Request permission to protect local storage from eviction (#7398)

* Request the browser to never evict the persistent storage

* Store browser storage persisted state in a tiddler

* Factor out some code into helper functions

* Display status of persistence request in the settings page

plugins/tiddlywiki/browser-storage/settings.tid

@@ -28,6 +28,16 @@ This setting allows a custom alert message to be displayed when an attempt to st
 
 <$link to="$:/config/BrowserStorage/QuotaExceededAlert">Quota Exceeded Alert</$link>: <$edit-text tiddler="$:/config/BrowserStorage/QuotaExceededAlert" default="" tag="input" size="50"/>
 
+! Prevent browser from evicting local storage
+
+Permission for local storage persistence: ''{{$:/info/browser/storage/persisted}}''
+
+The first time a tiddler is saved to local storage a request will be made to prevent automatic eviction of local storage for this site. This means the data will not be cleared unless the user manually clears it.
+
+Old browsers may not support this feature. New browsers might not support the feature if the wiki is hosted on a non-localhost unencrypted http connection.
+
+Some browsers will explicitly prompt the user for permission. Other browsers may automatically grant or deny the request based on site usage or based on whether the site is bookmarked.
+
 ! Startup Log
 
 The tiddler $:/temp/BrowserStorage/Log contains a log of the tiddlers that were loaded from local storage at startup:

plugins/tiddlywiki/browser-storage/startup.js

@@ -19,7 +19,8 @@ exports.after = ["startup"];
 exports.synchronous = true;
 
 var ENABLED_TITLE = "$:/config/BrowserStorage/Enabled",
-	SAVE_FILTER_TITLE = "$:/config/BrowserStorage/SaveFilter";
+	SAVE_FILTER_TITLE = "$:/config/BrowserStorage/SaveFilter",
+	PERSISTED_STATE_TITLE = "$:/info/browser/storage/persisted";
 
 var BrowserStorageUtil = require("$:/plugins/tiddlywiki/browser-storage/util.js").BrowserStorageUtil;
 
@@ -53,6 +54,48 @@ exports.startup = function() {
 		$tw.wiki.addTiddler({title: ENABLED_TITLE, text: "no"});
 		$tw.browserStorage.clearLocalStorage();
 	});
+	// Helpers for protecting storage from eviction
+	var setPersistedState = function(state) {
+			$tw.wiki.addTiddler({title: PERSISTED_STATE_TITLE, text: state});
+		},
+		requestPersistence = function() {
+			setPersistedState("requested");
+			navigator.storage.persist().then(function(persisted) {
+				console.log("Request for persisted storage " + (persisted ? "granted" : "denied"));
+				setPersistedState(persisted ? "granted" : "denied");
+			});
+		},
+		persistPermissionRequested = false,
+		requestPersistenceOnFirstSave = function() {
+			$tw.hooks.addHook("th-saving-tiddler", function(tiddler) {
+				if (!persistPermissionRequested) {
+					var filteredChanges = filterFn.call($tw.wiki, function(iterator) {
+						iterator(tiddler,tiddler.getFieldString("title"));
+					});
+					if (filteredChanges.length > 0) {
+						// The tiddler will be saved to local storage, so request persistence
+						requestPersistence();
+						persistPermissionRequested = true;
+					}
+				}
+				return tiddler;
+			});
+		};
+	// Request the browser to never evict the localstorage. Some browsers such as firefox
+	// will prompt the user. To make the decision easier for the user only prompt them
+	// when they click the save button on a tiddler which will be stored to localstorage.
+	if (navigator.storage && navigator.storage.persist) {
+		navigator.storage.persisted().then(function(isPersisted) {
+			if (!isPersisted) {
+				setPersistedState("not requested yet");
+				requestPersistenceOnFirstSave();
+			} else {
+				setPersistedState("granted");
+			}
+		});
+	} else {
+		setPersistedState("feature not available");
+	}
 	// Track tiddler changes
 	$tw.wiki.addEventListener("change",function(changes) {
 		// Bail if browser storage is disabled
@@ -76,6 +119,10 @@ exports.startup = function() {
 			if(title === ENABLED_TITLE) {
 				return;
 			}
+			// This should always be queried from the browser, so don't store it in local storage
+			if(title === PERSISTED_STATE_TITLE) {
+				return;
+			}
 			// Save the tiddler
 			$tw.browserStorage.saveTiddlerToLocalStorage(title);
 		});