test for #3920 (improper caching of tarballs in build)

See https://github.com/containers/buildah/pull/1955

I've confirmed that this test fails under podman-1.6.2-2.fc30
and passes under current master.

Signed-off-by: Ed Santiago <santiago@redhat.com>

test/system/070-build.bats

@@ -32,6 +32,54 @@ EOF
     run_podman rmi -f build_test
 }
 
+# Regression from v1.5.0. This test passes fine in v1.5.0, fails in 1.6
+@test "podman build - cache (#3920)" {
+    if is_remote && is_rootless; then
+        skip "unreliable with podman-remote and rootless; #2972"
+    fi
+
+    # Make an empty test directory, with a subdirectory used for tar
+    tmpdir=$PODMAN_TMPDIR/build-test
+    run mkdir -p $tmpdir/subtest || die "Could not mkdir $tmpdir/subtest"
+
+    echo "This is the ORIGINAL file" > $tmpdir/subtest/myfile1
+    run tar -C $tmpdir -cJf $tmpdir/myfile.tar.xz subtest
+
+    cat >$tmpdir/Dockerfile <<EOF
+FROM $IMAGE
+ADD myfile.tar.xz /
+EOF
+
+    # One of: ADD myfile /myfile or COPY . .
+    run_podman build  -t build_test -f $tmpdir/Dockerfile $tmpdir
+    is "$output" ".*STEP 3: COMMIT" "COMMIT seen in log"
+    if [[ "$output" =~ "Using cache" ]]; then
+        is "$output" "[no instance of 'Using cache']" "no cache used"
+    fi
+    iid=${lines[-1]}
+
+    run_podman run --rm build_test cat /subtest/myfile1
+    is "$output"   "This is the ORIGINAL file" "file contents, first time"
+
+    # Step 2: Recreate the tarfile, with new content. Rerun podman build.
+    echo "This is a NEW file" >| $tmpdir/subtest/myfile2
+    run tar -C $tmpdir -cJf $tmpdir/myfile.tar.xz subtest
+
+    run_podman build -t build_test -f $tmpdir/Dockerfile $tmpdir
+    is "$output" ".*STEP 3: COMMIT" "COMMIT seen in log"
+
+    # Since the tarfile is modified, podman SHOULD NOT use a cached layer.
+    if [[ "$output" =~ "Using cache" ]]; then
+        is "$output" "[no instance of 'Using cache']" "no cache used"
+    fi
+
+    # Pre-buildah-1906, this fails with ENOENT because the tarfile was cached
+    run_podman run --rm build_test cat /subtest/myfile2
+    is "$output"   "This is a NEW file" "file contents, second time"
+
+    run_podman rmi -f build_test $iid
+}
+
 function teardown() {
     # A timeout or other error in 'build' can leave behind stale images
     # that podman can't even see and which will cascade into subsequent