Undetected cyclic dependency between module extensions

This repository illustrates an issue where Bazel does not detect a cyclic dependency between imported module extensions and instead silently disregards tags on the affected extension.

Expected behavior

You can observe the expected behavior with the following command:

$ (cd module_b; bazel fetch @extension_2//:defs.bzl)
DEBUG: .../module_a~override/extension_1.bzl:7:18: MODULE module_b TAG module_b_tag_1
DEBUG: .../module_a~override/extension_1.bzl:7:18: MODULE module_b TAG module_b_tag_2
DEBUG: .../module_a~override/extension_1.bzl:7:18: MODULE module_a TAG module_a_tag_1
DEBUG: .../module_a~override/extension_1.bzl:7:18: MODULE module_a TAG module_a_tag_2
DEBUG: .../module_a~override/extension_2.bzl:5:10: extension_2 found hello from extension_1, 4 tags

Note that 4 tags of extension_1 are reported.

Faulty behavior

You can observe the faulty behavior by changing the extension imports in module_b as follows:

diff --git a/module_b/MODULE.bazel b/module_b/MODULE.bazel
index 16a8f90..8f57311 100644
--- a/module_b/MODULE.bazel
+++ b/module_b/MODULE.bazel
@@ -3,15 +3,15 @@ module(name = "module_b")
 bazel_dep(name = "module_a")
 local_path_override(module_name = "module_a", path = "../module_a")

-# extension_1 = use_extension("@module_a//:extensions.bzl", "extension_1")
-# extension_2 = use_extension("@module_a//:extensions.bzl", "extension_2")
+extension_1 = use_extension("@module_a//:extensions.bzl", "extension_1")
+extension_2 = use_extension("@module_a//:extensions.bzl", "extension_2")
 # $ (cd module_b; bazel fetch @extension_2//:defs.bzl)
 # DEBUG: .../module_a~override/extension_1.bzl:7:18: MODULE module_a TAG module_a_tag_1
 # DEBUG: .../module_a~override/extension_1.bzl:7:18: MODULE module_a TAG module_a_tag_2
 # DEBUG: .../module_a~override/extension_2.bzl:5:10: extension_2 found hello from extension_1, 2 tags

-extension_1 = use_extension("@module_a//:extension_1.bzl", "extension_1")
-extension_2 = use_extension("@module_a//:extension_2.bzl", "extension_2")
+# extension_1 = use_extension("@module_a//:extension_1.bzl", "extension_1")
+# extension_2 = use_extension("@module_a//:extension_2.bzl", "extension_2")
 # $ (cd module_b; bazel fetch @extension_2//:defs.bzl)
 # DEBUG: .../module_a~override/extension_1.bzl:7:18: MODULE module_b TAG module_b_tag_1
 # DEBUG: .../module_a~override/extension_1.bzl:7:18: MODULE module_b TAG module_b_tag_2

And then executing the following command:

$ (cd module_b; bazel fetch @extension_2//:defs.bzl)
DEBUG: .../module_a~override/extension_1.bzl:7:18: MODULE module_a TAG module_a_tag_1
DEBUG: .../module_a~override/extension_1.bzl:7:18: MODULE module_a TAG module_a_tag_2
DEBUG: .../module_a~override/extension_2.bzl:5:10: extension_2 found hello from extension_1, 2 tags

Note that no only 2 tags of extension_1 are reported. The tags defined in module_b are lost.

For reference

Bazel does detect the cyclic dependency from the context of module_a which defines the module extensions extension_1 and extension_2.

First modify the imports in module_a as follows:

diff --git a/module_a/MODULE.bazel b/module_a/MODULE.bazel
index 5ff4cb7..11259b0 100644
--- a/module_a/MODULE.bazel
+++ b/module_a/MODULE.bazel
@@ -1,6 +1,6 @@
 module(name = "module_a")

-# extension_1 = use_extension("//:extensions.bzl", "extension_1")
+extension_1 = use_extension("//:extensions.bzl", "extension_1")
 # ```
 # $ (cd module_a; bazel fetch @extension_1//:defs.bzl)
 # ERROR: Circular definition of repositories generated by module extensions and/or .bzl files:
@@ -13,7 +13,7 @@ module(name = "module_a")
 # ERROR: cycles detected during target parsing
 # ```

-extension_1 = use_extension("//:extension_1.bzl", "extension_1")
+# extension_1 = use_extension("//:extension_1.bzl", "extension_1")

 extension_1.tag(name = "module_a_tag_1")
 extension_1.tag(name = "module_a_tag_2")

Then execute the following command to trigger the error:

$ (cd module_a; bazel fetch @extension_1//:defs.bzl)
ERROR: Circular definition of repositories generated by module extensions and/or .bzl files:
.-> @_main~extension_1~extension_1
|   extension 'extension_1' defined in //:extensions.bzl
|   //:extensions.bzl
|   //:extension_2.bzl
|   @_main~extension_1~extension_1//:defs.bzl
`-- @_main~extension_1~extension_1
ERROR: cycles detected during target parsing