Use getpwuid_r() instead of getpwuid()

In the Netopeer2 test suite, two threads were calling getpwuid(), which
is not a reentrant function. Here's a trace from TSAN:

WARNING: ThreadSanitizer: data race (pid=3803971)
  Write of size 8 at 0x7fc95e073f40 by thread T2:
    #0 getpwuid <null> (test_parallel_sessions+0x4367d4)
    #1 nc_connect_unix libnetconf2/src/session_client.c:1270:10 (libnetconf2.so.2+0x1ba34)
    #2 send_get_rpc Netopeer2/tests/test_parallel_sessions.c:68:15 (test_parallel_sessions+0x4ea807)

  Previous write of size 8 at 0x7fc95e073f40 by thread T1:
    #0 getpwuid <null> (test_parallel_sessions+0x4367d4)
    #1 nc_connect_unix libnetconf2/src/session_client.c:1270:10 (libnetconf2.so.2+0x1ba34)
    #2 send_get_rpc Netopeer2/tests/test_parallel_sessions.c:68:15 (test_parallel_sessions+0x4ea807)

  Location is global 'resbuf.11357' of size 48 at 0x7fc95e073f40 (libc.so.6+0x1bcf40)

  Thread T2 (tid=3807449, running) created by main thread at:
    #0 pthread_create <null> (test_parallel_sessions+0x469032)
    #1 test_first Netopeer2/tests/test_parallel_sessions.c:115:9 (test_parallel_sessions+0x4ea6c2)
    #2 cmocka_run_one_test_or_fixture <null> (libcmocka.so.0+0x630e)

  Thread T1 (tid=3807448, running) created by main thread at:
    #0 pthread_create <null> (test_parallel_sessions+0x469032)
    #1 test_first Netopeer2/tests/test_parallel_sessions.c:115:9 (test_parallel_sessions+0x4ea6ad)
    #2 cmocka_run_one_test_or_fixture <null> (libcmocka.so.0+0x630e)

SUMMARY: ThreadSanitizer: data race (Netopeer2/tests/test_parallel_sessions+0x4367d4) in __interceptor_getpwuid

Fix that by switching to getpwuid_r(). The code is very similar to the
auth_password_getpwnam() wrapper from session_server_ssh.c. I chose to
put this into io.c even though it is not really an IO function, but the
existing nc_realloc() is in that file as well.

src/io.c

@@ -17,10 +17,12 @@
 #include <errno.h>
 #include <inttypes.h>
 #include <poll.h>
+#include <pwd.h>
 #include <signal.h>
 #include <stdarg.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/types.h>
 #include <time.h>
 #include <unistd.h>
 
@@ -1158,3 +1160,39 @@ nc_realloc(void *ptr, size_t size)
 
     return ret;
 }
+
+struct passwd *
+nc_getpwuid(uid_t uid, struct passwd *pwd_buf, char **buf, size_t *buf_size)
+{
+    struct passwd *pwd = NULL;
+    char *mem;
+    int r = 0;
+
+    do {
+        r = getpwuid_r(uid, pwd_buf, *buf, *buf_size, &pwd);
+        if (pwd) {
+            break;
+        }
+
+        if (r == ERANGE) {
+            if (!*buf_size) {
+                ssize_t size = sysconf(_SC_GETPW_R_SIZE_MAX);
+                if (size == -1) {
+                    *buf_size = 256;
+                } else {
+                    *buf_size = size;
+                }
+            } else {
+                *buf_size <<= 2;
+            }
+            mem = realloc(*buf, *buf_size);
+            if (!mem) {
+                ERRMEM;
+                return NULL;
+            }
+            *buf = mem;
+        }
+    } while (r == ERANGE);
+
+    return pwd;
+}

src/session_client.c

@@ -1238,16 +1238,18 @@ nc_connect_unix(const char *address, struct ly_ctx *ctx)
 {
     struct nc_session *session = NULL;
     struct sockaddr_un sun;
-    const struct passwd *pw;
+    struct passwd *pw, pw_buf;
     char *username;
     int sock = -1;
+    char *buf = NULL;
+    size_t buf_size = 0;
 
     if (address == NULL) {
         ERRARG("address");
         return NULL;
     }
 
-    pw = getpwuid(geteuid());
+    pw = nc_getpwuid(geteuid(), &pw_buf, &buf, &buf_size);
     if (pw == NULL) {
         ERR(NULL, "Failed to find username for euid=%u.\n", geteuid());
         goto fail;

