error: Infrastructure to track locations for error reporting

New struct Location holds a location.  So far, the only location is
LOC_NONE, so this doesn't do anything useful yet.

Passing the current location all over the place would be too
cumbersome.  Hide it away in static cur_loc instead, and provide
accessors.  Print it in error_report().

Store it in QError, and print it in qerror_print().

Store it in QemuOpt, for use by qemu_opts_foreach().  This makes
error_report() do the right thing when it runs within
qemu_opts_foreach().

We may still have to store it in other data structures holding user
input for better error messages.  Left for another day.

qemu-error.c

@@ -41,15 +41,98 @@ void error_printf(const char *fmt, ...)
     va_end(ap);
 }
 
+static Location std_loc = {
+    .kind = LOC_NONE
+};
+static Location *cur_loc = &std_loc;
+
+/*
+ * Push location saved in LOC onto the location stack, return it.
+ * The top of that stack is the current location.
+ * Needs a matching loc_pop().
+ */
+Location *loc_push_restore(Location *loc)
+{
+    assert(!loc->prev);
+    loc->prev = cur_loc;
+    cur_loc = loc;
+    return loc;
+}
+
+/*
+ * Initialize *LOC to "nowhere", push it onto the location stack.
+ * The top of that stack is the current location.
+ * Needs a matching loc_pop().
+ * Return LOC.
+ */
+Location *loc_push_none(Location *loc)
+{
+    loc->kind = LOC_NONE;
+    loc->prev = NULL;
+    return loc_push_restore(loc);
+}
+
+/*
+ * Pop the location stack.
+ * LOC must be the current location, i.e. the top of the stack.
+ */
+Location *loc_pop(Location *loc)
+{
+    assert(cur_loc == loc && loc->prev);
+    cur_loc = loc->prev;
+    loc->prev = NULL;
+    return loc;
+}
+
+/*
+ * Save the current location in LOC, return LOC.
+ */
+Location *loc_save(Location *loc)
+{
+    *loc = *cur_loc;
+    loc->prev = NULL;
+    return loc;
+}
+
+/*
+ * Change the current location to the one saved in LOC.
+ */
+void loc_restore(Location *loc)
+{
+    Location *prev = cur_loc->prev;
+    assert(!loc->prev);
+    *cur_loc = *loc;
+    cur_loc->prev = prev;
+}
+
+/*
+ * Change the current location to "nowhere in particular".
+ */
+void loc_set_none(void)
+{
+    cur_loc->kind = LOC_NONE;
+}
+
+/*
+ * Print current location to current monitor if we have one, else to stderr.
+ */
+void error_print_loc(void)
+{
+    switch (cur_loc->kind) {
+    default: ;
+    }
+}
+
 /*
  * Print an error message to current monitor if we have one, else to stderr.
- * Appends a newline to the message.
+ * Prepend the current location and append a newline.
  * It's wrong to call this in a QMP monitor.  Use qerror_report() there.
  */
 void error_report(const char *fmt, ...)
 {
     va_list ap;
 
+    error_print_loc();
     va_start(ap, fmt);
     error_vprintf(fmt, ap);
     va_end(ap);

qemu-error.h

@@ -13,8 +13,24 @@
 #ifndef QEMU_ERROR_H
 #define QEMU_ERROR_H
 
+typedef struct Location {
+    /* all members are private to qemu-error.c */
+    enum { LOC_NONE } kind;
+    int num;
+    const void *ptr;
+    struct Location *prev;
+} Location;
+
+Location *loc_push_restore(Location *loc);
+Location *loc_push_none(Location *loc);
+Location *loc_pop(Location *loc);
+Location *loc_save(Location *loc);
+void loc_restore(Location *loc);
+void loc_set_none(void);
+
 void error_vprintf(const char *fmt, va_list ap);
 void error_printf(const char *fmt, ...) __attribute__ ((format(printf, 1, 2)));
+void error_print_loc(void);
 void error_report(const char *fmt, ...) __attribute__ ((format(printf, 1, 2)));
 void qerror_report_internal(const char *file, int linenr, const char *func,
                             const char *fmt, ...)

