[PATCH gwproxy v1 1/2] log: Fix missing `gettid()` syscall on older glibc version

[Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>]

Compiling on GNU CC version 8.5.0 20210514 (Red Hat 8.5.0-26) with
glibc version 2.28 throws this error:

  src/gwproxy/log.c: In function ‘__pr_log’:
  src/gwproxy/log.c:51:56: warning: implicit declaration of function ‘gettid’; \
                                    did you mean ‘getgid’? [-Wimplicit-function-declaration]
    fprintf(handle, "[%s][%s][%08d]: %s\n", time_buf, ls, gettid(), pb);
                                                          ^~~~~~
                                                          getgid
  src/gwproxy/log.c:51: undefined reference to `gettid'
  collect2: error: ld returned 1 exit status

Fix it by defining our own `__sys_gettid()`.

Signed-off-by: Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>
---
 src/gwproxy/log.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/src/gwproxy/log.c b/src/gwproxy/log.c
index 95d1930..574c5e0 100644
--- a/src/gwproxy/log.c
+++ b/src/gwproxy/log.c
@@ -1,5 +1,6 @@
 #include <gwproxy/common.h>
 #include <gwproxy/log.h>
+#include <gwproxy/syscall.h>
 #include <time.h>
 #include <stdlib.h>
 #include <stdio.h>
@@ -7,6 +8,11 @@
 #include <unistd.h>
 #include <stdarg.h>
 
+static inline pid_t __sys_gettid(void)
+{
+	return (pid_t)__do_syscall0(__NR_gettid);
+}
+
 __attribute__((__format__(printf, 3, 4)))
 void __pr_log(FILE *handle, int level, const char *fmt, ...)
 {
@@ -48,7 +54,7 @@ void __pr_log(FILE *handle, int level, const char *fmt, ...)
 	else
 		time_buf[0] = '\0';
 
-	fprintf(handle, "[%s][%s][%08d]: %s\n", time_buf, ls, gettid(), pb);
+	fprintf(handle, "[%s][%s][%08d]: %s\n", time_buf, ls, __sys_gettid(), pb);
 	if (unlikely(pb != loc_buf))
 		free(pb);
 out:
-- 
Alviro Iskandar Setiawan