[PATCH v3] trace2: refactor to avoid gcc warning under -O3

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Refactor tr2_dst_try_uds_connect() to avoid a gcc warning[1] that
appears under -O3 (but not -O2). This makes the build pass under
DEVELOPER=1 without needing a DEVOPTS=no-error.

This can be reproduced with GCC Debian 8.3.0-6, but not e.g. with
clang 7.0.1-8+deb10u2. We've had this warning since
ee4512ed481 (trace2: create new combined trace facility, 2019-02-22).

As noted in [2] this warning happens because the compiler doesn't
assume that errno must be non-zero after a failed syscall.

Let's work around by using the well-established "saved_errno" pattern,
along with returning -1 ourselves instead of "errno". The caller can
thus rely on our "errno" on failure.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61846 for a related
bug report against GCC.

1.

    trace2/tr2_dst.c: In function ‘tr2_dst_get_trace_fd.part.5’:
    trace2/tr2_dst.c:296:10: warning: ‘fd’ may be used uninitialized in this function [-Wmaybe-uninitialized]
      dst->fd = fd;
      ~~~~~~~~^~~~
    trace2/tr2_dst.c:229:6: note: ‘fd’ was declared here
      int fd;
          ^~
2. https://lore.kernel.org/git/20200404142131.GA679473@coredump.intra.peff.net/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
---

On Thu, May 20 2021, Junio C Hamano wrote:

> Jeff King <peff@peff.net> writes:
>
>> I also wondered briefly why we needed the out-parameter at all, and not
>> just letting the caller look at errno. The answer is that we need to
>> preserve it across the close() call. The more usual thing in our code
>> base would be to use saved_errno, but not have it as an out-parameter.
>> [...]
> I think this alternative is more readable as well.
>
> I'll mark the topic to be "Expecting a reroll" in the what's cooking
> report.
>
> Thanks.

Here's that re-roll, thanks both.

The patch is entirely Jeff's at this point (from
<YJrIMbr6VkYGQMfs@coredump.intra.peff.net>), with my amended commit
message. So I added his SOB per his recent-ish ML "every patch of mine
can be assumed to have my SOB" message.

Range-diff against v2:
1:  782555daad ! 1:  2e41e3e4cb trace2: refactor to avoid gcc warning under -O3
    @@ Commit message
         ee4512ed481 (trace2: create new combined trace facility, 2019-02-22).
     
         As noted in [2] this warning happens because the compiler doesn't
    -    assume that errno must be non-zero after a failed syscall. Let's work
    -    around it as suggested in that analysis. We now return -1 ourselves on
    -    error, and save away the value of errno in a variable the caller
    -    passes in.
    +    assume that errno must be non-zero after a failed syscall.
    +
    +    Let's work around by using the well-established "saved_errno" pattern,
    +    along with returning -1 ourselves instead of "errno". The caller can
    +    thus rely on our "errno" on failure.
     
         See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61846 for a related
         bug report against GCC.
    @@ Commit message
         2. https://lore.kernel.org/git/20200404142131.GA679473@coredump.intra.peff.net/
     
         Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
    +    Signed-off-by: Jeff King <peff@peff.net>
     
      ## trace2/tr2_dst.c ##
    -@@ trace2/tr2_dst.c: static int tr2_dst_try_path(struct tr2_dst *dst, const char *tgt_value)
    - #define PREFIX_AF_UNIX_STREAM "af_unix:stream:"
    - #define PREFIX_AF_UNIX_DGRAM "af_unix:dgram:"
    - 
    --static int tr2_dst_try_uds_connect(const char *path, int sock_type, int *out_fd)
    -+static int tr2_dst_try_uds_connect(const char *path, int sock_type,
    -+				   int *out_fd, int *e)
    - {
    - 	int fd;
    - 	struct sockaddr_un sa;
    +@@ trace2/tr2_dst.c: static int tr2_dst_try_uds_connect(const char *path, int sock_type, int *out_fd)
      
      	fd = socket(AF_UNIX, sock_type, 0);
    --	if (fd == -1)
    + 	if (fd == -1)
     -		return errno;
    -+	if (fd == -1) {
    -+		*e = errno;
     +		return -1;
    -+	}
      
      	sa.sun_family = AF_UNIX;
      	strlcpy(sa.sun_path, path, sizeof(sa.sun_path));
      
      	if (connect(fd, (struct sockaddr *)&sa, sizeof(sa)) == -1) {
     -		int e = errno;
    -+		*e = errno;
    ++		int saved_errno = errno;
      		close(fd);
     -		return e;
    ++		errno = saved_errno;
     +		return -1;
      	}
      
