Background - validate upstream branch before creating the worktree (#308953)

* Background - validate upstream branch before creating the worktree

* Pull request feedback

Author: lszomoru

extensions/copilot/src/extension/chatSessions/vscode-node/chatSessionWorktreeServiceImpl.ts

@@ -72,6 +72,7 @@ export class ChatSessionWorktreeService extends Disposable implements IChatSessi
 
 			const autoCommit = this.configurationService.getConfig<boolean>(ConfigKey.Advanced.CLIAutoCommitEnabled);
 
+			let baseCommit: string | undefined = undefined;
 			const branch = await this.generateBranchName(branchName, activeRepository);
 
 			// When a base branch is provided, we attempt to resolve it, to see whether it has an
@@ -82,8 +83,19 @@ export class ChatSessionWorktreeService extends Disposable implements IChatSessi
 					// Attempt to resolve the provided base branch
 					const branchDetails = await this.gitService.getBranch(activeRepository.rootUri, baseBranch);
 					if (branchDetails?.upstream?.remote && branchDetails.upstream?.name) {
-						// If the base branch has an upstream, use it as the base for the worktree
-						baseBranch = `${branchDetails.upstream.remote}/${branchDetails.upstream.name}`;
+						const upstreamBranchName = `${branchDetails.upstream.remote}/${branchDetails.upstream.name}`;
+
+						try {
+							// Attempt to resolve the upstream branch before using it as the base for the worktree
+							const upstreamBranch = await this.gitService.getBranch(activeRepository.rootUri, upstreamBranchName);
+							if (upstreamBranch) {
+								baseBranch = upstreamBranchName;
+								baseCommit = upstreamBranch.commit;
+							}
+						} catch (error) {
+							const errorMessage = error instanceof Error ? error.message : String(error);
+							this.logService.warn(`[ChatSessionWorktreeService][_createWorktree] Failed to resolve upstream branch ${upstreamBranchName}. Error: ${errorMessage}`);
+						}
 					}
 				} catch (error) {
 					const errorMessage = error instanceof Error ? error.message : String(error);
@@ -97,8 +109,7 @@ export class ChatSessionWorktreeService extends Disposable implements IChatSessi
 				const baseBranchName = baseBranch ?? activeRepository.headBranchName;
 				const baseBranchProtected = await this.gitService.isBranchProtected(activeRepository.rootUri, baseBranchName);
 
-				let baseCommit: string | undefined = undefined;
-				if (baseBranch) {
+				if (baseBranch && !baseCommit) {
 					const refs = await this.gitService.getRefs(activeRepository.rootUri, { pattern: `refs/heads/${baseBranch}` });
 					baseCommit = refs.length === 1 && refs[0].commit ? refs[0].commit : undefined;
 				}