feat: support controlled rollouts by environment variable (#41)

* fix: support CSP_NONCE_DISTRIBUTION env var

* fix: more comments

* fix: udpated readme

* fix: correct scope in readme

* fix: missing second argument of Math.max

* fix: more logging

* fix: replace Netlify.env with Deno.env

* fix: more logging

* fix: actually use the correct syntax

* fix: disable logging

README.md

@@ -93,8 +93,14 @@ If your HTML does not contain the `nonce` attribute on the `<script>` tags that
 - The `content-type` response header starts with `text/html`
 - The path of the request is satisfied by the `path` config option, and not included in the `excludedPath` config option
 
-### Quickly enabling and disabling
+### Controlling rollout
 
-You may want to quickly enable/disable the plugin while monitoring violation reports. You can do so without modifying code.
+You may want to gradually rollout the effects of this plugin while you monitor violation reports, without modifying code.
 
-Simply set the `DISABLE_CSP_NONCE` environment variable to `true`, and your next deploy will skip running the plugin. Setting to `false` will re-enable the plugin. The environment variable needs to be scoped to `Builds`.
+You can ramp up or ramp down the inclusion of the headers this plugin enforces by setting the `CSP_NONCE_DISTRIBUTION` environment variable to a value between `0` and `1`.
+
+- If `0`, the plugin is completely skipped at build time, and no extra functions or edge functions get deployed. Functionally, this acts the same as if the plugin isn't installed at all.
+- If `1`, 100% of traffic for all matching paths will include the nonce. Functionally, this acts the same as if the `CSP_NONCE_DISTRIBUTION` environment variable was not defined.
+- Any value in between `0` and `1` will include the nonce in randomly distributed traffic. For example, a value of `0.25` will include the nonce 25% of the time for matching paths.
+
+The `CSP_NONCE_DISTRIBUTION` environment variable needs to be scoped to both `Builds` and `Functions`.

index.js

@@ -7,11 +7,28 @@ export const onPreBuild = async ({ inputs, netlifyConfig, utils }) => {
   const config = JSON.stringify(inputs, null, 2);
   const { build } = netlifyConfig;
 
+  // DISABLE_CSP_NONCE is undocumented (deprecated), but still supported
+  // -> superseded by CSP_NONCE_DISTRIBUTION
   if (build.environment.DISABLE_CSP_NONCE === "true") {
     console.log(`  DISABLE_CSP_NONCE environment variable is true, skipping.`);
     return;
   }
 
+  // CSP_NONCE_DISTRIBUTION is a number from 0 to 1,
+  // but 0 to 100 is also supported, along with a trailing %
+  const distribution = build.environment.CSP_NONCE_DISTRIBUTION;
+  if (!!distribution) {
+    const threshold =
+      distribution.endsWith("%") || parseFloat(distribution) > 1
+        ? Math.max(parseFloat(distribution) / 100, 0)
+        : Math.max(parseFloat(distribution), 0);
+    console.log(`  CSP_NONCE_DISTRIBUTION is set to ${threshold * 100}%`);
+    if (threshold === 0) {
+      console.log(`  Skipping.`);
+      return;
+    }
+  }
+
   console.log(`  Current working directory: ${process.cwd()}`);
   const basePath =
     build.environment.SITE_ID === SITE_ID

src/__csp-nonce.ts

@@ -39,6 +39,22 @@ const handler = async (request: Request, context: Context) => {
     return response;
   }
 
+  // CSP_NONCE_DISTRIBUTION is a number from 0 to 1,
+  // but 0 to 100 is also supported, along with a trailing %
+  // @ts-expect-error
+  const distribution = Netlify.env.get("CSP_NONCE_DISTRIBUTION");
+  if (!!distribution) {
+    const threshold =
+      distribution.endsWith("%") || parseFloat(distribution) > 1
+        ? Math.max(parseFloat(distribution) / 100, 0)
+        : Math.max(parseFloat(distribution), 0);
+    // if a roll of the dice is greater than our threshold, skip
+    const random = Math.random();
+    if (random > threshold && threshold <= 1) {
+      return response;
+    }
+  }
+
   const nonce = randomBytes(24).toString("base64");
   // `'strict-dynamic'` allows scripts to be loaded from trusted scripts
   // when `'strict-dynamic'` is present, `'unsafe-inline' 'self' https: http:` is ignored by browsers