v2sub-process: print the cmd when a capability is unsupported

> In handshake_capabilities() we use warning() when a capability
> is not supported, so the exit code of the function is 0 and no
> further error is shown. This is a problem because the warning
> message doesn't tell us which subprocess cmd failed.
> 
> On the contrary if we cannot write a packet from this function,
> we use error() and then subprocess_start() outputs:
> 
>      initialization for subprocess '<cmd>' failed
> 
> so we can know which subprocess cmd failed.
> 
> Let's improve the warning() message, so that we can know which
> subprocess cmd failed.
> 
> Helped-by: Lars Schneider <larsxschneider@gmail.com>
> Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
> ---
> Change since previous version:
> 
>    - Use process->argv[0] instead of adding a new parameter to
>      handshake_capabilities(), thanks to Lars.
> 
>   sub-process.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/sub-process.c b/sub-process.c
> index 6edb97c1c6..6ccfaaba99 100644
> --- a/sub-process.c
> +++ b/sub-process.c
> @@ -184,8 +184,8 @@ static int handshake_capabilities(struct child_process *process,
>   			if (supported_capabilities)
>   				*supported_capabilities |= capabilities[i].flag;
>   		} else {
> -			warning("external filter requested unsupported filter capability '%s'",
> -				p);
> +			warning("subprocess '%s' requested unsupported capability '%s'",
> +				process->argv[0], p);
>   		}
>   	}
>   
> 

>> -			warning("external filter requested unsupported filter capability '%s'",
>> -				p);
>> +			warning("subprocess '%s' requested unsupported capability '%s'",
>> +				process->argv[0], p);
>>   		}
>>   	}
>>   
>>
>
> This one is even cleaner.  Thanks Lars for pointing out the fact we
> already had the cmd name.  Looks good.

> Ben Peart <peartben@gmail.com> writes:
>
>>> -			warning("external filter requested unsupported filter capability '%s'",
>>> -				p);
>>> +			warning("subprocess '%s' requested unsupported capability '%s'",
>>> +				process->argv[0], p);
>>>   		}
>>>   	}
>>>   
>>>
>>
>> This one is even cleaner.  Thanks Lars for pointing out the fact we
>> already had the cmd name.  Looks good.
>
> Thanks, all.  Will queue.

> Junio C Hamano <gitster@pobox.com> writes:
> 
>> I still think we would want to turn warning() to die(), but it
>> probably is better to do so in a separate follow-up patch.  That
>> will give us a good place to record the reason why the current "just
>> call a warning() and pretend as if nothing bad happend" is wrong.
> 
> And here is such an update.  It seems that pretty much all comments
> in the original thread were "warning is wrong--we should die here",
> but nobody seems to have bothered following it through.
> 
> cf. <20170815111725.5d009b66@twelve2.svl.corp.google.com>
> 
> -- >8 --
> Subject: [PATCH] subprocess: loudly die when subprocess asks for an unsupported capability
> 
> The handshake_capabilities() function first advertises the set of
> capabilities it supports, so that the other side can pick and choose
> which ones to use and ask us to enable in its response.  Then we
> read the response that tells us what choice the other side made.  If
> we saw something that we never advertised, that indicates one of two
> things.  The other side, i.e. the "upgraded" filter, is not paying
> attention of the capabilities advertisement, and asking something
> its correct operation relies on, but we are not capable of giving
> that unknown feature and operate without it, so after that point the
> exchange of data is a garbage-in-garbage-out.  Or the other side
> wanted to ask for one of the capabilities we advertised, but the
> code has typo and their wish to enable a capability that its correct
> operation relies on is not understood on this end.  The result is
> the same garbage-in-garbage-out.
> 
> Instead of sweeping such a potential bug under the rug, die loudly
> when we see a request for an unsupported capability in order to
> force sloppily-written filter scripts to get corrected.
> 

> Signed-off-by: Junio C Hamano <gitster@pobox.com>
> ---
>   sub-process.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/sub-process.c b/sub-process.c
> index fcc4832c14..ec9a51b7b1 100644
> --- a/sub-process.c
> +++ b/sub-process.c
> @@ -181,8 +181,8 @@ static int handshake_capabilities(struct child_process *process,
>   			if (supported_capabilities)
>   				*supported_capabilities |= capabilities[i].flag;
>   		} else {
> -			warning("subprocess '%s' requested unsupported capability '%s'",
> -				process->argv[0], p);
> +			die("subprocess '%s' requested unsupported capability '%s'",
> +			    process->argv[0], p);
>   		}
>   	}
>   
> 

