Git - improve heuristics for determining branch base (#193986)

* Add getBranchBaseFromReflog

* Read/store branch merge base in the git config

* Add getBranchBase() extension api

extensions/git/src/api/api1.ts

@@ -177,6 +177,10 @@ export class ApiRepository implements Repository {
 		return this.repository.getBranches(query, cancellationToken);
 	}
 
+	getBranchBase(name: string): Promise<Branch | undefined> {
+		return this.repository.getBranchBase(name);
+	}
+
 	setBranchUpstream(name: string, upstream: string): Promise<void> {
 		return this.repository.setBranchUpstream(name, upstream);
 	}

extensions/git/src/api/git.d.ts

@@ -219,6 +219,7 @@ export interface Repository {
 	deleteBranch(name: string, force?: boolean): Promise<void>;
 	getBranch(name: string): Promise<Branch>;
 	getBranches(query: BranchQuery, cancellationToken?: CancellationToken): Promise<Ref[]>;
+	getBranchBase(name: string): Promise<Branch | undefined>;
 	setBranchUpstream(name: string, upstream: string): Promise<void>;
 
 	getRefs(query: RefQuery, cancellationToken?: CancellationToken): Promise<Ref[]>;

extensions/git/src/git.ts

@@ -1083,6 +1083,17 @@ export class Repository {
 		return parseGitCommits(result.stdout);
 	}
 
+	async reflog(ref: string, pattern: string): Promise<string[]> {
+		const args = ['reflog', ref, `--grep-reflog=${pattern}`];
+		const result = await this.exec(args);
+		if (result.exitCode) {
+			return [];
+		}
+
+		return result.stdout.split('\n')
+			.filter(entry => !!entry);
+	}
+
 	async bufferString(object: string, encoding: string = 'utf8', autoGuessEncoding = false): Promise<string> {
 		const stdout = await this.buffer(object);
 

extensions/git/src/repository.ts

@@ -1452,14 +1452,66 @@ export class Repository implements Disposable {
 
 	async getBranchBase(ref: string): Promise<Branch | undefined> {
 		const branch = await this.getBranch(ref);
+		const branchMergeBaseConfigKey = `branch.${branch.name}.vscode-merge-base`;
 
 		// Upstream
 		if (branch.upstream) {
-			return this.getBranch(`refs/remotes/${branch.upstream.remote}/${branch.upstream.name}`);
+			return await this.getBranch(`refs/remotes/${branch.upstream.remote}/${branch.upstream.name}`);
+		}
+
+		// Git config
+		try {
+			const mergeBase = await this.getConfig(branchMergeBaseConfigKey);
+			if (mergeBase) {
+				return await this.getBranch(mergeBase);
+			}
+		} catch (err) { }
+
+		// Reflog
+		const branchFromReflog = await this.getBranchBaseFromReflog(ref);
+		if (branchFromReflog) {
+			await this.setConfig(branchMergeBaseConfigKey, branchFromReflog.name!);
+			return branchFromReflog;
 		}
 
 		// Default branch
-		return await this.getDefaultBranch();
+		const defaultBranch = await this.getDefaultBranch();
+		if (defaultBranch) {
+			await this.setConfig(branchMergeBaseConfigKey, defaultBranch.name!);
+			return defaultBranch;
+		}
+
+		return undefined;
+	}
+
+	private async getBranchBaseFromReflog(ref: string): Promise<Branch | undefined> {
+		try {
+			const reflogEntries = await this.repository.reflog(ref, 'branch: Created from *.');
+			if (reflogEntries.length !== 1) {
+				return undefined;
+			}
+
+			// Branch created from an explicit branch
+			const match = reflogEntries[0].match(/branch: Created from (?<name>.*)$/);
+			if (match && match.length === 2 && match[1] !== 'HEAD') {
+				return await this.getBranch(match[1]);
+			}
+
+			// Branch created from HEAD
+			const headReflogEntries = await this.repository.reflog('HEAD', `checkout: moving from .* to ${ref.replace('refs/heads/', '')}`);
+			if (headReflogEntries.length === 0) {
+				return undefined;
+			}
+
+			const match2 = headReflogEntries[headReflogEntries.length - 1].match(/checkout: moving from ([^\s]+)\s/);
+			if (match2 && match2.length === 2) {
+				return await this.getBranch(match2[1]);
+			}
+
+		}
+		catch (err) { }
+
+		return undefined;
 	}
 
 	private async getDefaultBranch(): Promise<Branch | undefined> {