Sources/ShotdeckCore/Support/TransportSettings.swift

@@ -176,4 +176,29 @@ public enum OneDriveLocator {
         guard exists, isDirectory.boolValue else { return false }
         return fileManager.isWritableFile(atPath: url.path)
     }
+
+    /// Writes a tiny probe file into `folder`, fsyncs it, then removes it — the only
+    /// reliable way to catch a OneDrive Files-On-Demand directory whose provider domain
+    /// is signed out: such a directory can report as existing and POSIX-writable
+    /// (`isWritableDirectory` returns true) while an actual write fails. True only when
+    /// the write, fsync, AND removal of the probe file all succeed; any failure at any
+    /// of those steps means false, so the caller treats the folder as unavailable
+    /// rather than proceeding to compose a real PDF into it.
+    public static func probeWritable(
+        at folder: URL,
+        fileManager: FileManager = .default
+    ) -> Bool {
+        let probeURL = folder.appendingPathComponent(".redline-probe-\(UUID().uuidString)")
+        do {
+            try AtomicFile.write(Data(), to: probeURL)
+        } catch {
+            return false
+        }
+        do {
+            try fileManager.removeItem(at: probeURL)
+        } catch {
+            return false
+        }
+        return true
+    }
 }

Tests/ShotdeckCoreTests/TransportSettingsTests.swift

@@ -122,6 +122,44 @@ func isWritableDirectoryFalseForAPlainFileAndForANonexistentPath() throws {
     #expect(!OneDriveLocator.isWritableDirectory(at: dir.appendingPathComponent("does-not-exist")))
 }
 
+@Test
+func probeWritableTrueForAnOrdinaryWritableDirectoryAndLeavesNoProbeFileBehind() throws {
+    let dir = try makeTransportTemporaryDirectory(prefix: "shotdeck-probe-writable")
+    defer { try? FileManager.default.removeItem(at: dir) }
+
+    #expect(OneDriveLocator.probeWritable(at: dir))
+
+    let leftovers = try FileManager.default.contentsOfDirectory(atPath: dir.path)
+    #expect(leftovers.isEmpty)
+}
+
+@Test
+func probeWritableFalseForAChmod500Directory() throws {
+    // The File Provider edge case this probe exists for: isWritableDirectory can be
+    // true (as verified by the isWritableDirectory tests above) while an actual write
+    // still fails. A chmod 500 directory reproduces that "looks writable, isn't"
+    // shape closely enough to prove the probe itself does a real write, not just
+    // another permissions-bit check.
+    let dir = try makeTransportTemporaryDirectory(prefix: "shotdeck-probe-unwritable")
+    defer {
+        try? FileManager.default.setAttributes([.posixPermissions: 0o755], ofItemAtPath: dir.path)
+        try? FileManager.default.removeItem(at: dir)
+    }
+    try FileManager.default.setAttributes([.posixPermissions: 0o500], ofItemAtPath: dir.path)
+
+    #expect(!OneDriveLocator.probeWritable(at: dir))
+}
+
+@Test
+func probeWritableFalseForAPlainFilePath() throws {
+    let dir = try makeTransportTemporaryDirectory(prefix: "shotdeck-probe-file-parent")
+    defer { try? FileManager.default.removeItem(at: dir) }
+    let filePath = dir.appendingPathComponent("plain-file.txt")
+    FileManager.default.createFile(atPath: filePath.path, contents: Data("x".utf8))
+
+    #expect(!OneDriveLocator.probeWritable(at: filePath))
+}
+
 struct TransportDefaultsSuite {
     let name: String
     let defaults: UserDefaults