Escape special/non-printing characters in debug output (RHBZ#731744).

The default event handler in libguestfs was simply writing all debug
output directly to stderr.  However if the output contains
non-printable characters such as terminal control codes then these
would also be sent directly.

With newer SeaBIOS there is a lame attempt to implement a splash
screen using terminal control codes, thus when libguestfs tries to
display debugging output it would cause the screen to clear and debug
output to be lost.

This commit causes all non-printing characters to be escaped.
(\n and \r characters from the appliance are treated somewhat
specially).

Furthermore, instead of using write(2), use buffered stderr calls.

diff --git a/src/events.c b/src/events.c
index 3894349..aa30fd2 100644 (file)
--- a/src/events.c
+++ b/src/events.c
@@ -28,6 +28,7 @@
 #include <assert.h>
 #include <string.h>
 
+#include "c-ctype.h"
 #include "ignore-value.h"
 
 #include "guestfs.h"
@@ -116,9 +117,9 @@ guestfs___call_callbacks_message (guestfs_h *g, uint64_t event,
    * override print-on-stderr simply by registering a callback.
    */
   if (count == 0 && (g->verbose || event == GUESTFS_EVENT_TRACE)) {
-    const char *prefix = "libguestfs: ";
-    const char *trace = "trace: ";
-    const char *nl = "\n";
+    int from_appliance = event == GUESTFS_EVENT_APPLIANCE;
+    size_t i;
+    char c;
 
     /* APPLIANCE =>  <buf>
      * LIBRARY =>    libguestfs: <buf>\n
@@ -126,18 +127,47 @@ guestfs___call_callbacks_message (guestfs_h *g, uint64_t event,
      */
 
     if (event != GUESTFS_EVENT_APPLIANCE)
-      ignore_value (write (STDERR_FILENO, prefix, strlen (prefix)));
+      fputs ("libguestfs: ", stderr);
 
     if (event == GUESTFS_EVENT_TRACE)
-      ignore_value (write (STDERR_FILENO, trace, strlen (trace)));
-
-    ignore_value (write (STDERR_FILENO, buf, buf_len));
-
-    /* Messages from the appliance already contain \n characters, others
-     * need this to be appended.
+      fputs ("trace: ", stderr);
+
+    /* Special or non-printing characters in the buffer must be
+     * escaped (RHBZ#731744).  The buffer can contain any 8 bit
+     * character, even \0.
+     *
+     * Handling of \n and \r characters is complex:
+     *
+     * Case 1: Messages from the appliance: These messages already
+     * contain \n and \r characters at logical positions, so we just
+     * echo those out directly.
+     *
+     * Case 2: Messages from other sources: These messages should NOT
+     * contain \n or \r.  If they do, it is escaped.  However we also
+     * need to print a real end of line after these messages.
      */
-    if (event != GUESTFS_EVENT_APPLIANCE)
-      ignore_value (write (STDERR_FILENO, nl, strlen (nl)));
+    for (i = 0; i < buf_len; ++i) {
+      c = buf[i];
+      if (c_isprint (c) || (from_appliance && (c == '\n' || c == '\r')))
+        putc (c, stderr);
+      else {
+        switch (c) {
+        case '\0': fputs ("\\0", stderr); break;
+        case '\a': fputs ("\\a", stderr); break;
+        case '\b': fputs ("\\b", stderr); break;
+        case '\f': fputs ("\\f", stderr); break;
+        case '\n': fputs ("\\n", stderr); break;
+        case '\r': fputs ("\\r", stderr); break;
+        case '\t': fputs ("\\t", stderr); break;
+        case '\v': fputs ("\\v", stderr); break;
+        default:
+          fprintf (stderr, "\\x%x", (unsigned) c);
+        }
+      }
+    }
+
+    if (!from_appliance)
+      putc ('\n', stderr);
   }
 }