utilize Pattern Tree to decide the policy to use and modify HTTP response headers according to that policy

This commit also enhances the build script so that preprocessor conditionals can now use operators '&&' and '||'.
The features being developed are not yet included in the actual Haketilo build.
Some of the new source files contain similar functionality to other ones already existing in the source tree. At some point the latter will be removed.

4 files changed, 152 insertions, 33 deletions

diff --git a/common/indexeddb.js b/common/indexeddb.js
index 096391a..e54d1ca 100644
--- a/common/indexeddb.js
+++ b/common/indexeddb.js
@@ -62,7 +62,8 @@ const stores = 	[
     ["files",     {keyPath: "hash_key"}],
     ["file_uses", {keyPath: "hash_key"}],
     ["resources", {keyPath: "identifier"}],
-    ["mappings",  {keyPath: "identifier"}]
+    ["mappings",  {keyPath: "identifier"}],
+    ["settings",  {keyPath: "name"}]
 ];
 
 let db = null;
@@ -207,7 +208,7 @@ async function incr_file_uses(context, file_ref, by=1)
 
 const decr_file_uses = (ctx, file_ref) => incr_file_uses(ctx, file_ref, -1);
 
-async function finalize_items_transaction(context)
+async function finalize_transaction(context)
 {
     for (const uses of Object.values(context.file_uses)) {
 	if (uses.uses < 0)
@@ -248,7 +249,7 @@ async function finalize_items_transaction(context)
 
     return context.result;
 }
-#EXPORT finalize_items_transaction
+#EXPORT finalize_transaction
 
 /*
  * How a sample data argument to the function below might look like:
@@ -304,7 +305,7 @@ async function _save_items(resources, mappings, context)
     for (const item of resources.concat(mappings))
 	await save_item(item, context);
 
-    await finalize_items_transaction(context);
+    await finalize_transaction(context);
 }
 
 /*
@@ -314,9 +315,9 @@ async function _save_items(resources, mappings, context)
  * object with keys being of the form `sha256-<file's-sha256-sum>`.
  *
  * context should be one returned from start_items_transaction() and should be
- * later passed to finalize_items_transaction() so that files depended on are
- * added to IndexedDB and files that are no longer depended on after this
- * operation are removed from IndexedDB.
+ * later passed to finalize_transaction() so that files depended on are added to
+ * IndexedDB and files that are no longer depended on after this operation are
+ * removed from IndexedDB.
  */
 async function save_item(item, context)
 {
@@ -346,9 +347,9 @@ async function _remove_item(store_name, identifier, context)
  * Remove definition of a resource/mapping from IndexedDB.
  *
  * context should be one returned from start_items_transaction() and should be
- * later passed to finalize_items_transaction() so that files depended on are
- * added to IndexedDB and files that are no longer depended on after this
- * operation are removed from IndexedDB.
+ * later passed to finalize_transaction() so that files depended on are added to
+ * IndexedDB and files that are no longer depended on after this operation are
+ * removed from IndexedDB.
  */
 async function remove_item(store_name, identifier, context)
 {
@@ -363,26 +364,49 @@ const remove_resource = (id, ctx) => remove_item("resources", id, ctx);
 const remove_mapping = (id, ctx) => remove_item("mappings",  id, ctx);
 #EXPORT remove_mapping
 
+/* A simplified kind of transaction for modifying just the "settings" store. */
+async function start_settings_transaction()
+{
+    const db = await get_db();
+    return make_context(db.transaction("settings", "readwrite"), {});
+}
+
+async function set_setting(name, value)
+{
+    const context = await start_settings_transaction();
+    broadcast.prepare(context.sender, `idb_changes_settings`, name);
+    await idb_put(context.transaction, "settings", {name, value});
+    return finalize_transaction(context);
+}
+#EXPORT set_setting
+
+async function get_setting(name)
+{
+    const transaction = (await get_db()).transaction("settings");
+    return ((await idb_get(transaction, "settings", name)) || {}).value;
+}
+#EXPORT get_setting
+
 /* Callback used when listening to broadcasts while tracking db changes. */
-async function track_change(tracking, identifier)
+async function track_change(tracking, key)
 {
     const transaction = (await get_db()).transaction([tracking.store_name]);
-    const new_val = await idb_get(transaction, tracking.store_name, identifier);
+    const new_val = await idb_get(transaction, tracking.store_name, key);
 
-    tracking.onchange({identifier, new_val});
+    tracking.onchange({key, new_val});
 }
 
 /*
  * Monitor changes to `store_name` IndexedDB object store.
  *
- * `store_name` should be either "resources" or "mappings".
+ * `store_name` should be either "resources", "mappings" or "settings".
  *
  * `onchange` should be a callback that will be called when an item is added,
  * modified or removed from the store. The callback will be passed an object
  * representing the change as its first argument. This object will have the
  * form:
  * {
- *     identifier: "the identifier of modified resource/mapping",
+ *     key: "the identifier of modified resource/mapping or settings key",
  *     new_val: undefined // `undefined` if item removed, item object otherwise
  * }
  *
@@ -395,7 +419,7 @@ async function track_change(tracking, identifier)
  * actually modified or that it only gets called once after multiple quick
  * changes to an item.
  */
-async function track(store_name, onchange)
+async function start_tracking(store_name, onchange)
 {
     const tracking = {store_name, onchange};
     tracking.listener =
@@ -408,12 +432,10 @@ async function track(store_name, onchange)
     return [tracking, (await wait_request(all_req)).target.result];
 }
 
