Simplify ensure payload logic

* Always write the manifest when provided.
* Write the lock only when modified or when writing vendor/.
* Clarify what should be passed for the two locks args:
  old lock and new lock. Updated `dep init` to match this expectation.
* Add test case for writing a new lock

Author: carolynvs

cmd/dep/init.go

@@ -163,7 +163,7 @@ func (cmd *initCommand) Run(ctx *dep.Ctx, args []string) error {
 	vlogf("Writing manifest and lock files.")
 
 	var sw dep.SafeWriter
-	sw.Prepare(m, l, nil, true)
+	sw.Prepare(m, nil, l, true)
 	if err := sw.Write(root, sm); err != nil {
 		return errors.Wrap(err, "safe write of manifest and lock")
 	}

txn_writer.go

@@ -157,29 +157,23 @@ func (diff StringDiff) MarshalJSON() ([]byte, error) {
 //   and the vendor directory in the same way
 // - If the forceVendor param is true, then vendor/ will be unconditionally
 //   written out based on newLock if present, else lock, else error.
-func (sw *SafeWriter) Prepare(manifest *Manifest, lock *Lock, newLock *Lock, forceVendor bool) {
+func (sw *SafeWriter) Prepare(manifest *Manifest, oldLock *Lock, newLock *Lock, forceVendor bool) {
 	sw.Payload = &SafeWriterPayload{
-		Manifest:    manifest,
-		WriteVendor: forceVendor,
+		Manifest: manifest,
 	}
 
-	if newLock != nil {
-		if lock == nil {
-			sw.Payload.Lock = newLock
-			sw.Payload.WriteVendor = true
-		} else {
-			diff := diffLocks(lock, newLock)
-			if diff != nil {
-				sw.Payload.Lock = newLock
-				sw.Payload.LockDiff = diff
-				sw.Payload.WriteVendor = true
-			} else if forceVendor {
-				sw.Payload.Lock = newLock
-				sw.Payload.WriteVendor = true
-			}
-		}
-	} else if lock != nil {
-		sw.Payload.Lock = lock
+	if oldLock != nil && newLock != nil {
+		sw.Payload.LockDiff = diffLocks(oldLock, newLock)
+	}
+
+	if forceVendor || sw.Payload.LockDiff != nil {
+		sw.Payload.Lock = newLock
+		sw.Payload.WriteVendor = true
+	}
+
+	if oldLock == nil && newLock != nil {
+		sw.Payload.Lock = newLock
+		sw.Payload.WriteVendor = true
 	}
 }
 

txn_writer_test.go

@@ -373,6 +373,54 @@ func TestSafeWriter_ForceVendorWhenVendorAlreadyExists(t *testing.T) {
 	}
 }
 
+func TestSafeWriter_NewLock(t *testing.T) {
+	test.NeedsExternalNetwork(t)
+	test.NeedsGit(t)
+
+	h := test.NewHelper(t)
+	defer h.Cleanup()
+
+	pc := NewTestProjectContext(h, safeWriterProject)
+	defer pc.Release()
+	pc.Load()
+
+	var sw SafeWriter
+	lf := h.GetTestFile(safeWriterGoldenLock)
+	defer lf.Close()
+	newLock, err := readLock(lf)
+	h.Must(err)
+	sw.Prepare(nil, nil, newLock, false)
+
+	// Verify prepared actions
+	if sw.Payload.HasManifest() {
+		t.Fatal("Did not expect the payload to contain the manifest")
+	}
+	if !sw.Payload.HasLock() {
+		t.Fatal("Expected the payload to contain the lock")
+	}
+	if !sw.Payload.HasVendor() {
+		t.Fatal("Expected the payload to the vendor directory")
+	}
+
+	// Write changes
+	err = sw.Write(pc.Project.AbsRoot, pc.SourceManager)
+	h.Must(errors.Wrap(err, "SafeWriter.Write failed"))
+
+	// Verify file system changes
+	if err := pc.ManifestShouldNotExist(); err != nil {
+		t.Fatal(err)
+	}
+	if err := pc.LockShouldMatchGolden(safeWriterGoldenLock); err != nil {
+		t.Fatal(err)
+	}
+	if err := pc.VendorShouldExist(); err != nil {
+		t.Fatal(err)
+	}
+	if err := pc.VendorFileShouldExist("github.com/sdboyer/dep-test"); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func TestSafeWriter_DiffLocks(t *testing.T) {
 	test.NeedsExternalNetwork(t)
 	test.NeedsGit(t)
@@ -435,4 +483,4 @@ func TestSafeWriter_DiffLocks(t *testing.T) {
 	if err = pc.ShouldMatchGolden(goldenOutput, output); err != nil {
 		t.Fatal(err)
 	}
-}
+}