qemu-option.c

@@ -27,6 +27,7 @@
 #include <string.h>
 
 #include "qemu-common.h"
+#include "qemu-error.h"
 #include "qemu-option.h"
 
 /*
@@ -483,6 +484,7 @@ struct QemuOpt {
 struct QemuOpts {
     char *id;
     QemuOptsList *list;
+    Location loc;
     QTAILQ_HEAD(QemuOptHead, QemuOpt) head;
     QTAILQ_ENTRY(QemuOpts) next;
 };
@@ -653,6 +655,7 @@ QemuOpts *qemu_opts_create(QemuOptsList *list, const char *id, int fail_if_exist
         opts->id = qemu_strdup(id);
     }
     opts->list = list;
+    loc_save(&opts->loc);
     QTAILQ_INIT(&opts->head);
     QTAILQ_INSERT_TAIL(&list->head, opts, next);
     return opts;
@@ -810,13 +813,17 @@ int qemu_opts_validate(QemuOpts *opts, const QemuOptDesc *desc)
 int qemu_opts_foreach(QemuOptsList *list, qemu_opts_loopfunc func, void *opaque,
                       int abort_on_failure)
 {
+    Location loc;
     QemuOpts *opts;
     int rc = 0;
 
+    loc_push_none(&loc);
     QTAILQ_FOREACH(opts, &list->head, next) {
+        loc_restore(&opts->loc);
         rc |= func(opts, opaque);
         if (abort_on_failure  &&  rc != 0)
             break;
     }
+    loc_pop(&loc);
     return rc;
 }

qemu-tool.c

@@ -104,6 +104,30 @@ int64_t qemu_get_clock(QEMUClock *clock)
     return (tv.tv_sec * 1000000000LL + (tv.tv_usec * 1000)) / 1000000;
 }
 
+Location *loc_push_restore(Location *loc)
+{
+    return loc;
+}
+
+Location *loc_push_none(Location *loc)
+{
+    return loc;
+}
+
+Location *loc_pop(Location *loc)
+{
+    return loc;
+}
+
+Location *loc_save(Location *loc)
+{
+    return loc;
+}
+
+void loc_restore(Location *loc)
+{
+}
+
 void error_report(const char *fmt, ...)
 {
     va_list args;

qerror.c

@@ -224,6 +224,7 @@ QError *qerror_from_info(const char *file, int linenr, const char *func,
     QError *qerr;
 
     qerr = qerror_new();
+    loc_save(&qerr->loc);
     qerr->linenr = linenr;
     qerr->file = file;
     qerr->func = func;
@@ -321,10 +322,12 @@ QString *qerror_human(const QError *qerror)
  * it uses error_report() for this, so that the output is routed to the right
  * place (ie. stderr or Monitor's device).
  */
-void qerror_print(const QError *qerror)
+void qerror_print(QError *qerror)
 {
     QString *qstring = qerror_human(qerror);
+    loc_push_restore(&qerror->loc);
     error_report("%s", qstring_get_str(qstring));
+    loc_pop(&qerror->loc);
     QDECREF(qstring);
 }
 

qerror.h

@@ -14,6 +14,7 @@
 
 #include "qdict.h"
 #include "qstring.h"
+#include "qemu-error.h"
 #include <stdarg.h>
 
 typedef struct QErrorStringTable {
@@ -24,6 +25,7 @@ typedef struct QErrorStringTable {
 typedef struct QError {
     QObject_HEAD;
     QDict *error;
+    Location loc;
     int linenr;
     const char *file;
     const char *func;
@@ -34,7 +36,7 @@ QError *qerror_new(void);
 QError *qerror_from_info(const char *file, int linenr, const char *func,
                          const char *fmt, va_list *va);
 QString *qerror_human(const QError *qerror);
-void qerror_print(const QError *qerror);
+void qerror_print(QError *qerror);
 QError *qobject_to_qerror(const QObject *obj);
 
 /*