fix(software): don't treat an empty read-ahead buffer as the end of the file

CHANGELOG.md

@@ -10,7 +10,16 @@ the public-API contract.
 
 ## [Unreleased]
 
-_Nothing yet._
+### Fixed
+
+- **A seek that misses the software path's packet cache no longer ends playback at its landing.**
+  The miss empties the read-ahead store while holding the store's operation lock, and a demux-loop
+  read that began inside the seek's window could leave its wait at once (its reader was not current
+  yet), get the lock only after the seek had landed, find the store empty and return nil, which the
+  loop takes for end of stream: "end of media" at the landing. An empty store is now the end only
+  when the producer has seen the source's own end; before that the read throws `.interrupted` and the
+  loop reads again. Seen on DVD-Video, whose time-map seek settles at once: a seek past the first
+  cell ended playback in 7 of 8 runs, in none of 8 with the fix (48 seeks that missed the cache).
 
 ## [7.45.0] - 2026-10-21
 

Sources/AetherEngine/Native/SoftwarePacketReadAhead.swift

@@ -421,8 +421,16 @@ final class SoftwarePacketReadAhead: @unchecked Sendable {
         }
         if count == 0 {
             let error = failure
+            // Only the source's own end ends the stream. The wait above also stops for a reader
+            // that is not current, and a seek can make it current again while this read waits for
+            // `operations`, which a cache miss holds to empty the store: that store is about to
+            // refill from the new position, and reading it as the end stopped playback at the
+            // landing (a DVD seek past the first cell, whose time-map seek settles at once, hit it
+            // in 7 of 8 runs).
+            let sourceEnded = ended
             condition.unlock()
             if let error { throw error }
+            guard sourceEnded else { throw ReadError.interrupted }
             return nil
         }
         condition.unlock()

Tests/AetherEngineTests/SoftwarePacketReadAheadEndOfStreamTests.swift

@@ -0,0 +1,65 @@
+import Foundation
+import Testing
+@testable import AetherEngine
+
+/// An empty read-ahead store is the end of the stream only when the source said so.
+///
+/// A seek that misses the cache empties the store while holding `operations`. A consumer read that
+/// started inside the seek's window left its wait at once (its reader was not current), then waited
+/// for `operations`; by the time it got the lock the seek had landed, its reader was current again,
+/// and it found the store empty and returned nil, which the host takes for the end of the media.
+/// Playback stopped at the landing (a DVD seek past the first cell: 7 of 8 runs).
+@Suite("Software read-ahead end of stream", .offCooperativePool, .timeLimit(.minutes(1)))
+struct SoftwarePacketReadAheadEndOfStreamTests {
+
+    /// A reader that is not current on its first check and current on every later one: the
+    /// seek's window closing between the read's wait and its admission check.
+    private final class CurrentAfterFirstCheck: @unchecked Sendable {
+        private let lock = NSLock()
+        private var checks = 0
+        func check() -> Bool { lock.lock(); defer { lock.unlock() }; checks += 1; return checks > 1 }
+    }
+
+    private static func readAhead(_ root: URL,
+                                  source: @escaping @Sendable (@Sendable () -> Bool) throws -> SoftwareStoredPacket?
+    ) throws -> SoftwarePacketReadAhead {
+        let fifo = try SoftwarePacketDiskFIFO(chunkTargetBytes: 64 * 1024, retainConsumed: true,
+                                              parentDirectory: root)
+        return SoftwarePacketReadAhead(
+            video: .init(index: 0, numerator: 1, denominator: 1000), audio: nil,
+            byteBudget: 1 << 20, forwardSeconds: 10, initialSourceClock: 0, fifo: fifo,
+            readSource: source)
+    }
+
+    private static func temporaryRoot() throws -> URL {
+        let root = FileManager.default.temporaryDirectory
+            .appendingPathComponent("readahead-eos-\(UUID().uuidString)", isDirectory: true)
+        try FileManager.default.createDirectory(at: root, withIntermediateDirectories: true)
+        return root
+    }
+
+    @Test("an empty store whose reader becomes current again is not the end of the stream")
+    func emptyStoreBeforeTheSourceEndsIsInterrupted() throws {
+        let root = try Self.temporaryRoot()
+        defer { try? FileManager.default.removeItem(at: root) }
+        // Not started: nothing is read, the store stays empty, the source has not ended.
+        let source = try Self.readAhead(root) { _ in nil }
+        defer { source.close() }
+        let reader = CurrentAfterFirstCheck()
+        #expect(throws: SoftwarePacketReadAhead.ReadError.interrupted) {
+            _ = try source.read(isCurrent: { reader.check() })
+        }
+    }
+
+    @Test("the source's own end is the end of the stream")
+    func sourceEndIsEndOfStream() async throws {
+        let root = try Self.temporaryRoot()
+        defer { try? FileManager.default.removeItem(at: root) }
+        let source = try Self.readAhead(root) { _ in nil }
+        defer { source.close() }
+        source.start()
+        // The producer has to have seen the end before there is anything to measure.
+        try await waitFor { source.snapshot.sourceEnded }
+        #expect(try source.read() == nil)
+    }
+}