fix: open the composer for mailto links sent with SENDTO

Most apps start an email with `ACTION_SENDTO` and a `mailto:` URI. The
manifest offered the app for it, but React Native's Linking only
forwards VIEW intents, so the app opened on the inbox instead of the
composer.

MainActivity now rewrites a `SENDTO mailto:` intent into the equivalent
`VIEW mailto:` link before React Native sees it, on a cold start and
when the running app receives it. The EXTRA_EMAIL, EXTRA_CC, EXTRA_BCC,
EXTRA_SUBJECT and EXTRA_TEXT extras are folded into the link's query.
Bcc recipients of a mailto link now reach the composer too; they were
dropped for VIEW links as well.

android/app/src/main/java/com/anonymous/bulwarkmobile/MainActivity.kt

@@ -17,12 +17,14 @@ class MainActivity : ReactActivity() {
     // coloring the background, status bar, and navigation bar.
     // This is required for expo-splash-screen.
     setTheme(R.style.AppTheme);
+    ShareIntentStore.rewriteSendToAsView(intent)
     super.onCreate(null)
     NotificationTapStore.captureFromIntent(intent)
     ShareIntentStore.captureFromIntent(intent, contentResolver)
   }
 
   override fun onNewIntent(intent: Intent) {
+    ShareIntentStore.rewriteSendToAsView(intent)
     super.onNewIntent(intent)
     setIntent(intent)
     ShareIntentStore.captureFromIntent(intent, contentResolver)?.let { share ->

android/app/src/main/java/com/anonymous/bulwarkmobile/ShareIntentStore.kt

@@ -12,7 +12,8 @@ import com.facebook.react.bridge.WritableMap
 // Process-wide holder for the most recent ACTION_SEND / ACTION_SEND_MULTIPLE
 // intent (the system share sheet). MainActivity fills this on onCreate /
 // onNewIntent; JS drains it via BulwarkFcm.getInitialShare or reacts to the
-// live "app:share" event and opens the composer.
+// live "app:share" event and opens the composer. `SENDTO mailto:` intents are
+// rewritten into `VIEW mailto:` links instead (see rewriteSendToAsView).
 object ShareIntentStore {
     @Volatile
     private var pending: SharePayload? = null
@@ -56,6 +57,40 @@ object ShareIntentStore {
         return payload
     }
 
+    // `SENDTO mailto:` is how most apps start an email, but React Native's
+    // Linking only forwards VIEW intents, so the app opened on the inbox.
+    // Turn it into the equivalent `VIEW mailto:` link, folding the recipient,
+    // subject and text extras into the URI, so it reaches JS like any other
+    // mailto link: Linking.getInitialURL on a cold start, the "url" event
+    // while running. Must run before the intent is handed to React Native.
+    fun rewriteSendToAsView(intent: Intent?) {
+        val action = intent?.action ?: return
+        if (action != Intent.ACTION_SENDTO) return
+        val data = intent.data ?: return
+        if (!"mailto".equals(data.scheme, ignoreCase = true)) return
+
+        val params = mutableListOf<String>()
+        addressExtra(intent, Intent.EXTRA_EMAIL).forEach { params.add("to=${Uri.encode(it)}") }
+        addressExtra(intent, Intent.EXTRA_CC).forEach { params.add("cc=${Uri.encode(it)}") }
+        addressExtra(intent, Intent.EXTRA_BCC).forEach { params.add("bcc=${Uri.encode(it)}") }
+        intent.getCharSequenceExtra(Intent.EXTRA_SUBJECT)?.let {
+            params.add("subject=${Uri.encode(it.toString().take(512))}")
+        }
+        intent.getCharSequenceExtra(Intent.EXTRA_TEXT)?.let {
+            params.add("body=${Uri.encode(it.toString().take(MAX_TEXT))}")
+        }
+
+        val link = data.toString()
+        val separator = if (link.contains('?')) "&" else "?"
+        intent.action = Intent.ACTION_VIEW
+        intent.data = if (params.isEmpty()) data else Uri.parse(link + separator + params.joinToString("&"))
+    }
+
+    // EXTRA_EMAIL/CC/BCC are documented as String[], but some apps put a
+    // single String.
+    private fun addressExtra(intent: Intent, key: String): List<String> =
+        intent.getStringArrayExtra(key)?.filterNotNull() ?: listOfNotNull(intent.getStringExtra(key))
+
     private class SharedFile(val name: String, val mimeType: String, val size: Double)
 
