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Re: [Qemu-devel] [PATCH 2/3] block/ssh: Make ssh_grow_file() blocking


From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH 2/3] block/ssh: Make ssh_grow_file() blocking
Date: Wed, 14 Feb 2018 22:28:40 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0

On 2018-02-14 22:11, Eric Blake wrote:
> On 02/14/2018 02:49 PM, Max Reitz wrote:
>> At runtime (that is, during a future ssh_truncate()), the SSH session is
>> non-blocking.  However, ssh_truncate() (or rather, bdrv_truncate() in
>> general) is not a coroutine, so this resize operation needs to block.
>>
>> For ssh_create(), that is fine, too; the session is never set to
>> non-blocking anyway.
>>
>> Signed-off-by: Max Reitz <address@hidden>
>> ---
>>   block/ssh.c | 7 +++++++
>>   1 file changed, 7 insertions(+)
>>
>> diff --git a/block/ssh.c b/block/ssh.c
>> index 964e55f7fe..ff8576f21e 100644
>> --- a/block/ssh.c
>> +++ b/block/ssh.c
>> @@ -803,17 +803,24 @@ static int ssh_file_open(BlockDriverState *bs,
>> QDict *options, int bdrv_flags,
>>       return ret;
>>   }
>>   +/* Note: This is a blocking operation */
>>   static int ssh_grow_file(BDRVSSHState *s, int64_t offset, Error **errp)
>>   {
>>       ssize_t ret;
>>       char c[1] = { '\0' };
>> +    int was_blocking = libssh2_session_get_blocking(s->session);
>>         /* offset must be strictly greater than the current size so we do
>>        * not overwrite anything */
>>       assert(offset > 0 && offset > s->attrs.filesize);
>>   +    libssh2_session_set_blocking(s->session, 1);
>> +
>>       libssh2_sftp_seek64(s->sftp_handle, offset - 1);
>>       ret = libssh2_sftp_write(s->sftp_handle, c, 1);
>> +
>> +    libssh2_session_set_blocking(s->session, was_blocking);
> 
> Is it worth skipping the two libssh2_session_set_blocking() calls if
> was_blocking is 1?  But that's a micro-optimization that probably won't
> be noticeable, so I'm also fine with unconditional.

I was hoping libssh2 is clever enough for that itself. :-)

> Reviewed-by: Eric Blake <address@hidden>

Thanks!

Max

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