system tests: address COPY-hardlink flake

Possible cause: on Debian, maybe because of fuse-overlayfs(??),
we sometimes see unexpected inode numbers.

This PR tightens the test logic, so it runs one 'stat' command
in only one podman invocation, then cross-checks multiple lines
of output. I don't know if this will really fix the flake, but
even if it doesn't, it will at least give us much more useful
diagnostic output than before.

And, as long as I'm in here, clean up test, remove duplication,
make error messages distinct (hence more useful), and comment.

Fixes: #17979

Signed-off-by: Ed Santiago <[email protected]>

test/system/070-build.bats

@@ -941,29 +941,41 @@ EOF
 }
 
 @test "podman build COPY hardlinks " {
-    tmpdir=$PODMAN_TMPDIR/build-test
-    subdir=$tmpdir/subdir
-    subsubdir=$subdir/subsubdir
-    mkdir -p $subsubdir
+    local build_dir=$PODMAN_TMPDIR/build-test
 
-    dockerfile=$tmpdir/Dockerfile
+    mkdir -p $build_dir
+    dockerfile=$build_dir/Dockerfile
     cat >$dockerfile <<EOF
 FROM $IMAGE
 COPY . /test
 EOF
-    ln $dockerfile $tmpdir/hardlink1
-    ln $dockerfile $subdir/hardlink2
-    ln $dockerfile $subsubdir/hardlink3
 
-    run_podman build -t build_test $tmpdir
-    run_podman run --rm build_test stat -c '%i' /test/Dockerfile
-    dinode=$output
-    run_podman run --rm build_test stat -c '%i' /test/hardlink1
-    is "$output"   "$dinode"   "COPY hardlinks work"
-    run_podman run --rm build_test stat -c '%i' /test/subdir/hardlink2
-    is "$output"   "$dinode"   "COPY hardlinks work"
-    run_podman run --rm build_test stat -c '%i' /test/subdir/subsubdir/hardlink3
-    is "$output"   "$dinode"   "COPY hardlinks work"
+    # Create all our hardlinks, including their parent directories
+    local -a linkfiles=(hardlink1 subdir/hardlink2 subdir/subsubdir/hardlink3)
+    for l in "${linkfiles[@]}"; do
+        mkdir -p $(dirname $build_dir/$l)
+        ln $dockerfile $build_dir/$l
+    done
+
+    run_podman build -t build_test $build_dir
+
+    # Stat() all files in one fell swoop, because it seems impossible
+    # for inode numbers to change within the scope of one exec, but
+    # maybe they do across different runs?? fuse-overlay maybe?? #17979
+    run_podman run --rm build_test \
+               stat -c '%i %n' /test/Dockerfile "${linkfiles[@]/#//test/}"
+
+    # First output line is the inode of our reference file and its filename.
+    # Slash-replacement strips off everything after the space.
+    local dinode="${lines[0]/ */}"
+
+    # All subsequent inodes must match the first one. We check filename (%n)
+    # simply out of unwarranted paranoia.
+    local i=1
+    for l in "${linkfiles[@]}"; do
+        assert "${lines[$i]}" = "$dinode /test/$l" "line $i: inode of $l"
+        i=$((i + 1))
+    done
 
     run_podman rmi -f build_test
 }