src/cmd-compress: don't require image.json for decompression

b60dc1f caused the `cosa buildfetch --artifact=ostree && cosa decompress`
path to fail since there was no local ostree repo and no ociarchive
that got downloaded. Let's drop the image.json search into the compress
path only.

While we are here let's call import_ostree_commit (which also calls
extract_image_json) instead of extract_image_json and add a function
called get_image_json() that helps us iterate over the possible
architectures to find a build with all the pieces necessary to get
the image.json.

src/cmd-compress

@@ -13,7 +13,7 @@ sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
 from cosalib.builds import Builds
 from cosalib.meta import GenericBuildMeta
 from cosalib.cmdlib import (
-    extract_image_json,
+    import_ostree_commit,
     ncpu,
     rm_allow_noent,
     runcmd,
@@ -52,24 +52,8 @@ else:
 
 print(f"Targeting build: {build}")
 
-# that's the tool default
-gzip_level = 6
-if args.fast:
-    args.compressor = 'gzip'
-    gzip_level = 1
-elif not args.compressor:
-    workdir = os.getcwd()
-    arches = builds.get_build_arches(build)
-    # image.json isn't arch-dependent, so just extract it from the first arch
-    buildmeta = GenericBuildMeta(workdir=workdir, build=build, basearch=arches[0])
-    extract_image_json(workdir, buildmeta['ostree-commit'])
-    with open(os.path.join(workdir, 'tmp/image.json')) as f:
-        image_json = json.load(f)
-    args.compressor = image_json.get('compressor', DEFAULT_COMPRESSOR)
-
-# find extension for compressor
+# common extensions for known compressors
 ext_dict = {'xz': '.xz', 'gzip': '.gz'}
-ext = ext_dict[args.compressor]
 
 
 def get_cpu_param(param):
@@ -93,6 +77,9 @@ def compress_one_builddir(builddir):
     with open(buildmeta_path) as f:
         buildmeta = json.load(f)
 
+    # Find what extension to use based on the selected compressor
+    ext = ext_dict[args.compressor]
+
     tmpdir = 'tmp/compress'
     if os.path.isdir(tmpdir):
         shutil.rmtree(tmpdir)
@@ -252,8 +239,36 @@ def uncompress_one_builddir(builddir):
     return at_least_one
 
 
+def get_image_json():
+    # All arches might not be local. Find one that has the info.
+    workdir = os.getcwd()
+    for arch in builds.get_build_arches(build):
+        builddir = builds.get_build_dir(build, arch)
+        if not os.path.exists(os.path.join(builddir, 'meta.json')):
+            continue
+        buildmeta = GenericBuildMeta(workdir=workdir, build=build, basearch=arch)
+        if not os.path.exists(os.path.join(builddir, buildmeta['images']['ostree']['path'])):
+            continue
+        import_ostree_commit(workdir, builddir, buildmeta)  # runs extract_image_json()
+        with open(os.path.join(workdir, 'tmp/image.json')) as f:
+            return json.load(f)
+    # If we get here we were unsuccessful
+    print("Could not find/extract image.json. Please pass --compressor\n" +
+          "or make sure local ociarchive exists in the build directory.")
+    raise Exception("Could not find/extract image.json")
+
+
 changed = []
 if args.mode == "compress":
+    # Find what compressor we should use, either picking it up from
+    # CLI args or from image.json
+    gzip_level = 6
+    if args.fast:
+        args.compressor = 'gzip'
+        gzip_level = 1
+    elif not args.compressor:
+        image_json = get_image_json()
+        args.compressor = image_json.get('compressor', DEFAULT_COMPRESSOR)
     for arch in builds.get_build_arches(build):
         builddir = builds.get_build_dir(build, arch)
         changed.append(compress_one_builddir(builddir))