Handle non-seekable file descriptors

edolstra · edolstra · commit a70174cc1e8d · 2025-10-17T15:56:13.000+02:00
diff --git a/src/libutil/include/nix/util/serialise.hh b/src/libutil/include/nix/util/serialise.hh
@@ -179,6 +179,7 @@ struct FdSource : BufferedSource
     Descriptor fd;
     size_t read = 0;
     BackedStringView endOfFileError{"unexpected end-of-file"};
+    bool isSeekable = true;
 
     FdSource()
         : fd(INVALID_DESCRIPTOR)
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
@@ -202,6 +202,7 @@ bool FdSource::hasData()
 
 void FdSource::skip(size_t len)
 {
+#ifndef _WIN32
     /* Discard data in the buffer. */
     if (len && buffer && bufPosIn - bufPosOut) {
         if (len >= bufPosIn - bufPosOut) {
@@ -213,9 +214,21 @@ void FdSource::skip(size_t len)
         }
     }
 
-    /* Seek forward in the file to skip the rest. */
-    if (len && lseek(fd, len, SEEK_CUR) == -1)
-        throw SysError("seeking forward in file");
+    /* If we can, seek forward in the file to skip the rest. */
+    if (isSeekable && len) {
+        if (lseek(fd, len, SEEK_CUR) == -1) {
+            if (errno == ESPIPE)
+                isSeekable = false;
+            else
+                throw SysError("seeking forward in file");
+        } else
+            return;
+    }
+#endif
+
+    /* Otherwise, skip by reading. */
+    if (len)
+        BufferedSource::skip(len);
 }
 
 size_t StringSource::read(char * data, size_t len)
