_project.py: Add check for projects not written for BuildStream 1.

This checks the `min-version` added in BuildStream 2 to raise an
error to the user informing them of what BuildStream version they
should use for this project.

This also adds the _parse_version() utility to utils.py in order
to properly report the BuildStream version specified by project.conf.

2 files changed, 54 insertions, 0 deletions

diff --git a/buildstream/_project.py b/buildstream/_project.py
index 5530aa23d..0f327c66d 100644
--- a/buildstream/_project.py
+++ b/buildstream/_project.py
@@ -401,6 +401,37 @@ class Project():
                 "Project requested format version {}, but BuildStream {}.{} only supports up until format version {}"
                 .format(format_version, major, minor, BST_FORMAT_VERSION))
 
+        # Since BuildStream 2, project.conf is required to specify min-version.
+        #
+        # Detect this and raise an error, indicating which major version of BuildStream
+        # should be used for this project.
+        #
+        min_version = _yaml.node_get(pre_config_node, str, 'min-version', default_value=None)
+        if min_version:
+
+            # Handle case of malformed min-version
+            #
+            try:
+                major, minor = utils._parse_version(min_version)
+            except UtilError as e:
+                raise LoadError(
+                    LoadErrorReason.UNSUPPORTED_PROJECT,
+                    "This is not a BuildStream 1 project: {}".format(e)
+                ) from e
+
+            # Raise a helpful error indicating what the user should do to
+            # use this project.
+            #
+            raise LoadError(
+                LoadErrorReason.UNSUPPORTED_PROJECT,
+                "Tried to load a BuildStream {} project with BuildStream 1".format(major),
+
+                # TODO: Include a link to the appropriate documentation for parallel
+                # installing different BuildStream versions.
+                #
+                detail="Please install at least BuildStream {}.{} to use this project".format(major, minor)
+            )
+
         # FIXME:
         #
         #   Performing this check manually in the absense
diff --git a/buildstream/utils.py b/buildstream/utils.py
index d02777897..3a2372262 100644
--- a/buildstream/utils.py
+++ b/buildstream/utils.py
@@ -1207,3 +1207,26 @@ def _deduplicate(iterable, key=None):
             if k not in seen:
                 seen_add(k)
                 yield element
+
+
+# _parse_version():
+#
+# Args:
+#    version (str): The file name from which to determine compression
+#
+# Returns:
+#    A 2-tuple of form (major_version: int, minor_version: int)
+#
+# Raises:
+#    UtilError: In the case of a malformed version string
+#
+def _parse_version(version):
+
+    versions = version.split(".")
+    try:
+        major = int(versions[0])
+        minor = int(versions[1])
+    except (IndexError, ValueError):
+        raise UtilError("Malformed version string: {}".format(version),)
+
+    return (major, minor)