Re: [PATCH 7/7] t/helper/simple-ipc: convert test-simple-ipc to use start_bg_command

[Jeff Hostetler <git@jeffhostetler.com>]

On 9/16/21 1:06 AM, Taylor Blau wrote:
> On Wed, Sep 15, 2021 at 08:36:17PM +0000, Jeff Hostetler via GitGitGadget wrote:
>> From: Jeff Hostetler <jeffhost@microsoft.com>
>>
>> Convert test helper to use `start_bg_command()` when spawning a server
>> daemon in the background rather than blocks of platform-specific code.
>>
>> Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
>> ---
>>   t/helper/test-simple-ipc.c | 193 ++++++++-----------------------------
>>   1 file changed, 40 insertions(+), 153 deletions(-)
>>
>> diff --git a/t/helper/test-simple-ipc.c b/t/helper/test-simple-ipc.c
>> index 91345180750..59a950f3b00 100644
>> --- a/t/helper/test-simple-ipc.c
>> +++ b/t/helper/test-simple-ipc.c
>> @@ -9,6 +9,7 @@
>>   #include "parse-options.h"
>>   #include "thread-utils.h"
>>   #include "strvec.h"
>> +#include "run-command.h"
>>
>>   #ifndef SUPPORTS_SIMPLE_IPC
>>   int cmd__simple_ipc(int argc, const char **argv)
>> @@ -274,178 +275,64 @@ static int daemon__run_server(void)
>>   	return ret;
>>   }
>>
>> -#ifndef GIT_WINDOWS_NATIVE
>> -/*
>> - * This is adapted from `daemonize()`.  Use `fork()` to directly create and
>> - * run the daemon in a child process.
>> - */
>> -static int spawn_server(pid_t *pid)
>> -{
>> -	struct ipc_server_opts opts = {
>> -		.nr_threads = cl_args.nr_threads,
>> -	};
>> +static start_bg_wait_cb bg_wait_cb;
> 
> This whole patch is delightful to read, as the new implementation is so
> much cleaner as a result of the earlier work in this series.
> 
> Am I correct in assuming that this is to encourage a compiler error if
> bg_wait_cb does not satisfy the type of start_bg_wait_cb? If so, then I
> think we are already getting that by trying to pass bg_wait_cb to
> start_bg_command().

I use that trick to get the compiler to give me a compiler error at the
point of the function declaration.

For example, If I add an arg to the function that doesn't match what's
in the prototype definition, I get:

t/helper/test-simple-ipc.c:280:12: error: conflicting types for 'bg_wait_cb'
static int bg_wait_cb(const struct child_process *cp, void *cb_data, int 
foo)
            ^
t/helper/test-simple-ipc.c:278:25: note: previous declaration is here
static start_bg_wait_cb bg_wait_cb;
                         ^
1 error generated.

Yes, we may get an error when the function pointer is referenced in
start_bg_command() or if we're using it to initialize a vtable or
something, but those errors are further away from the actual error
(and sometimes they can be a little cryptic).

Also, it helps document that this function's signature is predefined
for a reason.

It's a quirky trick I know, but it has served me well over the years.
	
> 
> E.g., applying this (intentionally broken) diff on top:
> 
> --- 8< ---
> 
> diff --git a/t/helper/test-simple-ipc.c b/t/helper/test-simple-ipc.c
> index 59a950f3b0..3aed787206 100644
> --- a/t/helper/test-simple-ipc.c
> +++ b/t/helper/test-simple-ipc.c
> @@ -275,9 +275,7 @@ static int daemon__run_server(void)
>   	return ret;
>   }
> 
> -static start_bg_wait_cb bg_wait_cb;
> -
> -static int bg_wait_cb(void *cb_data, const struct child_process *cp)
> +static int bg_wait_cb(const void *cb_data, const struct child_process *cp)
>   {
>   	int s = ipc_get_active_state(cl_args.path);
> 
> --- >8 ---
> 
> and then compiling still warns of a mismatched type when calling
> start_bg_command().
> 
>> -	*pid = fork();
>> -
>> -	switch (*pid) {
>> -	case 0:
>> -		if (setsid() == -1)
>> -			error_errno(_("setsid failed"));
>> -		close(0);
>> -		close(1);
>> -		close(2);
>> -		sanitize_stdfds();
>> +static int bg_wait_cb(void *cb_data, const struct child_process *cp)
>> +{
>> +	int s = ipc_get_active_state(cl_args.path);
>>
>> -		return ipc_server_run(cl_args.path, &opts, test_app_cb,
>> -				      (void*)&my_app_data);
>> +	switch (s) {
>> +	case IPC_STATE__LISTENING:
>> +		/* child is "ready" */
>> +		return 0;
>>
>> -	case -1:
>> -		return error_errno(_("could not spawn daemon in the background"));
>> +	case IPC_STATE__NOT_LISTENING:
>> +	case IPC_STATE__PATH_NOT_FOUND:
>> +		/* give child more time */
>> +		return 1;
>>
>>   	default:
> 
> I'm always a little hesitant to have default cases when switch over enum
> types, since it suppresses the warning when there's a new value of that
> type. But we already have a similar default in client__probe_server().

Do all compilers now handle switching over an enum and detect unhandled
cases?  Once upon a time that wasn't the case IIRC.

>...
> 
> Ditto.
> 
> Thanks,
> Taylor
>