-const track_resources = onchange => track("resources", onchange);
-#EXPORT track_resources
-
-const track_mappings = onchange => track("mappings",  onchange);
-#EXPORT track_mappings
+const track = {};
+for (const store_name of ["resources", "mappings", "settings"])
+    track[store_name] = onchange => start_tracking(store_name, onchange);
+#EXPORT track
 
 const untrack = tracking => broadcast.close(tracking.listener);
 #EXPORT untrack
-
diff --git a/common/misc.js b/common/misc.js
index dc4a598..82f6cbf 100644
--- a/common/misc.js
+++ b/common/misc.js
@@ -67,17 +67,6 @@ function gen_nonce(length=16)
 }
 #EXPORT gen_nonce
 
-/* CSP rule that blocks scripts according to policy's needs. */
-function make_csp_rule(policy)
-{
-    let rule = "prefetch-src 'none'; script-src-attr 'none';";
-    const script_src = policy.nonce !== undefined ?
-	  `'nonce-${policy.nonce}'` : "'none'";
-    rule += ` script-src ${script_src}; script-src-elem ${script_src};`;
-    return rule;
-}
-#EXPORT make_csp_rule
-
 /* Check if some HTTP header might define CSP rules. */
 const csp_header_regex =
       /^\s*(content-security-policy|x-webkit-csp|x-content-security-policy)/i;
diff --git a/common/patterns_query_tree.js b/common/patterns_query_tree.js
index 1bbdb39..f8ec405 100644
--- a/common/patterns_query_tree.js
+++ b/common/patterns_query_tree.js
@@ -41,6 +41,8 @@
  * proprietary program, I am not going to enforce this in court.
  */
 
+// TODO! Modify the code to use `Object.create(null)` instead of `{}`.
+
 #FROM common/patterns.js IMPORT deconstruct_url
 
 /* "Pattern Tree" is how we refer to the data structure used for querying
diff --git a/common/policy.js b/common/policy.js
new file mode 100644
index 0000000..ebd663f
--- /dev/null
+++ b/common/policy.js
@@ -0,0 +1,106 @@
+/**
+ * This file is part of Haketilo.
+ *
+ * Function: Determining what to do on a given web page.
+ *
+ * Copyright (C) 2021 Wojtek Kosior
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * As additional permission under GNU GPL version 3 section 7, you
+ * may distribute forms of that code without the copy of the GNU
+ * GPL normally required by section 4, provided you include this
+ * license notice and, in case of non-source distribution, a URL
+ * through which recipients can access the Corresponding Source.
+ * If you modify file(s) with this exception, you may extend this
+ * exception to your version of the file(s), but you are not
+ * obligated to do so. If you do not wish to do so, delete this
+ * exception statement from your version.
+ *
+ * As a special exception to the GPL, any HTML file which merely
+ * makes function calls to this code, and for that purpose
+ * includes it by reference shall be deemed a separate work for
+ * copyright law purposes. If you modify this code, you may extend
+ * this exception to your version of the code, but you are not
+ * obligated to do so. If you do not wish to do so, delete this
+ * exception statement from your version.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ *
+ * I, Wojtek Kosior, thereby promise not to sue for violation of this file's
+ * license. Although I request that you do not make use this code in a
+ * proprietary program, I am not going to enforce this in court.
+ */
+
+#IMPORT common/patterns_query_tree.js AS pqt
+
+#FROM common/sha256.js IMPORT sha256
+
+/*
+ * CSP rule that either blocks all scripts or only allows scripts with specified
+ * nonce attached.
+ */
+function make_csp(nonce)
+{
+    const rule = nonce ? `nonce-${nonce}` : "none";
+    const csp_dict = {"prefetch-src": "none", "script-src-attr": "none"};
+    Object.assign(csp_dict, {"script-src": rule, "script-src-elem": rule});
+    return Object.entries(csp_dict).map(([a, b]) => `${a} '${b}';`).join(" ");
+}
+
+function decide_policy(patterns_tree, url, default_allow, secret)
+{
+    const policy = {allow: default_allow};
+
+    try {
+	var payloads = pqt.search(patterns_tree, url).next().value;
+    } catch (e) {
+	console.error(e);
+	policy.allow = false;
+	policy.error = true;
+    }
+
+    if (payloads !== undefined) {
+	policy.mapping = Object.keys(payloads).sort()[0];
+	const payload = payloads[policy.mapping];
+	if (payload.allow !== undefined) {
+	    policy.allow = payload.allow;
+	} else /* if (payload.identifier) */ {
+	    policy.allow = false;
+	    policy.payload = payload;
+	    /*
+	     * Hash a secret and other values into a string that's unpredictable
+	     * to someone who does not know these values. What we produce here
+	     * is not a true "nonce" because it might get produced multiple
+	     * times given the same url and mapping choice. Nevertheless, this
+	     * is reasonably good given the limitations WebExtension APIs and
+	     * environments give us. If we were using a true nonce, we'd have no
+	     * reliable way of passing it to our content scripts.
+	     */
+	    const nonce_source = [
+		policy.mapping,
+		policy.payload.identifier,
+		url,
+		secret
+	    ];
+	    policy.nonce = sha256(nonce_source.join(":"));
+	}
+    }
+
+    if (!policy.allow)
+	policy.csp = make_csp(policy.nonce);
+
+    return policy;
+}
+#EXPORT decide_policy
+
+#EXPORT  () => ({allow: false, csp: make_csp()})  AS fallback_policy