src/session_client_ssh.c

@@ -1545,8 +1545,10 @@ _nc_connect_libssh(ssh_session ssh_session, struct ly_ctx *ctx, struct nc_keepal
     char *host = NULL, *username = NULL, *ip_host;
     unsigned int port = 0;
     int sock;
-    struct passwd *pw;
+    struct passwd *pw, pw_buf;
     struct nc_session *session = NULL;
+    char *buf;
+    size_t buf_len;
 
     if (!ssh_session) {
         ERRARG("ssh_session");
@@ -1604,7 +1606,7 @@ _nc_connect_libssh(ssh_session ssh_session, struct ly_ctx *ctx, struct nc_keepal
         /* remember username */
         if (!username) {
             if (!opts->username) {
-                pw = getpwuid(getuid());
+                pw = nc_getpwuid(getuid(), &pw_buf, &buf, &buf_len);
                 if (!pw) {
                     ERR(NULL, "Unknown username for the SSH connection (%s).", strerror(errno));
                     goto fail;
@@ -1683,8 +1685,10 @@ nc_connect_ssh(const char *host, uint16_t port, struct ly_ctx *ctx)
     int sock;
     uint32_t port_uint;
     char *username, *ip_host = NULL;
-    struct passwd *pw;
+    struct passwd *pw, pw_buf;
     struct nc_session *session = NULL;
+    char *buf = NULL;
+    size_t buf_len = 0;
 
     /* process parameters */
     if (!host || strisempty(host)) {
@@ -1697,7 +1701,7 @@ nc_connect_ssh(const char *host, uint16_t port, struct ly_ctx *ctx)
     port_uint = port;
 
     if (!ssh_opts.username) {
-        pw = getpwuid(getuid());
+        pw = nc_getpwuid(getuid(), &pw_buf, &buf, &buf_len);
         if (!pw) {
             ERR(session, "Unknown username for the SSH connection (%s).", strerror(errno));
             return NULL;
@@ -1855,9 +1859,11 @@ nc_accept_callhome_ssh_sock(int sock, const char *host, uint16_t port, struct ly
 {
     const long ssh_timeout = NC_SSH_TIMEOUT;
     unsigned int uint_port;
-    struct passwd *pw;
+    struct passwd *pw, pw_buf;
     struct nc_session *session;
     ssh_session sess;
+    char *buf = NULL;
+    size_t buf_len = 0;
 
     sess = ssh_new();
     if (!sess) {
@@ -1873,7 +1879,7 @@ nc_accept_callhome_ssh_sock(int sock, const char *host, uint16_t port, struct ly
     ssh_options_set(sess, SSH_OPTIONS_PORT, &uint_port);
     ssh_options_set(sess, SSH_OPTIONS_TIMEOUT, &ssh_timeout);
     if (!ssh_ch_opts.username) {
-        pw = getpwuid(getuid());
+        pw = nc_getpwuid(getuid(), &pw_buf, &buf, &buf_len);
         if (!pw) {
             ERR(NULL, "Unknown username for the SSH connection (%s).", strerror(errno));
             ssh_free(sess);

src/session_p.h

@@ -514,6 +514,8 @@ struct nc_ntf_thread_arg {
 
 void *nc_realloc(void *ptr, size_t size);
 
+struct passwd *nc_getpwuid(uid_t uid, struct passwd *pwd_buf, char **buf, size_t *buf_size);
+
 NC_MSG_TYPE nc_send_msg_io(struct nc_session *session, int io_timeout, struct lyd_node *op);
 
 int nc_gettimespec_mono(struct timespec *ts);

src/session_server.c

@@ -1877,17 +1877,19 @@ static int
 nc_accept_unix(struct nc_session *session, int sock)
 {
 #if defined (SO_PEERCRED) || defined (HAVE_GETPEEREID)
-    const struct passwd *pw;
+    struct passwd *pw, pw_buf;
     char *username;
     session->ti_type = NC_TI_UNIX;
     uid_t uid = 0;
+    char *buf = NULL;
+    size_t buf_len = 0;
 
     if (nc_get_uid(sock, &uid)) {
         close(sock);
         return -1;
     }
 
-    pw = getpwuid(uid);
+    pw = nc_getpwuid(uid, &pw_buf, &buf, &buf_len);
     if (pw == NULL) {
         ERR(NULL, "Failed to find username for uid=%u (%s).\n", uid, strerror(errno));
         close(sock);