[Libguestfs] [nbdkit PATCH 1/4] errors: Avoid interleaved errors from parallel threads

[Eric Blake]

Since we construct our error/debug messages via multiple calls
to stdio primitives, we are at risk of multiple threads interleaving
their output if they try to output at once.  Add a mutex to group
related outputs into an atomic chunk.

Signed-off-by: Eric Blake <eblake at redhat.com>
---
 src/errors.c | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/src/errors.c b/src/errors.c
index 5f14315..2fc83ab 100644
--- a/src/errors.c
+++ b/src/errors.c
@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2013 Red Hat Inc.
+ * Copyright (C) 2013-2017 Red Hat Inc.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -38,10 +38,30 @@
 #include <stdarg.h>
 #include <string.h>
 #include <errno.h>
+#include <assert.h>
+#include <pthread.h>

 #include "nbdkit-plugin.h"
 #include "internal.h"

+/* Used to group piecemeal message construction into atomic output. */
+static pthread_mutex_t errors_lock = PTHREAD_MUTEX_INITIALIZER;
+
+static void
+lock (void)
+{
+  int r = pthread_mutex_lock(&errors_lock);
+  assert(!r);
+}
+
+static void
+unlock (void)
+{
+  int r = pthread_mutex_unlock(&errors_lock);
+  assert(!r);
+}
+
+/* Called with lock taken. */
 static void
 prologue (const char *type)
 {
@@ -69,11 +89,13 @@ nbdkit_vdebug (const char *fs, va_list args)
   if (!verbose)
     return;

+  lock ();
   prologue ("debug");

   vfprintf (stderr, fs, args);

   fprintf (stderr, "\n");
+  unlock ();

   errno = err;
 }
@@ -88,6 +110,7 @@ nbdkit_debug (const char *fs, ...)
   if (!verbose)
     return;

+  lock ();
   prologue ("debug");

   va_start (args, fs);
@@ -95,6 +118,7 @@ nbdkit_debug (const char *fs, ...)
   va_end (args);

   fprintf (stderr, "\n");
+  unlock ();

   errno = err;
 }
@@ -105,11 +129,13 @@ nbdkit_verror (const char *fs, va_list args)
 {
   int err = errno;

+  lock ();
   prologue ("error");

   vfprintf (stderr, fs, args);

   fprintf (stderr, "\n");
+  unlock ();

   errno = err;
 }
@@ -121,6 +147,7 @@ nbdkit_error (const char *fs, ...)
   va_list args;
   int err = errno;

+  lock ();
   prologue ("error");

   va_start (args, fs);
@@ -128,6 +155,7 @@ nbdkit_error (const char *fs, ...)
   va_end (args);

   fprintf (stderr, "\n");
+  unlock ();

   errno = err;
 }
-- 
2.13.6