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Re: [Qemu-devel] [Qemu-block] [PATCH] Add interactive mode to qemu-img c

From: Programmingkid
Subject: Re: [Qemu-devel] [Qemu-block] [PATCH] Add interactive mode to qemu-img command
Date: Mon, 30 Jul 2018 16:53:25 -0400

> On Jul 30, 2018, at 3:48 PM, Eric Blake <address@hidden> wrote:
> On 07/30/2018 02:14 PM, John Arbuckle wrote:
>> Changes qemu-img so if the user runs it without any
>> arguments, it will walk the user thru making an image
>> file.
> Please remember to cc qemu-devel on ALL patches, as suggested by 
> ./scripts/getmaintainer.pl.

Sorry about that. I did remember a few minutes
after sending the patch and sent it.

>> Signed-off-by: John Arbuckle <address@hidden>
>> ---
>>  qemu-img.c | 31 +++++++++++++++++++++++++++++--
>>  1 file changed, 29 insertions(+), 2 deletions(-)
> Missing documentation patches (at a minimum, 'man qemu-img' should be updated 
> to mention this new mode of operation).  I'm also going to be a stickler and 
> require an addition to tests/qemu-iotests to ensure this new feature gets 
> coverage and doesn't regress (at least, if others are even in favor of the 
> idea. Personally, I'm 60-40 against the bloat, as telling the user to read 
> --help is sufficient in my mind).

After talking to Max Reitz about this issue I'm
not even sure I should continue with this plan.

Another idea I have is to improve the
documentation to qemu-img. Right now it looks
very hard to look at. I'm thinking a few examples
might make things easier for the user.

>> diff --git a/qemu-img.c b/qemu-img.c
>> index 9b7506b8ae..aa3df3b431 100644
>> --- a/qemu-img.c
>> +++ b/qemu-img.c
>> @@ -4873,6 +4873,32 @@ out:
>>      return ret;
>>  }
>>  +/* Guides the user on making an image file */
>> +static int interactive_mode()
>> +{
>> +    char format[100];
>> +    char size[100];
>> +    char name[1000];
> Eww. Fixed size buffers for user-provided input are evil.  getline() is your 
> friend.
>> +
>> +    printf("\nInteractive mode (Enter Control-C to cancel)\n");
>> +    printf("Please select a format (qcow, qcow2, raw, vdi, vhdx, vmdk, 
>> vpc): ");
> Incomplete.  Better to generate the list dynamically by iterating over the 
> QAPI enum than it is to hard-code an incomplete list - particularly since 
> distros can white- or black-list particular drivers.
>> +    scanf("%100s", format);
> Eww. Repeating the limit of your fixed size buffer, without checking that you 
> actually read a complete line, or that the scanf actually succeeded in 
> reading.  That's dangerous if a later programmer changes one but not both of 
> the two lines that are supposed to be using the same fixed size.  Even if we 
> accept the direction that this patch is heading in, it would need to be done 
> using more robust code.
>> +    printf("Please enter a size (e.g. 100M, 10G): ");
>> +    scanf("%100s", size);
>> +    printf("Please enter a name: ");
>> +    scanf("%1000s", name);
> A limit of 1000 has no bearing on actual file name lengths; a more typical 
> limit of NAME_MAX and/or PATH_MAX may come into play first (as low as 256 on 
> many systems, but on some systems, that value is intentionally undefined 
> because there is no inherent limit).  Again, you should be using getline() 
> and malloc'ing your result rather than using arbitrary (and probably 
> wrong-size) fixed-size buffers.
>> +
>> +    const char *arguments[] = {"create", "-f", format, name, size};
> Although this is valid C99, qemu tends to prefer that you declare all 
> variables at the start of the scope rather than after statements.
> You did not do ANY error checking that you actually parsed arguments from the 
> user; that in turn could result in passing bogus arguments on to img_create() 
> that could not normally happen via command line usage.

I was going to do error handling until I realized
img_create() is very good at telling the user
what is wrong. Anything I would add would be
redundant to what img_create() does.

> Why are you hard-coding a create, rather than going FULLY interactive and 
> asking the user which sub-command (create, compare, ...) that they want to 
> use?

I was only thinking about what I use qemu-img for.
You do have a good point. A fully interactive
qemu-img command would be better.

>  Note that if we do want the level of interactivity to encompass the choice 
> of subcommand, that you still need to make things programmatic, not 
> hard-coded.  Also, John Snow had started some work on making qemu-img.c and 
> associated documentation saner, including making subcommands more uniform; 
> you may want to rebase your work on top of his cleanups, if there is even 
> demand for this.
>> +    int arg_count = 5;
>> +    int return_value;
>> +    return_value = img_create(arg_count, (char **)arguments);
> Can we come up with a way to not have to cast away the const, if we are 
> locally supplying the arguments?
>> +    if (return_value == 0) {
>> +        printf("Done creating image file\n");
>> +    }
>> +
>> +    return return_value;
>> +}
>> +
>>  static const img_cmd_t img_cmds[] = {
>>  #define DEF(option, callback, arg_string)        \
>>      { option, callback },
>> @@ -4912,8 +4938,9 @@ int main(int argc, char **argv)
>>        module_call_init(MODULE_INIT_QOM);
>>      bdrv_init();
>> -    if (argc < 2) {
>> -        error_exit("Not enough arguments");
>> +
>> +    if (argc == 1) { /* If no arguments passed to qemu-img */
>> +        return interactive_mode();
> This is placed early-enough in main() that you never use interactive mode if 
> the user passed options but no subcommand (such as 'qemu-img -T foo'), is 
> that intentional?


>  Conversely, your patch does not help if a user calls 'qemu-img create' 
> without options, while it can be argued that if we want (partial) 
> interactivity, why not be nice to the user and provide it at any step of the 
> way rather than just when no subcommand name was given.

I figure a user wants interactive help, or
not at all. Your idea is interesting. 

> -- 
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3266
> Virtualization:  qemu.org | libvirt.org

Thank you for looking at my patch.

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