core: fix crash on provider warning

When a provider validation only returns a warning, we were cutting the
evaltree short by returning an error. This is fine during a
`walkValidate` but was causing trouble during `walkPlan` and
`walkApply`.

fixes #2870

terraform/context_apply_test.go

@@ -78,6 +78,46 @@ func TestContext2Apply_providerAlias(t *testing.T) {
 	}
 }
 
+// GH-2870
+func TestContext2Apply_providerWarning(t *testing.T) {
+	m := testModule(t, "apply-provider-warning")
+	p := testProvider("aws")
+	p.ApplyFn = testApplyFn
+	p.DiffFn = testDiffFn
+	p.ValidateFn = func(c *ResourceConfig) (ws []string, es []error) {
+		ws = append(ws, "Just a warning")
+		return
+	}
+	ctx := testContext2(t, &ContextOpts{
+		Module: m,
+		Providers: map[string]ResourceProviderFactory{
+			"aws": testProviderFuncFixed(p),
+		},
+	})
+
+	if _, err := ctx.Plan(); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	state, err := ctx.Apply()
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	actual := strings.TrimSpace(state.String())
+	expected := strings.TrimSpace(`
+aws_instance.foo:
+  ID = foo
+	`)
+	if actual != expected {
+		t.Fatalf("got: \n%s\n\nexpected:\n%s", actual, expected)
+	}
+
+	if !p.ConfigureCalled {
+		t.Fatalf("provider Configure() was never called!")
+	}
+}
+
 func TestContext2Apply_emptyModule(t *testing.T) {
 	m := testModule(t, "apply-empty-module")
 	p := testProvider("aws")

terraform/evaltree_provider.go

@@ -40,9 +40,8 @@ func ProviderEvalTree(n string, config *config.RawConfig) EvalNode {
 		},
 	})
 
-	// Apply stuff
 	seq = append(seq, &EvalOpFilter{
-		Ops: []walkOperation{walkValidate, walkRefresh, walkPlan, walkApply},
+		Ops: []walkOperation{walkValidate},
 		Node: &EvalSequence{
 			Nodes: []EvalNode{
 				&EvalGetProvider{
@@ -70,6 +69,32 @@ func ProviderEvalTree(n string, config *config.RawConfig) EvalNode {
 		},
 	})
 
+	// Apply stuff
+	seq = append(seq, &EvalOpFilter{
+		Ops: []walkOperation{walkRefresh, walkPlan, walkApply},
+		Node: &EvalSequence{
+			Nodes: []EvalNode{
+				&EvalGetProvider{
+					Name:   n,
+					Output: &provider,
+				},
+				&EvalInterpolate{
+					Config: config,
+					Output: &resourceConfig,
+				},
+				&EvalBuildProviderConfig{
+					Provider: n,
+					Config:   &resourceConfig,
+					Output:   &resourceConfig,
+				},
+				&EvalSetProviderConfig{
+					Provider: n,
+					Config:   &resourceConfig,
+				},
+			},
+		},
+	})
+
 	// We configure on everything but validate, since validate may
 	// not have access to all the variables.
 	seq = append(seq, &EvalOpFilter{

terraform/test-fixtures/apply-provider-warning/main.tf

@@ -0,0 +1 @@
+resource "aws_instance" "foo" {}