addrs: limit source component length to 16

When specifying/installing plugins, a source URI is required for Packer
to be able to locate or install a plugin to the local plugin hierarchy.

The plugin hierarchy is based on the plugin source, where each component
in this hierarchy will become a directory.

In order to avoid sources with too many levels of nesting, causing a lot
of mkdirs, we limit the number of sources to 16 in this commit, this
should be long enough for most of our users.

hcl2template/addrs/plugin.go

@@ -218,7 +218,20 @@ func ParsePluginSourceString(str string) (*Plugin, hcl.Diagnostics) {
 		})
 	}
 
-	return &Plugin{
+	plug := &Plugin{
 		Source: str,
-	}, diags
+	}
+	if len(plug.Parts()) > 16 {
+		return nil, diags.Append(&hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Too many parts to source URL",
+			Detail: fmt.Sprintf("The source URL must have at most 16 components, and the one provided has %d.\n"+
+				"This is unsupported by Packer, please consider using a source that has less components to it.\n"+
+				"If this is a blocking issue for you, please open an issue to ask for supporting more "+
+				"components to the source URI.",
+				len(plug.Parts())),
+		})
+	}
+
+	return plug, diags
 }

hcl2template/addrs/plugin_test.go

@@ -20,6 +20,7 @@ func TestPluginParseSourceString(t *testing.T) {
 		{"invalid: only one component, rejected", "potato", nil, true},
 		{"valid: two components in name", "hashicorp/azr", &Plugin{"hashicorp/azr"}, false},
 		{"valid: three components, nothing superfluous", "github.com/hashicorp/azr", &Plugin{"github.com/hashicorp/azr"}, false},
+		{"valid: 16 components, nothing superfluous", "github.com/hashicorp/azr/a/b/c/d/e/f/g/h/i/j/k/l/m", &Plugin{"github.com/hashicorp/azr/a/b/c/d/e/f/g/h/i/j/k/l/m"}, false},
 		{"invalid: trailing slash", "github.com/hashicorp/azr/", nil, true},
 		{"invalid: reject because scheme specified", "https://github.com/hashicorp/azr", nil, true},
 		{"invalid: reject because query non nil", "github.com/hashicorp/azr?arg=1", nil, true},
@@ -28,6 +29,7 @@ func TestPluginParseSourceString(t *testing.T) {
 		{"invalid: leading slashes are removed", "/github.com/hashicorp/azr", nil, true},
 		{"invalid: plugin name contains packer-", "/github.com/hashicorp/packer-azr", nil, true},
 		{"invalid: plugin name contains packer-plugin-", "/github.com/hashicorp/packer-plugin-azr", nil, true},
+		{"invalid: 17 components, too many parts to URL", "github.com/hashicorp/azr/a/b/c/d/e/f/g/h/i/j/k/l/m/n", nil, true},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {

packer/plugin-getter/plugins.go

@@ -182,7 +182,22 @@ func (pr Requirement) getPluginBinaries(opts ListInstallationsOptions) ([]string
 		return nil, err
 	}
 
-	return matches, err
+	retMatches := make([]string, 0, len(matches))
+	// Don't keep plugins that are nested too deep in the hierarchy
+	for _, match := range matches {
+		dir := strings.Replace(filepath.Dir(match), opts.PluginDirectory, "", 1)
+		parts := strings.FieldsFunc(dir, func(r rune) bool {
+			return r == '/'
+		})
+		if len(parts) > 16 {
+			log.Printf("[WARN] plugin %q ignored, too many levels of depth: %d (max 16)", match, len(parts))
+			continue
+		}
+
+		retMatches = append(retMatches, match)
+	}
+
+	return retMatches, err
 }
 
 // ListInstallations lists unique installed versions of plugin Requirement pr