fix: MAJOR — send() race between concurrent transport switch and an in-flight send

Two issues: (1) directoryExists(at:) only checked existence + isDirectory, so
an existing-but-unwritable OneDrive folder skipped the
oneDriveFolderUnavailable branch and surfaced as a generic pdfCompositionFailed
message instead — now uses OneDriveLocator.isWritableDirectory(at:). (2)
send() read `self.outboxURL` again inside composePDFForSend() after at least
one await had already run, so a concurrent chooseTransport() call
(SettingsView.swift) could flip transport/outboxURL/recordUncommented
mid-send, landing the PDF under one transport's folder while the
archive/status branch ran the other's.

Fix: send() now snapshots BOTH transport and the destination folder into
local `let`s once, before any await, and passes the folder explicitly into
the renamed composePDFForSend(outbox:) — which no longer reads
self.outboxURL at all. The archive/status switch already used the frozen
`transport` local. (chooseTransport/chooseOneDriveFolder additionally refuse
outright while isSending is true — see the SettingsView.swift commit — so in
practice this race can no longer even be triggered, but the snapshot is the
actual structural fix regardless.)

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012ZiTXPbPCSjzPVsfoweAbp

Sources/Shotdeck/SendController.swift

@@ -16,17 +16,28 @@ extension AppModel: SendCapable {
         guard !session.isEmpty, !isSending else { return }
         setSending(true)
 
+        // Snapshot BOTH the transport AND the destination folder into local `let`s
+        // ONCE, before any `await` in this function. chooseTransport/chooseOneDriveFolder
+        // now refuse (status "Finish the current send first.") while isSending is true,
+        // but this snapshot is the actual fix for the race: even without that guard,
+        // everything below operates on these frozen values — composePDFForSend(outbox:)
+        // takes the folder as a parameter and never re-reads `self.outboxURL` after a
+        // suspension point, so a concurrent transport switch mid-send can no longer land
+        // the PDF under one transport's folder while the archive/status branch (which
+        // switches on the same frozen `transport` local) runs the other's.
         let transport = TransportSettings.transport()
+        let destinationFolder: URL
 
-        // OneDrive mode: verify the real destination exists RIGHT NOW, before composing
+        // OneDrive mode: verify the real destination exists AND is writable RIGHT NOW,
-        // anything. `outboxURL` is kept in sync with the resolved OneDrive folder by
+        // before composing anything. `outboxURL` is kept in sync with the resolved
-        // bootstrap/chooseTransport/chooseOneDriveFolder, but this is re-resolved fresh
+        // OneDrive folder by bootstrap/chooseTransport/chooseOneDriveFolder, but this is
-        // here (never trusted stale) so a folder that vanished since then (OneDrive
+        // re-resolved fresh here (never trusted stale) so a folder that vanished or lost
-        // signed out, external volume unmounted, folder deleted) is caught instead of
+        // its permissions since then (OneDrive signed out, external volume unmounted,
-        // silently writing into whatever `outboxURL` happens to hold.
+        // folder deleted, chmod'd unwritable) is caught instead of silently attempted
+        // and surfacing as a generic PDF-composition failure.
         if transport == .oneDrive {
             guard let folder = OneDriveLocator.resolveOneDriveFolder(),
-                  Self.directoryExists(at: folder)
+                  OneDriveLocator.isWritableDirectory(at: folder)
             else {
                 let path = OneDriveLocator.resolveOneDriveFolder()?.path
                     ?? TransportSettings.storedOneDriveFolderPath()
@@ -36,16 +47,19 @@ extension AppModel: SendCapable {
                 setSending(false)
                 return
             }
+            destinationFolder = folder
             setResolvedOneDriveFolder(folder)
             if outboxURL != folder || watchFolderURL != folder {
                 setFolderURLs(outbox: folder, watch: folder)
                 try? await watcher.updateWatchFolder(folder)
             }
+        } else {
+            destinationFolder = outboxURL
         }
 
         let pending: ComposedSend
         do {
-            pending = try await composePDFForSend()
+            pending = try await composePDFForSend(outbox: destinationFolder)
         } catch {
             // Never unlink the published PDF, and never unlink the temp file either:
             // a rename failure would leave the complete document at the temp name.
@@ -93,19 +107,14 @@ extension AppModel: SendCapable {
         }
     }
 
-    private static func directoryExists(at url: URL) -> Bool {
+    /// Writes the PDF to `outboxDir` and records its path. Does not archive the session
-        var isDirectory: ObjCBool = false
-        let exists = FileManager.default.fileExists(atPath: url.path, isDirectory: &isDirectory)
-        return exists && isDirectory.boolValue
-    }
-
-    /// Writes the PDF to the outbox and records its path. Does not archive the session
     /// and does not present AirDrop — that happens only after the share completes.
-    func composePDFForSend() async throws -> ComposedSend {
+    /// `outboxDir` is passed in (a value `send(anchor:)` snapshotted before any await)
+    /// rather than read from `self.outboxURL` here, so a concurrent transport switch
+    /// mid-send can never redirect an in-flight compose to a different folder.
+    func composePDFForSend(outbox outboxDir: URL) async throws -> ComposedSend {
         let workingSession = session
         let composer = self.composer
-        // Live outbox (FolderSettings), not `paths.outbox` — Settings changes take effect.
-        let outboxDir = outboxURL
         let sourceDir = paths.sessionDirectory(workingSession.id)
         let fileName = PDFComposer.fileName(for: workingSession)
         let finalURL = outboxDir.appendingPathComponent(fileName)