On Fri, Sep 17 2021, Jeff Hostetler wrote:
> 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@xxxxxxxxxxxxx>
>>>
>>> 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@xxxxxxxxxxxxx>
>>> ---
>>> 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.
I haven't seen this idiom before. I think it's best to avoid patterns
designed to massage messages out of any specific compilers/versions.
It seems inevitable that it'll either be counter-productive or
redundant. Here with clang v11 doing this makes the warning
worse. I.e. without the forward declaration:
t/helper/test-simple-ipc.c:315:31: error: incompatible function
pointer types passing 'int (void *, const struct child_process *,
int)' to parameter of type ' start_bg_wait_cb *' (aka 'int (*)(void
*, const struct child_process *)')
[-Werror,-Wincompatible-function-pointer-types]
sbgr = start_bg_command(&cp, bg_wait_cb, NULL, cl_args.max_wait_sec);
^~~~~~~~~~
./run-command.h:564:29: note: passing argument to parameter 'wait_cb' here
start_bg_wait_cb *wait_cb,
^
1 error generated.
I.e. we get the specific warning category for this type of error
(-Werror,-Wincompatible-function-pointer-types), and we're pointed at
the caller in question (which to be fair, it seems you don't prefer),
but also a reference to the run-command.h definition.
Most importantly, we get quoted what the type is/should be, which is
missing with the forward declaration. It's the equivalent of saying "you
did bad!" instead of "you did bad X, do Y instead!".
>> 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.
I don't think so, but the ones we widely use do, i.e. clang and gcc at
least.
For this sort of thing it really doesn't matter if *all* compilers
support it, since we'll only need to catch such "missing enum arm"
issues with one of them.
E.g. in my 338abb0f045 (builtins + test helpers: use return instead of
exit() in cmd_*, 2021-06-08) I fixed something that I've only gotten
Oracle SunCC to emit (gcc and clang don't detect it), but as long as
that one compiler does & someone checks it regularly...
By having a "default" case you're hiding that detection from the
compilers capable of detecting a logic error in this code, whereas if
the compiler can't do that it'll just ignore it.
