build: use docker_squash lib, not docker-squash script

The docker-squash script used to print the squashed <image id> on
stderr, but recently it was changed so the image id is printed to
stdout.  To be able to parse image id from both docker_squash
versions (before and after this change), we'd have to have some
ugly if-fork to detect the docker_squash version (or play with FD
redirection).

So rather let's do as upstream suggested in [1] and use the
docker_squash library directly.

Also add a simple CI test for the ./squash.py file.

[1] goldmann/docker-squash#145

Fixes: #101

Makefile

@@ -27,6 +27,10 @@ test: check
 
 check-failures: check-test-lib
 	cd tests/failures/check && make tag && ! make check && make clean
+	cd tests/failures/check && make tag SKIP_SQUASH=0
 
-check: check-failures
+check-squash:
+	./tests/squash/squash.sh
+
+check: check-failures check-squash
 	TESTED_IMAGES="$(TESTED_IMAGES)" tests/remote-containers.sh

build.sh

@@ -8,6 +8,9 @@
 
 set -e
 
+script_name=$(readlink -f "$0")
+script_dir=$(dirname "$script_name")
+
 OS=${1-$OS}
 VERSION=${2-$VERSION}
 
@@ -111,13 +114,13 @@ function docker_build_with_version {
   clean_image
   echo "$IMAGE_ID" > .image-id.raw
 
-  squash "${dockerfile}" "$IMAGE_ID"
+  squash "${dockerfile}"
   echo "$IMAGE_ID" > .image-id
 }
 
 # squash DOCKERFILE
 # -----------------
-# Install the docker squashing tool[1] and squash the result image
+# Use python library docker_squash[1] and squash the result image
 # when necessary.
 # [1] https://github.com/goldmann/docker-squash
 # Reads:
@@ -129,8 +132,6 @@ squash ()
   local base squashed_from squashed= unsquashed=$IMAGE_ID
   test "$SKIP_SQUASH" = 1 && return 0
 
-  docker-squash --help &>/dev/null || error "docker-squash is required"
-
   if test -f .image-id.squashed; then
       squashed=$(cat .image-id.squashed)
       # We (maybe) already have squashed file.
@@ -139,6 +140,7 @@ squash ()
           if test "$squashed_from" = "$IMAGE_ID"; then
               # $squashed is up2date
               IMAGE_ID=$squashed
+              echo "Image '$unsquashed' already squashed as '$squashed'"
               return 0
           fi
       fi
@@ -149,9 +151,9 @@ squash ()
   fi
 
   base=$(awk '/^FROM/{print $2}' "$1")
-  parse_output "docker-squash -f '$base' '$unsquashed'" \
-               "awk '/New squashed image ID is/{print \$NF}'" \
-               IMAGE_ID stderr
+
+  echo "Squashing the image '$unsquashed' from '$base' layer"
+  IMAGE_ID=$("${PYTHON-python3}" "$script_dir"/squash.py "$unsquashed" "$base")
 
   echo "$unsquashed" > .image-id.squashed_from
   echo "$IMAGE_ID" > .image-id.squashed

squash.py

@@ -0,0 +1,23 @@
+#! /bin/python
+
+import sys
+import logging
+from platform import python_version
+
+try:
+    from docker_squash import squash
+except ImportError:
+    logging.fatal("please install 'docker_squash' for Python {0}".format(python_version()))
+    sys.exit(1)
+
+
+if __name__ == "__main__":
+    if len(sys.argv) != 3:
+        logging.fatal("%s: %s", sys.argv[0], msg)
+        sys.exit(1)
+
+    print(squash.Squash(
+        log=logging.getLogger(),
+        image=sys.argv[1],
+        from_layer=sys.argv[2]).run()
+    )

tests/squash/.gitignore

@@ -0,0 +1 @@
+Dockerfile

tests/squash/squash.sh

@@ -0,0 +1,18 @@
+#! /bin/sh
+
+set -e
+
+origin=busybox
+squash=$(dirname "$(readlink -f "$0")")/../../squash.py
+cd "$(dirname "$0")"
+
+cat > Dockerfile <<EOF
+FROM $origin
+ENV test=test
+CMD /bin/echo test
+EOF
+out=`docker build . | awk '/Successfully built/{print $NF}'`
+echo "$out"
+squashed=$("${PYTHON-python3}" "$squash" "$out" "$origin")
+output=$(docker run --rm $squashed)
+test "$output" = "test"