      	*out_fd = fd;
    +@@ trace2/tr2_dst.c: static int tr2_dst_try_unix_domain_socket(struct tr2_dst *dst,
    + {
    + 	unsigned int uds_try = 0;
    + 	int fd;
    +-	int e;
    + 	const char *path = NULL;
    + 
    + 	/*
     @@ trace2/tr2_dst.c: static int tr2_dst_try_unix_domain_socket(struct tr2_dst *dst,
      	}
      
      	if (uds_try & TR2_DST_UDS_TRY_STREAM) {
     -		e = tr2_dst_try_uds_connect(path, SOCK_STREAM, &fd);
     -		if (!e)
    -+		if (!tr2_dst_try_uds_connect(path, SOCK_STREAM, &fd, &e))
    ++		if (!tr2_dst_try_uds_connect(path, SOCK_STREAM, &fd))
      			goto connected;
    - 		if (e != EPROTOTYPE)
    +-		if (e != EPROTOTYPE)
    ++		if (errno != EPROTOTYPE)
      			goto error;
      	}
      	if (uds_try & TR2_DST_UDS_TRY_DGRAM) {
     -		e = tr2_dst_try_uds_connect(path, SOCK_DGRAM, &fd);
     -		if (!e)
    -+		if (!tr2_dst_try_uds_connect(path, SOCK_DGRAM, &fd, &e))
    ++		if (!tr2_dst_try_uds_connect(path, SOCK_DGRAM, &fd))
      			goto connected;
      	}
      
    +@@ trace2/tr2_dst.c: static int tr2_dst_try_unix_domain_socket(struct tr2_dst *dst,
    + 	if (tr2_dst_want_warning())
    + 		warning("trace2: could not connect to socket '%s' for '%s' tracing: %s",
    + 			path, tr2_sysenv_display_name(dst->sysenv_var),
    +-			strerror(e));
    ++			strerror(errno));
    + 
    + 	tr2_dst_trace_disable(dst);
    + 	return 0;

 trace2/tr2_dst.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/trace2/tr2_dst.c b/trace2/tr2_dst.c
index ae052a07fe..bda283e7f4 100644
--- a/trace2/tr2_dst.c
+++ b/trace2/tr2_dst.c
@@ -204,15 +204,16 @@ static int tr2_dst_try_uds_connect(const char *path, int sock_type, int *out_fd)
 
 	fd = socket(AF_UNIX, sock_type, 0);
 	if (fd == -1)
-		return errno;
+		return -1;
 
 	sa.sun_family = AF_UNIX;
 	strlcpy(sa.sun_path, path, sizeof(sa.sun_path));
 
 	if (connect(fd, (struct sockaddr *)&sa, sizeof(sa)) == -1) {
-		int e = errno;
+		int saved_errno = errno;
 		close(fd);
-		return e;
+		errno = saved_errno;
+		return -1;
 	}
 
 	*out_fd = fd;
@@ -227,7 +228,6 @@ static int tr2_dst_try_unix_domain_socket(struct tr2_dst *dst,
 {
 	unsigned int uds_try = 0;
 	int fd;
-	int e;
 	const char *path = NULL;
 
 	/*
@@ -271,15 +271,13 @@ static int tr2_dst_try_unix_domain_socket(struct tr2_dst *dst,
 	}
 
 	if (uds_try & TR2_DST_UDS_TRY_STREAM) {
-		e = tr2_dst_try_uds_connect(path, SOCK_STREAM, &fd);
-		if (!e)
+		if (!tr2_dst_try_uds_connect(path, SOCK_STREAM, &fd))
 			goto connected;
-		if (e != EPROTOTYPE)
+		if (errno != EPROTOTYPE)
 			goto error;
 	}
 	if (uds_try & TR2_DST_UDS_TRY_DGRAM) {
-		e = tr2_dst_try_uds_connect(path, SOCK_DGRAM, &fd);
-		if (!e)
+		if (!tr2_dst_try_uds_connect(path, SOCK_DGRAM, &fd))
 			goto connected;
 	}
 
@@ -287,7 +285,7 @@ static int tr2_dst_try_unix_domain_socket(struct tr2_dst *dst,
 	if (tr2_dst_want_warning())
 		warning("trace2: could not connect to socket '%s' for '%s' tracing: %s",
 			path, tr2_sysenv_display_name(dst->sysenv_var),
-			strerror(e));
+			strerror(errno));
 
 	tr2_dst_trace_disable(dst);
 	return 0;
-- 
2.32.0.rc0.406.g73369325f8d