xap_helper.h: limit stderr assignment to glibc+FreeBSD

This fixes the C++ xap_helper compilation on OpenBSD.
Assignable `FILE *' pointers appear to only be supported on
FreeBSD and glibc.  Based on my reading of musl and NetBSD
source code, this should also fix builds on those platforms.

1 files changed, 49 insertions, 6 deletions

diff --git a/lib/PublicInbox/xap_helper.h b/lib/PublicInbox/xap_helper.h
index e10527d1..92210511 100644
--- a/lib/PublicInbox/xap_helper.h
+++ b/lib/PublicInbox/xap_helper.h
@@ -50,9 +50,18 @@
 #        define SET_MAX_EXPANSION set_max_wildcard_expansion
 #endif
 
+#if defined(__FreeBSD__) || defined(__GLIBC__)
+#        define STDERR_ASSIGNABLE (1)
+#else
+#        define STDERR_ASSIGNABLE (0)
+#endif
+
 static const int sock_fd = 0; // SOCK_SEQPACKET as stdin :P
 static pid_t parent_pid;
+#if STDERR_ASSIGNABLE
 static FILE *orig_err = stderr;
+#endif
+static int orig_err_fd = -1;
 static void *srch_tree; // tsearch + tdelete + twalk
 static pid_t *worker_pids; // nr => pid
 static unsigned long nworker;
@@ -820,6 +829,37 @@ static void cleanup_pids(void)
         worker_pids = NULL;
 }
 
+static void stderr_set(FILE *tmp_err)
+{
+#if STDERR_ASSIGNABLE
+        if (my_setlinebuf(tmp_err))
+                perror("W: setlinebuf(tmp_err)");
+        stderr = tmp_err;
+        return;
+#endif
+        int fd = fileno(tmp_err);
+        if (fd < 0) err(EXIT_FAILURE, "BUG: fileno(tmp_err)");
+        while (dup2(fd, STDERR_FILENO) < 0) {
+                if (errno != EINTR)
+                        err(EXIT_FAILURE, "dup2(%d => 2)", fd);
+        }
+}
+
+static void stderr_restore(FILE *tmp_err)
+{
+#if STDERR_ASSIGNABLE
+        stderr = orig_err;
+        return;
+#endif
+        if (ferror(stderr) | fflush(stderr))
+                perror("ferror|fflush stderr");
+        while (dup2(orig_err_fd, STDERR_FILENO) < 0) {
+                if (errno != EINTR)
+                        err(EXIT_FAILURE, "dup2(%d => 2)", orig_err_fd);
+        }
+        clearerr(stderr);
+}
+
 static void recv_loop(void) // worker process loop
 {
         static char rbuf[4096 * 33]; // per-process
@@ -828,18 +868,15 @@ static void recv_loop(void) // worker process loop
                 struct req req = {};
                 if (!recv_req(&req, rbuf, &len))
                         continue;
-                if (req.fp[1]) {
-                        if (my_setlinebuf(req.fp[1]))
-                                perror("W: setlinebuf(req.fp[1])");
-                        stderr = req.fp[1];
-                }
+                if (req.fp[1])
+                        stderr_set(req.fp[1]);
                 req.argc = (int)SPLIT2ARGV(req.argv, rbuf, len);
                 if (req.argc > 0)
                         dispatch(&req);
                 if (ferror(req.fp[0]) | fclose(req.fp[0]))
                         perror("ferror|fclose fp[0]");
                 if (req.fp[1]) {
-                        stderr = orig_err;
+                        stderr_restore(req.fp[1]);
                         if (ferror(req.fp[1]) | fclose(req.fp[1]))
                                 perror("ferror|fclose fp[1]");
                 }
@@ -895,6 +932,12 @@ int main(int argc, char *argv[])
         code_nrp_init();
         atexit(cleanup_all);
 
+        if (!STDERR_ASSIGNABLE) {
+                orig_err_fd = dup(STDERR_FILENO);
+                if (orig_err_fd < 0)
+                        err(EXIT_FAILURE, "dup(2)");
+        }
+
         nworker = 0;
 #ifdef _SC_NPROCESSORS_ONLN
         long j = sysconf(_SC_NPROCESSORS_ONLN);