libutil: treat EIO as EOF in readLine

src/libutil-tests/file-descriptor.cc

@@ -6,6 +6,11 @@
 
 #include <cstring>
 
+#ifndef _WIN32
+#  include <fcntl.h>
+#  include <stdlib.h>
+#endif
+
 namespace nix {
 
 // BufferedSource with configurable small buffer for precise boundary testing.
@@ -113,6 +118,55 @@ TEST(ReadLine, LineWithNullBytes)
             3));
 }
 
+#ifndef _WIN32
+TEST(ReadLine, TreatsEioAsEof)
+{
+    // Open a pty master. When the slave side is closed (or never opened),
+    // reading from the master returns EIO, which readLine should treat as EOF.
+    int master = posix_openpt(O_RDWR | O_NOCTTY);
+    ASSERT_NE(master, -1);
+    ASSERT_EQ(grantpt(master), 0);
+    ASSERT_EQ(unlockpt(master), 0);
+
+    // Open and immediately close the slave to trigger EIO on the master.
+    int slave = open(ptsname(master), O_RDWR | O_NOCTTY);
+    ASSERT_NE(slave, -1);
+    close(slave);
+
+    // With eofOk=true, readLine should return empty string (treating EIO as EOF).
+    EXPECT_EQ(readLine(master, /*eofOk=*/true), "");
+
+    // With eofOk=false, readLine should throw EndOfFile.
+    EXPECT_THROW(readLine(master), EndOfFile);
+
+    close(master);
+}
+
+// macOS (BSD) discards buffered pty data on slave close and returns normal
+// EOF (0) instead of EIO, so partial data never reaches the master.
+#  ifdef __linux__
+TEST(ReadLine, PartialLineBeforeEio)
+{
+    int master = posix_openpt(O_RDWR | O_NOCTTY);
+    ASSERT_NE(master, -1);
+    ASSERT_EQ(grantpt(master), 0);
+    ASSERT_EQ(unlockpt(master), 0);
+
+    int slave = open(ptsname(master), O_RDWR | O_NOCTTY);
+    ASSERT_NE(slave, -1);
+
+    // Write a partial line (no terminator) from the slave, then close it.
+    ASSERT_EQ(::write(slave, "partial", 7), 7);
+    close(slave);
+
+    // readLine should return the partial data when eofOk=true.
+    EXPECT_EQ(readLine(master, /*eofOk=*/true), "partial");
+
+    close(master);
+}
+#  endif
+#endif
+
 TEST(BufferedSourceReadLine, ReadsLinesFromPipe)
 {
     Pipe pipe;

src/libutil/file-descriptor.cc

@@ -76,8 +76,18 @@ std::string readLine(Descriptor fd, bool eofOk, char terminator)
         checkInterrupt();
         char ch;
         // FIXME: inefficient
-        auto rd =
-            retryOnBlock(fd, PollDirection::In, [&]() { return read(fd, {reinterpret_cast<std::byte *>(&ch), 1}); });
+        auto rd = retryOnBlock(fd, PollDirection::In, [&]() -> size_t {
+            try {
+                return read(fd, {reinterpret_cast<std::byte *>(&ch), 1});
+            } catch (SystemError & e) {
+                // On pty masters, EIO signals that the slave side closed,
+                // which is semantically EOF. Map it to a zero-length read
+                // so the existing EOF path handles it.
+                if (e.is(std::errc::io_error))
+                    return 0;
+                throw;
+            }
+        });
         if (rd == 0) {
             if (eofOk)
                 return s;