import: Handle small files

When systemd-pull encountered a file shorter than the compression magic
headers it looks for, then it would complete the download in the
analysis state and fail.
When we are still in the analysis state and the download is done, we
know there is no compression and we should leave the analysis state and
continue writing out to disk as usual.

test/units/TEST-72-SYSUPDATE.sh

@@ -552,4 +552,28 @@ EOF
 done
 done
 
+# Make sure the processing of compressed streams still handles uncompressed streams shorter than
+# COMPRESSION_MAGIC_BYTES_MAX correctly.
+rm -rf "$CONFIGDIR" "$WORKDIR/blobs"
+mkdir -p "$CONFIGDIR" "$WORKDIR/blobs"
+printf 'xx' >"$WORKDIR/source/tiny-v1.bin"
+(cd "$WORKDIR/source" && sha256sum tiny-v1.bin >SHA256SUMS)
+cat >"$CONFIGDIR/01-tiny-url.transfer" <<EOF
+[Source]
+Type=url-file
+Path=file://$WORKDIR/source
+MatchPattern=tiny-@v.bin
+
+[Target]
+Type=regular-file
+Path=$WORKDIR/blobs
+MatchPattern=tiny-@v.bin
+InstancesMax=1
+EOF
+"$SYSUPDATE" --verify=no update
+cmp "$WORKDIR/source/tiny-v1.bin" "$WORKDIR/blobs/tiny-v1.bin"
+rm "$CONFIGDIR/01-tiny-url.transfer"
+rm "$WORKDIR/source/tiny-v1.bin"
+rm "$WORKDIR/source/SHA256SUMS"
+
 touch /testok