Change exit codes for updated and publish (#807)

It is now possible to run `lerna publish` in CI unconditionally, only publishing when changes are actually detected, and never failing when it decides to not publish anything.

Previously:
- `lerna publish` when there are no updates to publish would throw an error
- `lerna updated` when there are no updates would `exit 0`, making it ineffective as a chained filter (e.g., `lerna updated && lerna publish`)

Now:
- `lerna publish` when there are no updates is a no-op, exiting successfully with a helpful log message
- `lerna updated` when there are no updates will exit non-zero (but _not_ throw an error), enabling it to be an effective filter

Fixes #802

src/commands/PublishCommand.js

@@ -124,7 +124,8 @@ export default class PublishCommand extends Command {
       : [packagesToPublish];
 
     if (!this.updates.length) {
-      callback(new Error("No updated packages to publish."));
+      this.logger.info("No updated packages to publish.");
+      callback(null, true);
       return;
     }
 

src/commands/UpdatedCommand.js

@@ -19,7 +19,13 @@ export default class UpdatedCommand extends Command {
   initialize(callback) {
     const updatedPackagesCollector = new UpdatedPackagesCollector(this);
     this.updates = updatedPackagesCollector.getUpdates();
-    callback(null, true);
+
+    const proceedWithUpdates = this.updates.length > 0;
+    if (!proceedWithUpdates) {
+      this.logger.info("No packages need updating");
+    }
+
+    callback(null, proceedWithUpdates);
   }
 
   get otherCommandConfigs() {

test/UpdatedCommand.js

@@ -221,6 +221,17 @@ describe("UpdatedCommand", () => {
         }
       }));
     });
+
+    it("should return a non-zero exit code when there are no changes", (done) => {
+      gitTag({ cwd: testDir });
+
+      const updatedCommand = new UpdatedCommand([], {}, testDir);
+
+      updatedCommand.runValidations();
+      updatedCommand.runPreparations();
+
+      updatedCommand.runCommand(exitWithCode(1, done));
+    });
   });
 
   /** =========================================================================
@@ -330,6 +341,17 @@ describe("UpdatedCommand", () => {
         }
       }));
     });
+
+    it("should return a non-zero exit code when there are no changes", (done) => {
+      gitTag({ cwd: testDir });
+
+      const updatedCommand = new UpdatedCommand([], {}, testDir);
+
+      updatedCommand.runValidations();
+      updatedCommand.runPreparations();
+
+      updatedCommand.runCommand(exitWithCode(1, done));
+    });
   });
 });
 

test/integration/__snapshots__/lerna-publish.test.js.snap

@@ -83,6 +83,14 @@ Array [
 ]
 `;
 
+exports[`stderr: exists with a exit code 0 when there are no updates 1`] = `
+"lerna info version __TEST_VERSION__
+lerna info current version 1.0.0
+lerna info Checking for updated packages...
+lerna info Comparing with tag v1.0.0
+lerna info No updated packages to publish. "
+`;
+
 exports[`stderr: updates fixed versions 1`] = `
 "lerna info version __TEST_VERSION__
 lerna info current version 1.0.0
@@ -101,6 +109,8 @@ lerna info Comparing with initial commit.
 lerna info auto-confirmed "
 `;
 
+exports[`stdout: exists with a exit code 0 when there are no updates 1`] = `""`;
+
 exports[`stdout: updates fixed versions 1`] = `
 "
 Changes:

test/integration/lerna-publish.test.js

@@ -11,6 +11,23 @@ const lastCommitMessage = (cwd) =>
   execa.stdout("git", ["log", "-1", "--format=%B"], { cwd }).then(normalizeNewline);
 
 describe("lerna publish", () => {
+  test.concurrent("exists with a exit code 0 when there are no updates", async () => {
+    const cwd = await initFixture("PublishCommand/normal");
+    await execa("git", ["tag", "-a", "v1.0.0", "-m", "v1.0.0"], { cwd });
+
+    const args = [
+      "publish",
+      "--skip-npm",
+      "--cd-version=patch",
+      "--yes",
+    ];
+
+    const { stdout, stderr } = await execa(LERNA_BIN, args, { cwd });
+
+    expect(stdout).toMatchSnapshot("stdout: exists with a exit code 0 when there are no updates");
+    expect(stderr).toMatchSnapshot("stderr: exists with a exit code 0 when there are no updates");
+  });
+
   test.concurrent("updates fixed versions", async () => {
     const cwd = await initFixture("PublishCommand/normal");
     const args = [