11s
 3/46 test-command-handler          OK              0.10s
 4/46 test-utils-file               OK              0.10s
 5/46 test-option                   OK              0.09s
 6/46 test-lang-detector            OK              0.09s
 7/46 test-html-to-text             OK              0.08s
 8/46 test-error                    OK              0.07s
 9/46 test-mu-utils                 OK              0.07s
10/46 test-contact                  OK              0.06s
11/46 test-fields                   OK              0.04s
12/46 test-flags                    OK              0.03s
13/46 test-priority                 OK              0.02s
14/46 test-message                  OK              0.21s
15/46 test-message-file             OK              0.20s
16/46 test-message-part             OK              0.19s
17/46 test-threads                  OK              0.18s
18/46 test-contacts-cache           OK              0.17s
19/46 test-config                   OK              0.16s
20/46 test-query-macros             OK              0.15s
21/46 test-query-processor          OK              0.14s
22/46 test-query-parser             OK              0.13s
23/46 test-maildir                  OK              0.24s
24/46 test-query-xapianizer         OK              0.32s
25/46 test-document                 OK              0.46s
26/46 test-msg                      OK              0.23s
27/46 test-cmd-mkdir                OK              0.07s
28/46 test-cmd-verify               OK              0.05s
29/46 test-cmd-extract              OK              0.26s
30/46 test-xapian-db                OK              0.48s
31/46 test-cmd-view                 OK              0.17s
32/46 test-logger                   OK              1.01s
33/46 test-scanner                  FAIL            0.80s   killed by signal 6 SIGABRT
[snip]
34/46 test-cmd-init                 OK              0.90s
35/46 test-query                    OK              2.42s
36/46 test-cmd-remove               OK              2.45s
37/46 test-cmd-cfind                OK              2.99s
38/46 test-cmd-find                 OK              3.29s
39/46 test-cmd-add                  OK              3.77s
40/46 test-store                    FAIL            8.67s   killed by signal 6 SIGABRT
[snip]
41/46 test-mu-guile                 OK              9.11s
42/46 test-indexer                  OK             10.37s
43/46 test-cmd-index                OK             10.45s
44/46 test-cmd-move                 OK             10.89s
45/46 test-cmd-query                OK             12.33s
46/46 test-store-query              OK             18.12s

Summary of Failures:

33/46 test-scanner          FAIL            0.80s   killed by signal 6 SIGABRT
40/46 test-store            FAIL            8.67s   killed by signal 6 SIGABRT

Ok:                 44
Expected Fail:      0
Fail:               2
Unexpected Pass:    0
Skipped:            0
Timeout:            0

Full log written to /space/azazel/work/git/repos/github/a3a3el/mu-disordered/build/meson-logs/testlog.txt
FAILED: meson-internal__test
/usr/bin/meson test --no-rebuild --print-errorlogs
ninja: build stopped: subcommand failed.

In both cases, scanning testdir2 returned no entries, e.g.:

# mu-DEBUG: starting scan @ /space/azazel/work/git/repos/github/a3a3el/mu-disordered/testdata/testdir2
# mu-DEBUG: finished scan of /space/azazel/work/git/repos/github/a3a3el/mu-disordered/testdata/testdir2 in 0 ms
not ok /scanner/count-maildirs - ERROR:../lib/mu-scanner.cc:365:void test_count_maildirs(): assertion failed (dirs.size() == 3): (0 == 3)
Bail out!

Here's an example Debian CI test failure: https://salsa.debian.org/emacsen-team/maildir-utils/-/jobs/5635549

djcb / mu

scanner: don't skip directory entries with type `DT_UNKNOWN` #2703