missing constraints passed erroniously

ConstrainVersions was documented as returning nil, but it was instead
returning an empty set. Use the Count() method to check for nil or
empty. Add test to verify failed constraints will show up as missing.

command/init_test.go

@@ -10,6 +10,7 @@ import (
 	"testing"
 
 	"github.com/hashicorp/terraform/helper/copy"
+	"github.com/hashicorp/terraform/plugin/discovery"
 	"github.com/mitchellh/cli"
 )
 
@@ -620,6 +621,40 @@ func TestInit_getProviderMissing(t *testing.T) {
 	}
 }
 
+func TestInit_getProviderHaveLegacyVersion(t *testing.T) {
+	// Create a temporary working directory that is empty
+	td := tempDir(t)
+	copy.CopyDir(testFixturePath("init-providers-lock"), td)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	if err := ioutil.WriteFile("terraform-provider-test", []byte("provider bin"), 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	// provider test has a version constraint in the config, which should
+	// trigger the getProvider error below.
+	ui := new(cli.MockUi)
+	c := &InitCommand{
+		Meta: Meta{
+			testingOverrides: metaOverridesForProvider(testProvider()),
+			Ui:               ui,
+		},
+		getProvider: func(dst, provider string, req discovery.Constraints, protoVersion uint) error {
+			return fmt.Errorf("EXPECTED PROVIDER ERROR %s", provider)
+		},
+	}
+
+	args := []string{}
+	if code := c.Run(args); code == 0 {
+		t.Fatalf("expceted error, got output: \n%s", ui.OutputWriter.String())
+	}
+
+	if !strings.Contains(ui.ErrorWriter.String(), "EXPECTED PROVIDER ERROR test") {
+		t.Fatalf("unexpected error output: %s", ui.ErrorWriter)
+	}
+}
+
 func TestInit_providerLockFile(t *testing.T) {
 	// Create a temporary working directory that is empty
 	td := tempDir(t)

command/plugins.go

@@ -108,6 +108,10 @@ func (m *Meta) providerPluginSet() discovery.PluginMetaSet {
 	plugins := discovery.FindPlugins("provider", m.pluginDirs())
 	plugins, _ = plugins.ValidateVersions()
 
+	for p := range plugins {
+		log.Printf("[DEBUG] found valid plugin: %q", p.Name)
+	}
+
 	return plugins
 }
 
@@ -121,10 +125,14 @@ func (m *Meta) providerResolver() terraform.ResourceProviderResolver {
 func (m *Meta) missingPlugins(avail discovery.PluginMetaSet, reqd discovery.PluginRequirements) discovery.PluginRequirements {
 	missing := make(discovery.PluginRequirements)
 
+	for n, r := range reqd {
+		log.Printf("[DEBUG] plugin requirements: %q=%q", n, r.Versions)
+	}
+
 	candidates := avail.ConstrainVersions(reqd)
 
 	for name, versionSet := range reqd {
-		if metas := candidates[name]; metas == nil {
+		if metas := candidates[name]; metas.Count() == 0 {
 			missing[name] = versionSet
 		}
 	}