Ensure matching modules with metadata when requested (#21640)

When a constraint is defined including build metadata, go-version ignores
this, and if a user explicitly requests a build version, this will now return that version.
Also covers cases when a user adds build metadata to a non-equality constraint,
or in a constraint with multiple conditions (ex. >0.8.0+abc, <1.0.0), and gives
and error when this occurs.

config/module/versions.go

@@ -3,14 +3,18 @@ package module
 import (
 	"errors"
 	"fmt"
+	"regexp"
 	"sort"
+	"strings"
 
 	version "github.com/hashicorp/go-version"
 	"github.com/hashicorp/terraform/registry/response"
 )
 
 const anyVersion = ">=0.0.0"
 
+var explicitEqualityConstraint = regexp.MustCompile("^=[0-9]")
+
 // return the newest version that satisfies the provided constraint
 func newest(versions []string, constraint string) (string, error) {
 	if constraint == "" {
@@ -21,6 +25,30 @@ func newest(versions []string, constraint string) (string, error) {
 		return "", err
 	}
 
+	// Find any build metadata in the constraints, and
+	// store whether the constraint is an explicit equality that
+	// contains a build metadata requirement, so we can return a specific,
+	// if requested, build metadata version
+	var constraintMetas []string
+	var equalsConstraint bool
+	for i := range cs {
+		constraintMeta := strings.SplitAfterN(cs[i].String(), "+", 2)
+		if len(constraintMeta) > 1 {
+			constraintMetas = append(constraintMetas, constraintMeta[1])
+		}
+	}
+
+	if len(cs) == 1 {
+		equalsConstraint = explicitEqualityConstraint.MatchString(cs.String())
+	}
+
+	// If the version string includes metadata, this is valid in go-version,
+	// However, it's confusing as to what expected behavior should be,
+	// so give an error so the user can do something more logical
+	if (len(cs) > 1 || !equalsConstraint) && len(constraintMetas) > 0 {
+		return "", fmt.Errorf("Constraints including build metadata must have explicit equality, or are otherwise too ambiguous: %s", cs.String())
+	}
+
 	switch len(versions) {
 	case 0:
 		return "", errors.New("no versions found")
@@ -58,6 +86,12 @@ func newest(versions []string, constraint string) (string, error) {
 			continue
 		}
 		if cs.Check(v) {
+			// Constraint has metadata and is explicit equality
+			if equalsConstraint && len(constraintMetas) > 0 {
+				if constraintMetas[0] != v.Metadata() {
+					continue
+				}
+			}
 			return versions[i], nil
 		}
 	}

config/module/versions_test.go

@@ -58,3 +58,33 @@ func TestNewestInvalidModuleVersion(t *testing.T) {
 		t.Fatalf("expected version %q, got %q", expected, m.Version)
 	}
 }
+
+func TestNewestModulesWithMetadata(t *testing.T) {
+	mpv := &response.ModuleProviderVersions{
+		Source: "registry/test/module",
+		Versions: []*response.ModuleVersion{
+			{Version: "0.9.0"},
+			{Version: "0.9.0+def"},
+			{Version: "0.9.0+abc"},
+			{Version: "0.9.0+xyz"},
+		},
+	}
+
+	// with metadata and explicit version request
+	expected := "0.9.0+def"
+	m, _ := newestVersion(mpv.Versions, "=0.9.0+def")
+	if m.Version != expected {
+		t.Fatalf("expected version %q, got %q", expected, m.Version)
+	}
+
+	// respect explicit equality, but >/</~, or metadata in multiple constraints, will give an error
+	_, err := newestVersion(mpv.Versions, "~>0.9.0+abc")
+	if err == nil {
+		t.Fatalf("expected an error, but did not get one")
+	}
+
+	_, err = newestVersion(mpv.Versions, ">0.8.0+abc, <1.0.0")
+	if err == nil {
+		t.Fatalf("expected an error, but did not get one")
+	}
+}