     // Ask the provider for the file's real name, type and size: a MediaStore

src/navigation/__tests__/linking.test.ts

@@ -30,12 +30,32 @@ describe('parseDeepLink', () => {
       kind: 'compose',
       to: [{ email: 'a@b.co' }],
       cc: [{ email: 'c@d.co' }],
+      bcc: [],
       subject: 'Hi',
       body: undefined,
     });
     expect(parseDeepLink('mailto:nope')).toBeNull();
   });
 
+  it('reads the link MainActivity builds from a SENDTO mailto: intent and its extras', () => {
+    // SENDTO mailto: + EXTRA_EMAIL/CC/BCC/SUBJECT/TEXT, each value Uri.encode()d.
+    expect(parseDeepLink(
+      'mailto:?to=bob%40partner.example&cc=carol%40partner.example&bcc=dave%40partner.example'
+      + '&subject=Hi%20there&body=Line%201%0ALine%202',
+    )).toEqual({
+      kind: 'compose',
+      to: [{ email: 'bob@partner.example' }],
+      cc: [{ email: 'carol@partner.example' }],
+      bcc: [{ email: 'dave@partner.example' }],
+      subject: 'Hi there',
+      body: 'Line 1\nLine 2',
+    });
+    // An address in the URI itself plus EXTRA_EMAIL.
+    expect(parseDeepLink('mailto:bob@partner.example?to=eve%40partner.example')).toMatchObject({
+      to: [{ email: 'bob@partner.example' }, { email: 'eve@partner.example' }],
+    });
+  });
+
   it('rejects unknown links', () => {
     expect(parseDeepLink('bulwarkmobile://whatever')).toBeNull();
     expect(parseDeepLink('garbage')).toBeNull();
@@ -94,6 +114,17 @@ describe('handleDeepLink', () => {
     });
   });
 
+  it('passes Bcc recipients through to the composer', async () => {
+    const navigation = nav();
+    await handleDeepLink(
+      { kind: 'compose', to: [{ email: 'a@b.co' }], cc: [], bcc: [{ email: 'c@d.co' }] },
+      { navigation: navigation as never, resolveThreadId: async () => null },
+    );
+    expect(navigation.navigate).toHaveBeenCalledWith('Compose', expect.objectContaining({
+      prefillBcc: [{ email: 'c@d.co' }],
+    }));
+  });
+
   it('refuses when the linked account is not signed in', async () => {
     const navigation = nav();
     const ok = await handleDeepLink(

src/navigation/linking.ts

@@ -19,7 +19,7 @@ export type DeepLink =
   | { kind: 'contacts' }
   | { kind: 'files' }
   | { kind: 'settings'; tab?: string }
-  | { kind: 'compose'; to: EmailAddress[]; cc: EmailAddress[]; subject?: string; body?: string };
+  | { kind: 'compose'; to: EmailAddress[]; cc: EmailAddress[]; bcc?: EmailAddress[]; subject?: string; body?: string };
 
 function decodeSegment(value: string): string {
   try {
@@ -65,6 +65,7 @@ export function parseDeepLink(url: string): DeepLink | null {
       kind: 'compose',
       to: toAddresses(parsed.to),
       cc: toAddresses(parsed.cc),
+      bcc: toAddresses(parsed.bcc),
       subject: parsed.subject,
       body: parsed.body,
     };
@@ -168,6 +169,7 @@ export async function handleDeepLink(link: DeepLink, nav: DeepLinkNavigator): Pr
       navigation.navigate('Compose', {
         prefillTo: link.to,
         prefillCc: link.cc.length > 0 ? link.cc : undefined,
+        prefillBcc: link.bcc?.length ? link.bcc : undefined,
         prefillSubject: link.subject,
         prefillBody: link.body,
       });