fix(branching): cross-repo PR builds fail

build.gradle.kts

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2024 Figure Technologies
+ * Copyright (C) 2024-2025 Figure Technologies
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.

settings.gradle.kts

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2024 Figure Technologies
+ * Copyright (C) 2024-2025 Figure Technologies
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.

src/functionalTest/kotlin/com/figure/gradle/semver/specs/CrossRepositoryPullRequestSpec.kt

@@ -0,0 +1,127 @@
+/*
+ * Copyright (C) 2024 Figure Technologies
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.figure.gradle.semver.specs
+
+import com.figure.gradle.semver.internal.environment.Env
+import com.figure.gradle.semver.kotest.GradleProjectsExtension
+import com.figure.gradle.semver.kotest.shouldOnlyHave
+import com.figure.gradle.semver.projects.RegularProject
+import com.figure.gradle.semver.projects.SettingsProject
+import com.figure.gradle.semver.projects.SubprojectProject
+import io.kotest.core.extensions.install
+import io.kotest.core.spec.style.FunSpec
+import io.kotest.extensions.system.OverrideMode
+import io.kotest.extensions.system.withEnvironment
+import org.gradle.util.GradleVersion
+
+class CrossRepositoryPullRequestSpec : FunSpec({
+    val projects = install(
+        GradleProjectsExtension(
+            RegularProject(projectName = "regular-project"),
+            SettingsProject(projectName = "settings-project"),
+            SubprojectProject(projectName = "subproject-project"),
+        ),
+    )
+
+    val mainBranch = "main"
+    val developmentBranch = "develop"
+    val forkedFeatureBranch = "fix/builds-fail-cross-repo"
+
+    test("should calculate next version for cross-repository/forked pull request") {
+        withEnvironment(
+            environment = mapOf(
+                Env.CI to "true",
+                Env.GITHUB_HEAD_REF to forkedFeatureBranch,
+            ),
+            mode = OverrideMode.SetOrOverride,
+        ) {
+            // Given: Simulate a cross-repo PR scenario where the feature branch doesn't exist locally
+            // This mimics what happens when GitHub Actions checks out a forked pull request
+            projects.git {
+                initialBranch = mainBranch
+                actions = actions {
+                    commit(message = "1 commit on $mainBranch", tag = "1.0.0")
+
+                    checkout(developmentBranch)
+                    commit(message = "1 commit on $developmentBranch")
+
+                    // Simulate being on main branch (as in cross-repo PRs) but with GITHUB_HEAD_REF set
+                    // to the forked branch name that doesn't exist locally
+                    checkout(mainBranch)
+                }
+            }
+
+            // When: Building should not fail even though the branch name from GITHUB_HEAD_REF doesn't exist locally
+            projects.build(GradleVersion.current())
+
+            // Then: Should generate a version using the forked branch name from GITHUB_HEAD_REF
+            projects.versions shouldOnlyHave "1.0.1-fix-builds-fail-cross-repo.0"
+        }
+    }
+
+    test("should handle cross-repo PR with both GITHUB_HEAD_REF and GITHUB_REF_NAME set") {
+        withEnvironment(
+            environment = mapOf(
+                Env.CI to "true",
+                Env.GITHUB_HEAD_REF to forkedFeatureBranch,
+                Env.GITHUB_REF_NAME to "123/merge", // Typical PR merge ref
+            ),
+            mode = OverrideMode.SetOrOverride,
+        ) {
+            // Given: Similar setup but with both environment variables set
+            projects.git {
+                initialBranch = mainBranch
+                actions = actions {
+                    commit(message = "1 commit on $mainBranch", tag = "1.0.0")
+                    checkout(mainBranch) // Stay on main to simulate cross-repo PR checkout
+                }
+            }
+
+            // When: Building should prioritize GITHUB_HEAD_REF over GITHUB_REF_NAME
+            projects.build(GradleVersion.current())
+
+            // Then: Should use the forked branch name, not the merge ref
+            projects.versions shouldOnlyHave "1.0.1-fix-builds-fail-cross-repo.0"
+        }
+    }
+
+    test("should fallback to GITHUB_REF_NAME when GITHUB_HEAD_REF is empty") {
+        withEnvironment(
+            environment = mapOf(
+                Env.CI to "true",
+                Env.GITHUB_HEAD_REF to "", // Empty but present
+                Env.GITHUB_REF_NAME to "feature-branch-fallback",
+            ),
+            mode = OverrideMode.SetOrOverride,
+        ) {
+            // Given: GITHUB_HEAD_REF is empty (not a PR) but GITHUB_REF_NAME is set
+            projects.git {
+                initialBranch = mainBranch
+                actions = actions {
+                    commit(message = "1 commit on $mainBranch", tag = "1.0.0")
+                    checkout("feature-branch-fallback")
+                    commit(message = "1 commit on feature branch")
+                }
+            }
+
+            // When: Building should use GITHUB_REF_NAME since GITHUB_HEAD_REF is empty
+            projects.build(GradleVersion.current())
+
+            // Then: Should use the ref name from GITHUB_REF_NAME with correct commit count
+            projects.versions shouldOnlyHave "1.0.1-feature-branch-fallback.1"
+        }
+    }
+})

