#909: Use default branch as Pull Request base when target does not exist

The resolution of Pull Request details currently uses the project
default branch when a pull request is analysed without specifying a
target branch. However, when a target branch is specified, the plugin
expects that branch to exist and throws an exception if the target
branch is not found. To overcome this, the target branch is being
resolved to the default branch where no target branch is specified or
the requested target branch does not exist. As the API used to gather
branch information does not return PULL_REQUEST details, the result of
this searching is that a pull request will either target a branch from
the main branch, or target the main branch, there is no further nesting
or pull requests and branches.

src/main/java/com/github/mc1arke/sonarqube/plugin/scanner/BranchConfigurationFactory.java

@@ -41,7 +41,7 @@ public BranchConfiguration createBranchConfiguration(String branchName, ProjectB
 
     public BranchConfiguration createPullRequestConfiguration(String pullRequestKey, String pullRequestBranch, String pullRequestBase, ProjectBranches branches) {
         String targetBranch = Optional.ofNullable(pullRequestBase).orElse(branches.defaultBranchName());
-        String referenceBranch = findReferenceBranch(targetBranch, branches);
+        String referenceBranch = findReferenceBranch(Optional.ofNullable(branches.get(targetBranch)).map(BranchInfo::name).orElse(branches.defaultBranchName()), branches);
         return new CommunityBranchConfiguration(pullRequestBranch, BranchType.PULL_REQUEST, referenceBranch, targetBranch, pullRequestKey);
     }
 
@@ -58,6 +58,7 @@ private static String findReferenceBranch(String targetBranch, ProjectBranches b
             throw MessageException.of(String.format("The branch '%s' of type %s does not have a target", target.name(), target.type()));
         }
 
-        return findReferenceBranch(targetBranchTarget, branches);
+        return Optional.ofNullable(branches.get(targetBranchTarget)).map(BranchInfo::name)
+                .orElseThrow(() -> MessageException.of(String.format("The branch '%s' of type %s has a target '%s' which does not exist", target.name(), target.type(), targetBranchTarget)));
     }
 }

src/test/java/com/github/mc1arke/sonarqube/plugin/scanner/BranchConfigurationFactoryTest.java

@@ -118,6 +118,20 @@ void shouldReturnPullRequestWithTargetOfDefaultBranchIfTargetNotSpecifiedAndDefa
         assertThat(actual).usingRecursiveComparison().isEqualTo(new CommunityBranchConfiguration("source", BranchType.PULL_REQUEST, "defaultBranch", "defaultBranch", "key"));
     }
 
+    @Test
+    void shouldReturnPullRequestWithTargetOfDefaultBranchIfTargetDoesNotExistAndDefaultExists() {
+        ProjectBranches projectBranches = mock();
+        when(projectBranches.isEmpty()).thenReturn(false);
+        when(projectBranches.defaultBranchName()).thenReturn("defaultBranch");
+        BranchInfo branchInfo = new BranchInfo("defaultBranch", BranchType.BRANCH, true, null);
+        when(projectBranches.get("defaultBranch")).thenReturn(branchInfo);
+
+        BranchConfigurationFactory underTest = new BranchConfigurationFactory();
+        BranchConfiguration actual = underTest.createPullRequestConfiguration("key", "source", "missing", projectBranches);
+
+        assertThat(actual).usingRecursiveComparison().isEqualTo(new CommunityBranchConfiguration("source", BranchType.PULL_REQUEST, "defaultBranch", "missing", "key"));
+    }
+
     @Test
     void shouldReturnPullRequestWithTargetOfTargetAsReferenceIfTargetBranchExists() {
         ProjectBranches projectBranches = mock(ProjectBranches.class);