fix consecutive backward seeks in seekable read buffers

src/Compression/CachedCompressedReadBuffer.cpp

@@ -105,7 +105,7 @@ void CachedCompressedReadBuffer::seek(size_t offset_in_compressed_file, size_t o
         /// We will discard our working_buffer, but have to account rest bytes
         bytes += offset();
         /// No data, everything discarded
-        pos = working_buffer.end();
+        resetWorkingBuffer();
         owned_cell.reset();
 
         /// Remember required offset in decompressed block which will be set in

src/Compression/CompressedReadBufferFromFile.cpp

@@ -80,7 +80,7 @@ void CompressedReadBufferFromFile::seek(size_t offset_in_compressed_file, size_t
         /// We will discard our working_buffer, but have to account rest bytes
         bytes += offset();
         /// No data, everything discarded
-        pos = working_buffer.end();
+        resetWorkingBuffer();
         size_compressed = 0;
         /// Remember required offset in decompressed block which will be set in
         /// the next ReadBuffer::next() call

src/Disks/IO/AsynchronousReadIndirectBufferFromRemoteFS.cpp

@@ -243,7 +243,7 @@ off_t AsynchronousReadIndirectBufferFromRemoteFS::seek(off_t offset_, int whence
         prefetch_future = {};
     }
 
-    pos = working_buffer.end();
+    resetWorkingBuffer();
 
     /**
     * Lazy ignore. Save number of bytes to ignore and ignore it either for prefetch buffer or current buffer.

src/Disks/IO/ReadIndirectBufferFromRemoteFS.cpp

@@ -64,7 +64,7 @@ off_t ReadIndirectBufferFromRemoteFS::seek(off_t offset_, int whence)
         throw Exception("Only SEEK_SET or SEEK_CUR modes are allowed.", ErrorCodes::CANNOT_SEEK_THROUGH_FILE);
 
     impl->reset();
-    pos = working_buffer.end();
+    resetWorkingBuffer();
 
     return impl->file_offset_of_buffer_end;
 }

src/IO/AsynchronousReadBufferFromFileDescriptor.cpp

@@ -181,8 +181,8 @@ off_t AsynchronousReadBufferFromFileDescriptor::seek(off_t offset, int whence)
 
         off_t offset_after_seek_pos = new_pos - seek_pos;
 
-        /// First put position at the end of the buffer so the next read will fetch new data to the buffer.
-        pos = working_buffer.end();
+        /// First reset the buffer so the next read will fetch new data to the buffer.
+        resetWorkingBuffer();
 
         /// Just update the info about the next position in file.
 

src/IO/BufferBase.h

@@ -97,6 +97,15 @@ public:
     bool isPadded() const { return padded; }
 
 protected:
+    void resetWorkingBuffer()
+    {
+        /// Move position to the end of buffer to trigger call of 'next' on next reading.
+        /// Discard all data in current working buffer to prevent wrong assumtions on content
+        /// of buffer, e.g. for optimizations of seeks in seekable buffers.
+        working_buffer.resize(0);
+        pos = working_buffer.end();
+    }
+
     /// Read/write position.
     Position pos;
 

src/IO/ReadBufferFromEncryptedFile.cpp

@@ -56,7 +56,7 @@ off_t ReadBufferFromEncryptedFile::seek(off_t off, int whence)
         offset = new_pos;
 
         /// No more reading from the current working buffer until next() is called.
-        pos = working_buffer.end();
+        resetWorkingBuffer();
         assert(!hasPendingData());
     }
 

src/IO/ReadBufferFromFileDescriptor.cpp

@@ -111,7 +111,6 @@ bool ReadBufferFromFileDescriptor::nextImpl()
         ProfileEvents::increment(ProfileEvents::ReadBufferFromFileDescriptorReadBytes, bytes_read);
         working_buffer = internal_buffer;
         working_buffer.resize(bytes_read);
-        buffer_is_dirty = false;
     }
     else
         return false;
@@ -153,10 +152,10 @@ off_t ReadBufferFromFileDescriptor::seek(off_t offset, int whence)
     }
 
     /// Position is unchanged.
-    if (!buffer_is_dirty && (new_pos + (working_buffer.end() - pos) == file_offset_of_buffer_end))
+    if (new_pos + (working_buffer.end() - pos) == file_offset_of_buffer_end)
         return new_pos;
 
-    if (!buffer_is_dirty && file_offset_of_buffer_end - working_buffer.size() <= static_cast<size_t>(new_pos)
+    if (file_offset_of_buffer_end - working_buffer.size() <= static_cast<size_t>(new_pos)
         && new_pos <= file_offset_of_buffer_end)
     {
         /// Position is still inside the buffer.
@@ -177,12 +176,8 @@ off_t ReadBufferFromFileDescriptor::seek(off_t offset, int whence)
 
         off_t offset_after_seek_pos = new_pos - seek_pos;
 
-        /// First put position at the end of the buffer so the next read will fetch new data to the buffer.
-        pos = working_buffer.end();
-
-        /// Mark buffer as dirty to disallow further seek optimizations, because fetching data to the buffer
-        /// is delayed to the next call of 'nextImpl', but it may be not called before next seek.
-        buffer_is_dirty = true;
+        /// First reset the buffer so the next read will fetch new data to the buffer.
+        resetWorkingBuffer();
 
         /// In case of using 'pread' we just update the info about the next position in file.
         /// In case of using 'read' we call 'lseek'.
@@ -234,7 +229,6 @@ void ReadBufferFromFileDescriptor::rewind()
     working_buffer.resize(0);
     pos = working_buffer.begin();
     file_offset_of_buffer_end = 0;
-    buffer_is_dirty = true;
 }
 
 

src/IO/ReadBufferFromFileDescriptor.h

@@ -62,9 +62,6 @@ public:
 private:
     /// Assuming file descriptor supports 'select', check that we have data to read or wait until timeout.
     bool poll(size_t timeout_microseconds);
-
-    /// If it's true then we cannot assume on content of buffer to optimize seek calls.
-    bool buffer_is_dirty = true;
 };
 
 

src/IO/ReadBufferFromS3.cpp

@@ -187,7 +187,7 @@ off_t ReadBufferFromS3::seek(off_t offset_, int whence)
             }
         }
 
-        pos = working_buffer.end();
+        resetWorkingBuffer();
         if (impl)
         {
             ProfileEvents::increment(ProfileEvents::ReadBufferSeekCancelConnection);

src/IO/ReadWriteBufferFromHTTP.h

@@ -498,7 +498,7 @@ namespace detail
                 impl.reset();
             }
 
-            pos = working_buffer.end();
+            resetWorkingBuffer();
             read_range.begin = offset_;
             read_range.end = std::nullopt;
             offset_from_begin_pos = 0;

src/Storages/HDFS/ReadBufferFromHDFS.cpp

@@ -173,7 +173,7 @@ off_t ReadBufferFromHDFS::seek(off_t offset_, int whence)
         return getPosition();
     }
 
-    pos = working_buffer.end();
+    resetWorkingBuffer();
     impl->seek(offset_, whence);
     return impl->getPosition();
 }