src/main/kotlin/com/figure/gradle/semver/internal/command/Branch.kt

@@ -19,6 +19,7 @@ import com.figure.gradle.semver.internal.command.extension.shortName
 import com.figure.gradle.semver.internal.environment.Env
 import org.eclipse.jgit.api.Git
 import org.eclipse.jgit.lib.Constants
+import org.eclipse.jgit.lib.ObjectId
 import org.eclipse.jgit.lib.Ref
 
 class Branch(
@@ -34,7 +35,23 @@ class Branch(
                 Env.isCI -> Env.githubHeadRef ?: Env.githubRefName
                 else -> git.repository.branch
             }
-            return branchList.find(refName) ?: error("Could not find current branch: $refName")
+            val foundRef = branchList.find(refName)
+            if (foundRef != null) {
+                return foundRef
+            }
+
+            // Handle cross-repo PRs where the branch doesn't exist locally
+            // This happens when GitHub Actions checks out the target repository but the
+            // branch name from GITHUB_HEAD_REF (from forked repo) doesn't exist locally
+            return if (Env.isCI && Env.githubHeadRef != null) {
+                // Create a synthetic ref object that preserves the original branch name
+                // This ensures version calculation uses the actual feature branch name
+                // instead of falling back to HEAD or the merge ref
+                SyntheticRef(name = "refs/heads/${Env.githubHeadRef}", target = headRef.objectId)
+            } else {
+                // For non-CI environments or when not in a PR, use HEAD
+                headRef
+            }
         }
 
     fun isOnMainBranch(providedMainBranch: String? = null): Boolean =
@@ -51,3 +68,28 @@ class Branch(
             .setForce(true)
             .call()
 }
+
+/**
+ * Synthetic Ref implementation for cross-repository PRs where the branch doesn't exist locally
+ * but we want to preserve the original branch name for version calculation
+ */
+private class SyntheticRef(
+    private val name: String,
+    private val target: ObjectId,
+) : Ref {
+    override fun getName(): String = name
+
+    override fun isSymbolic(): Boolean = false
+
+    override fun getLeaf(): Ref = this
+
+    override fun getTarget(): Ref? = null
+
+    override fun getObjectId(): ObjectId = target
+
+    override fun isPeeled(): Boolean = true
+
+    override fun getPeeledObjectId(): ObjectId? = target
+
+    override fun getStorage(): Ref.Storage = Ref.Storage.LOOSE
+}

src/main/kotlin/com/figure/gradle/semver/internal/command/BranchList.kt

@@ -93,6 +93,7 @@ class BranchList(
             ?: git.repository.resolve(baseBranchName)
 
         val targetBranch: ObjectId = git.repository.resolve(targetBranchName)
+            ?: git.repository.resolve("HEAD") // Fall back to HEAD for synthetic refs in cross-repo PRs
 
         return git.revWalk { revWalk ->
             revWalk.apply {