error: New error_prepend(), error_reportf_err()

Instead of simply propagating an error verbatim, we sometimes want to
add to its message, like this:

    frobnicate(arg, &err);
    error_setg(errp, "Can't frobnicate %s: %s",
                     arg, error_get_pretty(err));
    error_free(err);

This is suboptimal, because it loses err's hint (if any).  Moreover,
when errp is &error_abort or is subsequently propagated to
&error_abort, the abort message points to the place where we last
added to the error, not to the place where it originated.

To avoid these issues, provide means to add to an error's message in
place:

    frobnicate(arg, errp);
    error_prepend(errp, "Can't frobnicate %s: ", arg);

Likewise, reporting an error like

    frobnicate(arg, &err);
    error_report("Can't frobnicate %s: %s", arg, error_get_pretty(err));

can lose err's hint.  To avoid:

    error_reportf_err(err, "Can't frobnicate %s: ", arg);

The next commits will put these functions to use.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1450452927-8346-10-git-send-email-armbru@redhat.com>

include/qapi/error.h

@@ -31,6 +31,9 @@
  *     error_report_err(err);
  * This frees the error object.
  *
+ * Report an error to stderr with additional text prepended:
+ *     error_reportf_err(err, "Could not frobnicate '%s': ", name);
+ *
  * Report an error somewhere else:
  *     const char *msg = error_get_pretty(err);
  *     do with msg what needs to be done...
@@ -48,6 +51,10 @@
  *     error_propagate(errp, err);
  * where Error **errp is a parameter, by convention the last one.
  *
+ * Pass an existing error to the caller with the message modified:
+ *     error_propagate(errp, err);
+ *     error_prepend(errp, "Could not frobnicate '%s': ", name);
+ *
  * Create a new error and pass it to the caller:
  *     error_setg(errp, "situation normal, all fouled up");
  *
@@ -108,9 +115,10 @@
 #ifndef ERROR_H
 #define ERROR_H
 
+#include <stdarg.h>
+#include <stdbool.h>
 #include "qemu/compiler.h"
 #include "qapi-types.h"
-#include <stdbool.h>
 
 /*
  * Opaque error object.
@@ -208,7 +216,20 @@ void error_setg_win32_internal(Error **errp,
  */
 void error_propagate(Error **dst_errp, Error *local_err);
 
-/**
+/*
+ * Prepend some text to @errp's human-readable error message.
+ * The text is made by formatting @fmt, @ap like vprintf().
+ */
+void error_vprepend(Error **errp, const char *fmt, va_list ap);
+
+/*
+ * Prepend some text to @errp's human-readable error message.
+ * The text is made by formatting @fmt, ... like printf().
+ */
+void error_prepend(Error **errp, const char *fmt, ...)
+    GCC_FMT_ATTR(2, 3);
+
+/*
  * Append a printf-style human-readable explanation to an existing error.
  * @errp may be NULL, but not &error_fatal or &error_abort.
  * Trivially the case if you call it only after error_setg() or
@@ -250,6 +271,12 @@ void error_free_or_abort(Error **errp);
  */
 void error_report_err(Error *err);
 
+/*
+ * Convenience function to error_prepend(), error_report() and free @err.
+ */
+void error_reportf_err(Error *err, const char *fmt, ...)
+    GCC_FMT_ATTR(2, 3);
+
 /*
  * Just like error_setg(), except you get to specify the error class.
  * Note: use of error classes other than ERROR_CLASS_GENERIC_ERROR is

util/error.c

@@ -122,6 +122,29 @@ void error_setg_file_open_internal(Error **errp,
                               "Could not open '%s'", filename);
 }
 
+void error_vprepend(Error **errp, const char *fmt, va_list ap)
+{
+    GString *newmsg;
+
+    if (!errp) {
+        return;
+    }
+
+    newmsg = g_string_new(NULL);
+    g_string_vprintf(newmsg, fmt, ap);
+    g_string_append(newmsg, (*errp)->msg);
+    (*errp)->msg = g_string_free(newmsg, 0);
+}
+
+void error_prepend(Error **errp, const char *fmt, ...)
+{
+    va_list ap;
+
+    va_start(ap, fmt);
+    error_vprepend(errp, fmt, ap);
+    va_end(ap);
+}
+
 void error_append_hint(Error **errp, const char *fmt, ...)
 {
     va_list ap;
@@ -209,6 +232,16 @@ void error_report_err(Error *err)
     error_free(err);
 }
 
+void error_reportf_err(Error *err, const char *fmt, ...)
+{
+    va_list ap;
+
+    va_start(ap, fmt);
+    error_vprepend(&err, fmt, ap);
+    va_end(ap);
+    error_report_err(err);
+}
+
 void error_free(Error *err)
 {
     if (err) {