conn.copy_to_table() empties source file and throws exception

[sergeyspatar]

• asyncpg version: 0.20.1
• PostgreSQL version: 12.2
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: I use local PostgreSQL
• Python version: 3.8.3rc1
• Platform: Linux billing 5.4.0-4-cloud-amd64 Other examples? #1 SMP Debian 5.4.19-1 (2020-02-13) x86_64 GNU/Linux
• Do you use pgbouncer?: No
• Did you install asyncpg with pip?: Yes
• If you built asyncpg locally, which version of Cython did you use?:
• Can the issue be reproduced under both asyncio and uvloop?: Yes

Steps to reproduce:

1. Create CSV file "copy_test.csv":

echo "Hello,World" > copy_test.csv

2. This script creates table "copy_test" with two columns and tries to import file by passing file path to conn.copy_to_table():

import asyncio
import asyncpg


async def main():
    conn = await asyncpg.connect(...)
    await conn.execute("create table copy_test (c1 varchar, c2 varchar)")
    await conn.copy_to_table(
        'copy_test',
        source='copy_test.csv',
        format='csv'
    )


#import uvloop
#asyncio.set_event_loop(uvloop.new_event_loop())

asyncio.run(main())

Script throws exception:

$ python3 test_asyncpg_copy_to_table.py
Traceback (most recent call last):
  File "test_asyncpg_copy_to_table.py", line 18, in <module>
    asyncio.run(main())
  File "/usr/lib/python3.8/asyncio/runners.py", line 43, in run
    return loop.run_until_complete(main)
  File "/usr/lib/python3.8/asyncio/base_events.py", line 616, in run_until_complete
    return future.result()
  File "test_asyncpg_copy_to_table.py", line 8, in main
    await conn.copy_to_table(
  File "/usr/local/lib/python3.8/dist-packages/asyncpg/connection.py", line 666, in copy_to_table
    return await self._copy_in(copy_stmt, source, timeout)
  File "/usr/local/lib/python3.8/dist-packages/asyncpg/connection.py", line 845, in _copy_in
    return await self._protocol.copy_in(
  File "asyncpg/protocol/protocol.pyx", line 482, in copy_in
  File "asyncpg/protocol/protocol.pyx", line 455, in asyncpg.protocol.protocol.BaseProtocol.copy_in
  File "asyncpg/protocol/protocol.pyx", line 456, in asyncpg.protocol.protocol.BaseProtocol.copy_in
  File "/usr/lib/python3.8/asyncio/tasks.py", line 455, in wait_for
    return await fut
  File "/usr/local/lib/python3.8/dist-packages/asyncpg/connection.py", line 836, in __anext__
    data = await run_in_executor(None, f.read, 524288)
  File "/usr/lib/python3.8/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
io.UnsupportedOperation: read

And the file "copy_test.csv" is now 0 bytes.

[elprans]

This was fixed in #555.