log: use non-blocking atomic writes to stderr fifos

Buggy container runtimes are known to do partial reads on container's
stderr and emit torn logs [1]. This is due to write(2) allowing partial
data transfer. One way to avoid this when stderr is a FIFO (which is the
case for container runtimes) is to changed the file flags to set
O_NONBLOCK which alters the behavior of write(2) when the write size is
less than PIPE_BUF. This makes a write guaranteed to be atomic.

The small complexity added is to poll on stderr in this case whenever a
write(2) fails with EAGAIN. This does not block the log flush thread in
any new way. It would always block when the stderr FIFO was full.

[1] containers/conmon#242

Fixes: cef04bc
Fixes: https://tracker.ceph.com/issues/57923
Related: https://bugzilla.redhat.com/show_bug.cgi?id=1996599
Related: https://bugzilla.redhat.com/show_bug.cgi?id=2108228
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>

src/log/Log.cc

@@ -18,10 +18,13 @@
 #include "LogClock.h"
 #include "SubsystemMap.h"
 
+#include <boost/container/vector.hpp>
+
 #include <errno.h>
 #include <fcntl.h>
 #include <syslog.h>
 
+#include <algorithm>
 #include <iostream>
 #include <set>
 
@@ -49,6 +52,7 @@ Log::Log(const SubsystemMap *s)
     m_recent(DEFAULT_MAX_RECENT)
 {
   m_log_buf.reserve(MAX_LOG_BUF);
+  _configure_stderr();
 }
 
 Log::~Log()
@@ -64,6 +68,36 @@ Log::~Log()
   }
 }
 
+void Log::_configure_stderr()
+{
+  struct stat info;
+  if (int rc = fstat(STDERR_FILENO, &info); rc == -1) {
+    std::cerr << "failed to stat stderr: " << cpp_strerror(errno) << std::endl;
+    return;
+  }
+
+  if (S_ISFIFO(info.st_mode)) {
+    /* Set O_NONBLOCK on FIFO stderr file. We want to ensure atomic debug log
+     * writes so they do not get partially read by e.g. buggy container
+     * runtimes. See also IEEE Std 1003.1-2017 and Log::_log_stderr below.
+     */
+    int flags = fcntl(STDERR_FILENO, F_GETFL);
+    if (flags == -1) {
+      std::cerr << "failed to get fcntl flags for stderr: " << cpp_strerror(errno) << std::endl;
+      return;
+    }
+    if (!(flags & O_NONBLOCK)) {
+      flags |= O_NONBLOCK;
+      flags = fcntl(STDERR_FILENO, F_SETFL, flags);
+      if (flags == -1) {
+        std::cerr << "failed to set fcntl flags for stderr: " << cpp_strerror(errno) << std::endl;
+        return;
+      }
+    }
+    do_stderr_poll = true;
+  }
+}
+
 
 ///
 void Log::set_coarse_timestamps(bool coarse) {
@@ -260,6 +294,63 @@ void Log::_log_safe_write(std::string_view sv)
   }
 }
 
+void Log::_log_stderr(std::string_view strv)
+{
+  if (do_stderr_poll) {
+    auto& prefix = m_log_stderr_prefix;
+    size_t const len = prefix.size() + strv.size();
+    boost::container::small_vector<char, PIPE_BUF> buf;
+    buf.resize(len+1, '\0');
+    memcpy(buf.data(), prefix.c_str(), prefix.size());
+    memcpy(buf.data()+prefix.size(), strv.data(), strv.size());
+
+    char const* const start = buf.data();
+    char const* current = start;
+    while ((size_t)(current-start) < len) {
+      auto chunk = std::min<ssize_t>(PIPE_BUF, len-(ssize_t)(current-start));
+      while (1) {
+        ssize_t rc = write(STDERR_FILENO, current, chunk);
+        if (rc == chunk) {
+          current += chunk;
+          break;
+        } else if (rc > 0) {
+          /* According to IEEE Std 1003.1-2017, this cannot happen:
+           *
+           * Write requests to a pipe or FIFO shall be handled in the same way as a regular file with the following exceptions:
+           * ...
+           *   If the O_NONBLOCK flag is set ...
+           *   ...
+           *     A write request for {PIPE_BUF} or fewer bytes shall have the
+           *     following effect: if there is sufficient space available in
+           *     the pipe, write() shall transfer all the data and return the
+           *     number of bytes requested. Otherwise, write() shall transfer
+           *     no data and return -1 with errno set to [EAGAIN].
+           *
+           * In any case, handle misbehavior gracefully by incrementing current.
+           */
+          current += rc;
+          break;
+        } else if (rc == -1) {
+          if (errno == EAGAIN) {
+            struct pollfd pfd[1];
+            pfd[0].fd = STDERR_FILENO;
+            pfd[0].events = POLLOUT;
+            poll(pfd, 1, -1);
+            /* ignore errors / success, just retry the write */
+          } else if (errno == EINTR) {
+            continue;
+          } else {
+            /* some other kind of error, no point logging if stderr writes fail */
+            return;
+          }
+        }
+      }
+    }
+  } else {
+    fmt::print(std::cerr, "{}{}", m_log_stderr_prefix, strv);
+  }
+}
+
 void Log::_flush_logbuf()
 {
   if (m_log_buf.size()) {
@@ -319,7 +410,7 @@ void Log::_flush(EntryVector& t, bool crash)
       pos[used++] = '\n';
 
       if (do_stderr) {
-        fmt::print(std::cerr, "{}{}", m_log_stderr_prefix, std::string_view(pos, used));
+        _log_stderr(std::string_view(pos, used));
       }
 
       if (do_fd) {

src/log/Log.h

@@ -116,6 +116,7 @@ class Log : private Thread
   int m_journald_log = -3, m_journald_crash = -3;
 
   std::string m_log_stderr_prefix;
+  bool do_stderr_poll = false;
 
   std::shared_ptr<Graylog> m_graylog;
   std::unique_ptr<JournaldLogger> m_journald;
@@ -133,6 +134,9 @@ class Log : private Thread
   void _log_safe_write(std::string_view sv);
   void _flush_logbuf();
   void _log_message(std::string_view s, bool crash);
+  void _configure_stderr();
+  void _log_stderr(std::string_view strv);
+
 
 
 };