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

On Fri, Sep 17 2021, Jeff Hostetler wrote:

On 9/16/21 1:06 AM, Taylor Blau wrote:

quoted

On Wed, Sep 15, 2021 at 08:36:17PM +0000, Jeff Hostetler via GitGitGadget wrote:

quoted

From: Jeff Hostetler <redacted>

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 <redacted>
---
  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!".

quoted

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().

quoted

-	*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.