packer: support loading pre-release plugins

When a pre-release version of a plugin is locally installed, it may or
may not be loaded depending on the constraints expressed in the template
being executed.

If the template contains constraints for loading the plugin, it would be
ignored, while if that wasn't present, it would be loaded.

This is inconsistent, and deserves to be addressed, which is what this
commit does.

With this change, plugin pre-releases are now loaded, provided the
version reported matches the constraints, independently from its
pre-release suffix `-dev'.
Also, if a release with the same version is also installed alongside the
pre-release version of a plugin, it will have precedence over the
pre-release.

packer/plugin-getter/plugins.go

@@ -96,6 +96,8 @@ func (opts BinaryInstallationOptions) FilenameSuffix() string {
 	return "_" + opts.OS + "_" + opts.ARCH + opts.Ext
 }
 
+var pluginVersionRegex = regexp.MustCompile(`^v([0-9]+\.[0-9]+\.[0-9]+)(-dev)?$`)
+
 // ListInstallations lists unique installed versions of plugin Requirement pr
 // with opts as a filter.
 //
@@ -152,21 +154,37 @@ func (pr Requirement) ListInstallations(opts ListInstallationsOptions) (InstallL
 		// versionsStr now looks like v1.2.3_x5.1 or amazon_v1.2.3_x5.1
 		parts := strings.SplitN(versionsStr, "_", 2)
 		pluginVersionStr, protocolVerionStr := parts[0], parts[1]
-		pv, err := version.NewVersion(pluginVersionStr)
+		_, err = version.NewVersion(pluginVersionStr)
 		if err != nil {
 			// could not be parsed, ignoring the file
 			log.Printf("found %q with an incorrect %q version, ignoring it. %v", path, pluginVersionStr, err)
 			continue
 		}
 
-		if strings.Replace(pluginVersionStr, "v", "", -1) != describeInfo.Version {
-			log.Printf("plugin %q reported version %s while its name implies version %s, ignoring", path, describeInfo.Version, pluginVersionStr)
+		matches := pluginVersionRegex.FindStringSubmatch(pluginVersionStr)
+		if matches == nil {
+			log.Printf("invalid version found: %q, ignoring", pluginVersionStr)
+			continue
+		}
+
+		absVersion := matches[1]
+		if len(matches) == 3 {
+			absVersion = fmt.Sprintf("%s%s", absVersion, matches[2])
+		}
+
+		if absVersion != describeInfo.Version {
+			log.Printf("plugin %q reported version %s while its name implies version %s, ignoring", path, describeInfo.Version, absVersion)
 			continue
 		}
 
+		rawVersion, _ := version.NewVersion(matches[1])
 		// no constraint means always pass, this will happen for implicit
 		// plugin requirements and when we list all plugins.
-		if !pr.VersionConstraints.Check(pv) {
+		//
+		// Note: we use the raw version name here, without the pre-release
+		// suffix, as otherwise constraints reject them, which is not
+		// what we want by default.
+		if !pr.VersionConstraints.Check(rawVersion) {
 			log.Printf("[TRACE] version %q of file %q does not match constraint %q", pluginVersionStr, path, pr.VersionConstraints.String())
 			continue
 		}