[PATCH v2] 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 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.

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>
---

On Tue, May 11 2021, Junio C Hamano wrote:

> What's the concensus if any on this topic?

Having read Johannes's comments I think it's still most readable to
just return -1 unconditionally. The resulting code isn't weird, I'd
argue that it's a better pattern to save away errno like this, but the
commit messages notes that we're working around a GCC bug.

> In any case, this needs to be signed off before it gets carved into
> our history.

Done, and also changed the variable name to minimize the size of the
diff. A shorter name allowed for less re-flowing of lines.

Range-diff against v1:
1:  87d9bcf1095 ! 1:  782555daade trace2: refactor to avoid gcc warning under -O3
    @@ Commit message
         error, and save away the value of errno in a variable the caller
         passes in.
     
    +    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’:
    @@ Commit message
                   ^~
         2. https://lore.kernel.org/git/20200404142131.GA679473@coredump.intra.peff.net/
     
    +    Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
    +
      ## 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:"
    @@ trace2/tr2_dst.c: static int tr2_dst_try_path(struct tr2_dst *dst, const char *t
      
     -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 *saved_errno)
    ++				   int *out_fd, int *e)
      {
      	int fd;
      	struct sockaddr_un sa;
    @@ trace2/tr2_dst.c: static int tr2_dst_try_path(struct tr2_dst *dst, const char *t
     -	if (fd == -1)
     -		return errno;
     +	if (fd == -1) {
    -+		*saved_errno = errno;
    ++		*e = errno;
     +		return -1;
     +	}
      
    @@ trace2/tr2_dst.c: static int tr2_dst_try_path(struct tr2_dst *dst, const char *t
      
      	if (connect(fd, (struct sockaddr *)&sa, sizeof(sa)) == -1) {
     -		int e = errno;
    -+		*saved_errno = errno;
    ++		*e = errno;
      		close(fd);
     -		return e;
     +		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;
    -+	int saved_errno;
    - 	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,
    -+					     &saved_errno))
    ++		if (!tr2_dst_try_uds_connect(path, SOCK_STREAM, &fd, &e))
      			goto connected;
    --		if (e != EPROTOTYPE)
    -+		if (saved_errno != EPROTOTYPE)
    + 		if (e != 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,
    -+					     &saved_errno))
    ++		if (!tr2_dst_try_uds_connect(path, SOCK_DGRAM, &fd, &e))
      			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(saved_errno));
    - 
    - 	tr2_dst_trace_disable(dst);
    - 	return 0;

 trace2/tr2_dst.c | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/trace2/tr2_dst.c b/trace2/tr2_dst.c
index ae052a07fe2..a44fe6b73e0 100644
--- a/trace2/tr2_dst.c
+++ b/trace2/tr2_dst.c
@@ -197,22 +197,25 @@ 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;
 
 	fd = socket(AF_UNIX, sock_type, 0);
-	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;
 		close(fd);
-		return e;
+		return -1;
 	}
 
 	*out_fd = fd;
@@ -271,15 +274,13 @@ 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, &e))
 			goto connected;
 		if (e != 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))
 			goto connected;
 	}
 
-- 
2.31.1.909.g789bb6d90e