From db8a6cd2dca941d42ad9245368e74c8ea87f0de8 Mon Sep 17 00:00:00 2001
From: Martin Pulec <martin.pulec@cesnet.cz>
Date: Mon, 13 Nov 2023 12:51:52 +0100
Subject: [PATCH] crash_signal_handler: avoid backtrace interleaving

Previously, if eg. multiple worker crashed, backtraces were interleaved
- use temporary file to print it. It seems that the new implementation
prints backtrace just from one thread, which shouldn't matter, anyways,
because it is improbable that there are two different crashes at the
same time.
---
 src/host.cpp | 54 +++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 47 insertions(+), 7 deletions(-)

diff --git a/src/host.cpp b/src/host.cpp
index 796636b88..f52b6d8ce 100644
--- a/src/host.cpp
+++ b/src/host.cpp
@@ -46,6 +46,7 @@
 
 #ifndef _WIN32
 #include <execinfo.h>
+#include <fcntl.h>
 #endif // defined WIN32
 
 #include "host.h"
@@ -64,6 +65,7 @@
 #include "messaging.h"
 #include "module.h"
 #include "utils/color_out.h"
+#include "utils/fs.h"
 #include "utils/misc.h" // unit_evaluate
 #include "utils/random.h"
 #include "utils/string.h"
@@ -1051,19 +1053,57 @@ bool running_in_debugger(){
         return false;
 }
 
+static void
+print_backtrace()
+{
+#ifndef _WIN32
+        // print to a temporary file to avoid interleaving from multiple
+        // threads
+#ifdef __APPLE__
+        char path[MAX_PATH_SIZE];
+        snprintf(path, sizeof path, "%s/ug-%u", get_temp_dir(),
+                 pthread_mach_thread_np(pthread_self()));
+        int fd = open(path, O_CLOEXEC | O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
+        unlink(path);
+#else
+        int fd = memfd_create("ultragrid_backtrace", MFD_CLOEXEC);
+#endif
+        if (fd == -1) {
+                fd = STDERR_FILENO;
+        }
+        char backtrace_msg[] = "Backtrace:\n";
+        write_all(fd, sizeof backtrace_msg, backtrace_msg);
+        array<void *, 256> addresses{};
+        const int num_symbols = backtrace(addresses.data(), addresses.size());
+        backtrace_symbols_fd(addresses.data(), num_symbols, fd);
+        if (fd == STDERR_FILENO) {
+                return;
+        }
+
+        lseek(fd, 0, SEEK_SET);
+        char buf[STR_LEN];
+        ssize_t rbytes = 0;
+        while ((rbytes = read(fd, buf, sizeof buf)) > 0) {
+                ssize_t written = 0;
+                ssize_t wbytes  = 0;
+                while (written < rbytes &&
+                       (wbytes = write(STDERR_FILENO, buf + written,
+                                       rbytes - written)) > 0) {
+                        written += wbytes;
+                }
+        }
+        close(fd);
+#endif // defined WIN32
+}
+
 void crash_signal_handler(int sig)
 {
+        print_backtrace();
+
         char buf[1024];
         char *ptr = buf;
         char *ptr_end = buf + sizeof buf;
         strappend(&ptr, ptr_end, "\n" PACKAGE_NAME " has crashed");
-#ifndef WIN32
-        char backtrace_msg[] = "Backtrace:\n";
-        write_all(STDERR_FILENO, sizeof backtrace_msg, backtrace_msg);
-        array<void *, 256> addresses{};
-        int num_symbols = backtrace(addresses.data(), addresses.size());
-        backtrace_symbols_fd(addresses.data(), num_symbols, 2);
-#endif // defined WIN32
 
         append_sig_desc(&ptr, ptr_end, sig);