From 66f4ab2be323119488a5671142790381f193a25d Mon Sep 17 00:00:00 2001
From: Andreas Herrmann <andreas.herrmann@tweag.io>
Date: Thu, 23 Feb 2023 15:56:42 +0100
Subject: [PATCH] Add README.md to explain the issue

---
 README.md | 109 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 109 insertions(+)
 create mode 100644 README.md

diff --git a/README.md b/README.md
new file mode 100644
index 0000000..551a82d
--- /dev/null
+++ b/README.md
@@ -0,0 +1,109 @@
+# 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
+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
+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
+```