From b70856af8aa34b1af6fafcc6d86f840d4f47c0fa Mon Sep 17 00:00:00 2001
From: Jan-Erik Rediger <jrediger@mozilla.com>
Date: Thu, 19 Aug 2021 14:42:02 +0200
Subject: [PATCH] Force a compile failure when multiple Glean versions are
 detected in the build

Because of the native code shipped it's essential that all downstream
users rely on the same Glean version.
Gradle can be overeager in chosing a single version when multiple are in
the dependency tree, always picking the latest, even across potentially
breaking-change (that is: major) releases.

Gradle has a way to influence dependency resolution and can even
disallow differing versions using `failOnVersionConflict`.
But that applies to all dependencies and there's no native way to
enforce that for a single dependency.
So the recommended solution[1] is to "manually" check if a dependency was
resolved from multiple versions and raise an issue.

For the ease of consumption by users, such as Fenix, we can apply this
in the Gradle plugin, because that's guaranteed to execute as part of
the build (because if not how are they even using Glean?)

[1]: https://github.com/gradle/gradle/issues/8813
---
 CHANGELOG.md                                           |  1 +
 .../glean-gradle-plugin/GleanGradlePlugin.groovy       | 10 ++++++++++
 2 files changed, 11 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 039fd0073..973580ddd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,7 @@
 
 * Android
   * Updated to Kotlin 1.5, Android Gradle Plugin 4.2.2 and Gradle 6.7.1 ([#1747](https://github.com/mozilla/glean/pull/1747))
+  * The `glean-gradle-plugin` now forces a compile failure when multiple Glean versions are detected in the build ([#1756](https://github.com/mozilla/glean/pull/1756))
 
 # v40.0.0 (2021-07-28)
 
diff --git a/gradle-plugin/src/main/groovy/mozilla/telemetry/glean-gradle-plugin/GleanGradlePlugin.groovy b/gradle-plugin/src/main/groovy/mozilla/telemetry/glean-gradle-plugin/GleanGradlePlugin.groovy
index 2e6392aac..05fb7f1cc 100644
--- a/gradle-plugin/src/main/groovy/mozilla/telemetry/glean-gradle-plugin/GleanGradlePlugin.groovy
+++ b/gradle-plugin/src/main/groovy/mozilla/telemetry/glean-gradle-plugin/GleanGradlePlugin.groovy
@@ -567,6 +567,16 @@ except:
                 }
                 because 'use GeckoView Glean instead of standalone Glean'
             }
+
+            incoming.afterResolve {
+                resolutionResult.allComponents {
+                    if (selectionReason.conflictResolution && moduleVersion != null) {
+                        if (moduleVersion.group.contains("org.mozilla.telemetry") && moduleVersion.name.contains("glean")) {
+                            throw AssertionError("Cannot have a conflict on Glean ${selectionReason}")
+                        }
+                    }
+                }
+            }
         }
 
         if (project.android.hasProperty('applicationVariants')) {