> On 11 Sep 2017, at 05:27, Junio C Hamano <gitster@pobox.com> wrote:
> 
> Junio C Hamano <gitster@pobox.com> writes:
> 
>> I still think we would want to turn warning() to die(), but it
>> probably is better to do so in a separate follow-up patch.  That
>> will give us a good place to record the reason why the current "just
>> call a warning() and pretend as if nothing bad happend" is wrong.
> 
> And here is such an update.  It seems that pretty much all comments
> in the original thread were "warning is wrong--we should die here",
> but nobody seems to have bothered following it through.
> 
> cf. <20170815111725.5d009b66@twelve2.svl.corp.google.com>
> 
> -- >8 --
> Subject: [PATCH] subprocess: loudly die when subprocess asks for an unsupported capability
> 
> The handshake_capabilities() function first advertises the set of
> capabilities it supports, so that the other side can pick and choose
> which ones to use and ask us to enable in its response.  Then we
> read the response that tells us what choice the other side made.  If
> we saw something that we never advertised, that indicates one of two
> things.  The other side, i.e. the "upgraded" filter, is not paying
> attention of the capabilities advertisement, and asking something
> its correct operation relies on, but we are not capable of giving
> that unknown feature and operate without it, so after that point the
> exchange of data is a garbage-in-garbage-out.  Or the other side
> wanted to ask for one of the capabilities we advertised, but the
> code has typo and their wish to enable a capability that its correct
> operation relies on is not understood on this end.  The result is
> the same garbage-in-garbage-out.
> 
> Instead of sweeping such a potential bug under the rug, die loudly
> when we see a request for an unsupported capability in order to
> force sloppily-written filter scripts to get corrected.
> 
> Signed-off-by: Junio C Hamano <gitster@pobox.com>
> ---
> sub-process.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/sub-process.c b/sub-process.c
> index fcc4832c14..ec9a51b7b1 100644
> --- a/sub-process.c
> +++ b/sub-process.c
> @@ -181,8 +181,8 @@ static int handshake_capabilities(struct child_process *process,
> 			if (supported_capabilities)
> 				*supported_capabilities |= capabilities[i].flag;
> 		} else {
> -			warning("subprocess '%s' requested unsupported capability '%s'",
> -				process->argv[0], p);
> +			die("subprocess '%s' requested unsupported capability '%s'",
> +			    process->argv[0], p);

> On 16 Aug 2017, at 14:40, Christian Couder <christian.couder@gmail.com> wrote:
> 
> In handshake_capabilities() we use warning() when a capability
> is not supported, so the exit code of the function is 0 and no
> further error is shown. This is a problem because the warning
> message doesn't tell us which subprocess cmd failed.
> 
> On the contrary if we cannot write a packet from this function,
> we use error() and then subprocess_start() outputs:
> 
>    initialization for subprocess '<cmd>' failed
> 
> so we can know which subprocess cmd failed.
> 
> Let's improve the warning() message, so that we can know which
> subprocess cmd failed.
> 
> Helped-by: Lars Schneider <larsxschneider@gmail.com>
> Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
> ---
> Change since previous version:
> 
>  - Use process->argv[0] instead of adding a new parameter to
>    handshake_capabilities(), thanks to Lars.
> 
> sub-process.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/sub-process.c b/sub-process.c
> index 6edb97c1c6..6ccfaaba99 100644
> --- a/sub-process.c
> +++ b/sub-process.c
> @@ -184,8 +184,8 @@ static int handshake_capabilities(struct child_process *process,
> 			if (supported_capabilities)
> 				*supported_capabilities |= capabilities[i].flag;
> 		} else {
> -			warning("external filter requested unsupported filter capability '%s'",
> -				p);
> +			warning("subprocess '%s' requested unsupported capability '%s'",
> +				process->argv[0], p);
> 		}
> 	}
> 
> -- 
> 2.14.1.146.g7de11f915a
>