cmd-build: Always use fresh image.json

When importing the previous commit during `cosa build`, we would clobber
`image.json` file we just derived with the one from the imported commit.
Fix this by telling `import_ostree_commit()` to skip JSON extraction on
this execution path, but leave it as-is for other execution paths.

src/cmd-build

@@ -193,7 +193,8 @@ if [ -n "${previous_commit}" ]; then
     commitpartial=${tmprepo}/state/${previous_commit}.commitpartial
     if [ ! -f "${commitpath}" ] || [ -f "${commitpartial}" ]; then
         if [ -f "${previous_builddir}/${previous_ostree_tarfile_path}" ]; then
-            import_ostree_commit_for_build "${previous_build}"
+            # don't extract the image.json though, keep the one we generated during prepare_build above
+            import_ostree_commit_for_build "${previous_build}" 0
         else
             # ok, just fallback to importing the commit object only
             mkdir -p "$(dirname "${commitpath}")"

src/cmdlib.sh

@@ -1048,9 +1048,10 @@ cmdlib.write_image_json('${srcfile}', '${outfile}')")
 
 # API to prepare image builds.
 # Ensures that the tmp/repo ostree repo is initialized,
-# and also writes tmp/image.json.
+# and also writes tmp/image.json if arg2 is unset or set to 1
 import_ostree_commit_for_build() {
     local buildid=$1; shift
+    local extractjson=${1:-1}; shift
     (python3 -c "
 import sys
 sys.path.insert(0, '${DIR}')
@@ -1060,7 +1061,7 @@ workdir = '${workdir:-$(pwd)}'
 builds = Builds(workdir)
 builddir = builds.get_build_dir('${buildid}')
 buildmeta = builds.get_build_meta('${buildid}')
-cmdlib.import_ostree_commit(workdir, builddir, buildmeta)
+cmdlib.import_ostree_commit(workdir, builddir, buildmeta, ${extractjson})
 ")
 }
 

src/cosalib/cmdlib.py

@@ -277,7 +277,7 @@ def extract_image_json(workdir, commit):
 # a metal image, we may not have preserved that cache.
 #
 # Call this function to ensure that the ostree commit for a given build is in tmp/repo.
-def import_ostree_commit(workdir, buildpath, buildmeta):
+def import_ostree_commit(workdir, buildpath, buildmeta, extract_json=1):
     tmpdir = os.path.join(workdir, 'tmp')
     with Lock(os.path.join(workdir, 'tmp/repo.import.lock'),
               lifetime=LOCK_DEFAULT_LIFETIME):
@@ -294,7 +294,8 @@ def import_ostree_commit(workdir, buildpath, buildmeta):
                             stdout=subprocess.DEVNULL,
                             stderr=subprocess.DEVNULL) == 0
                 and not os.path.isfile(commitpartial)):
-            extract_image_json(workdir, commit)
+            if extract_json == 1:
+                extract_image_json(workdir, commit)
             return
 
         print(f"Extracting {commit}")
@@ -320,7 +321,8 @@ def import_ostree_commit(workdir, buildpath, buildmeta):
                 subprocess.check_call(['ostree', f'--repo={repo}', 'pull-local', tmpd, buildmeta['buildid']])
 
         # Also extract image.json since it's commonly needed by image builds
-        extract_image_json(workdir, commit)
+        if extract_json == 1:
+            extract_image_json(workdir, commit)
 
 
 def get_basearch():