terraform: Avoid having multiple primaries

terraform/context.go

@@ -1264,17 +1264,46 @@ func (c *walkContext) persistState(r *Resource) {
 	rs.Dependencies = r.Dependencies
 
 	// Assign the instance state to the proper location
-	if r.Flags&FlagTainted != 0 {
+	if r.Flags&FlagDeposed != 0 {
+		// We were previously the primary and have been deposed, so
+		// now we are the final tainted resource
+		r.TaintedIndex = len(rs.Tainted) - 1
+		rs.Tainted[r.TaintedIndex] = r.State
+
+	} else if r.Flags&FlagTainted != 0 {
 		if r.TaintedIndex >= 0 {
 			// Tainted with a pre-existing index, just update that spot
 			rs.Tainted[r.TaintedIndex] = r.State
+
+		} else if r.Flags&FlagReplacePrimary != 0 {
+			// We just replaced the primary, so restore the primary
+			rs.Primary = rs.Tainted[len(rs.Tainted)-1]
+
+			// Set ourselves as tainted
+			rs.Tainted[len(rs.Tainted)-1] = r.State
+
 		} else {
 			// Newly tainted, so append it to the list, update the
 			// index, and remove the primary.
 			rs.Tainted = append(rs.Tainted, r.State)
-			rs.Primary = nil
 			r.TaintedIndex = len(rs.Tainted) - 1
+			rs.Primary = nil
 		}
+
+	} else if r.Flags&FlagReplacePrimary != 0 {
+		// If the ID is blank (there was an error), then we leave
+		// the primary that exists, and do not store this as a tainted
+		// instance
+		if r.State.ID == "" {
+			return
+		}
+
+		// Push the old primary into the tainted state
+		rs.Tainted = append(rs.Tainted, rs.Primary)
+
+		// Set this as the new primary
+		rs.Primary = r.State
+
 	} else {
 		// The primary instance, so just set it directly
 		rs.Primary = r.State

terraform/graph.go

@@ -557,8 +557,11 @@ func graphAddDiff(g *depgraph.Graph, d *ModuleDiff) error {
 					Target: newN,
 				})
 
-				// Set the CreateBeforeDestroy flag on the old noun
-				rn.Resource.Flags |= FlagCreateBeforeDestroy
+				// Set the ReplacePrimary flag on the new instance so that
+				// it will become the new primary, and Diposed flag on the
+				// existing instance so that it will step down
+				rn.Resource.Flags |= FlagReplacePrimary
+				newNode.Resource.Flags |= FlagDeposed
 
 			} else {
 				dep := &depgraph.Dependency{

terraform/graph_test.go

@@ -704,10 +704,15 @@ func TestGraphAddDiff_createBeforeDestroy(t *testing.T) {
 		t.Fatalf("bad:\n\n%s\n\nexpected:\n\n%s", actual, expected)
 	}
 
-	// Verify the flag is set
+	// Verify the flags are set
 	r := g.Noun("aws_instance.bar")
-	if r.Meta.(*GraphNodeResource).Resource.Flags&FlagCreateBeforeDestroy == 0 {
-		t.Fatalf("missing FlagCreateBeforeDestroy")
+	if r.Meta.(*GraphNodeResource).Resource.Flags&FlagReplacePrimary == 0 {
+		t.Fatalf("missing FlagReplacePrimary")
+	}
+
+	r = g.Noun("aws_instance.bar (destroy)")
+	if r.Meta.(*GraphNodeResource).Resource.Flags&FlagDeposed == 0 {
+		t.Fatalf("missing FlagDeposed")
 	}
 
 	// Verify that our original structure has not been modified

terraform/resource.go

@@ -47,7 +47,8 @@ const (
 	FlagTainted
 	FlagOrphan
 	FlagHasTainted
-	FlagCreateBeforeDestroy
+	FlagReplacePrimary
+	FlagDeposed
 )
 
 // InstanceInfo is used to hold information about the instance and/or