error: Fix use of error_prepend() with &error_fatal, &error_abort

From include/qapi/error.h:

  * Pass an existing error to the caller with the message modified:
  *     error_propagate(errp, err);
  *     error_prepend(errp, "Could not frobnicate '%s': ", name);

Fei Li pointed out that doing error_propagate() first doesn't work
well when @errp is &error_fatal or &error_abort: the error_prepend()
is never reached.

Since I doubt fixing the documentation will stop people from getting
it wrong, introduce error_propagate_prepend(), in the hope that it
lures people away from using its constituents in the wrong order.
Update the instructions in error.h accordingly.

Convert existing error_prepend() next to error_propagate to
error_propagate_prepend().  If any of these get reached with
&error_fatal or &error_abort, the error messages improve.  I didn't
check whether that's the case anywhere.

Cc: Fei Li <fli@suse.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20181017082702.5581-2-armbru@redhat.com>

block.c

@@ -4697,9 +4697,9 @@ bool bdrv_op_is_blocked(BlockDriverState *bs, BlockOpType op, Error **errp)
     assert((int) op >= 0 && op < BLOCK_OP_TYPE_MAX);
     if (!QLIST_EMPTY(&bs->op_blockers[op])) {
         blocker = QLIST_FIRST(&bs->op_blockers[op]);
-        error_propagate(errp, error_copy(blocker->reason));
+        error_propagate_prepend(errp, error_copy(blocker->reason),
-        error_prepend(errp, "Node '%s' is busy: ",
+                                "Node '%s' is busy: ",
-                      bdrv_get_device_or_node_name(bs));
+                                bdrv_get_device_or_node_name(bs));
         return true;
     }
     return false;

block/qcow2.c

@@ -2208,8 +2208,8 @@ static void coroutine_fn qcow2_co_invalidate_cache(BlockDriverState *bs,
     qemu_co_mutex_unlock(&s->lock);
     qobject_unref(options);
     if (local_err) {
-        error_propagate(errp, local_err);
+        error_propagate_prepend(errp, local_err,
-        error_prepend(errp, "Could not reopen qcow2 layer: ");
+                                "Could not reopen qcow2 layer: ");
         bs->drv = NULL;
         return;
     } else if (ret < 0) {

block/qed.c

@@ -1606,8 +1606,8 @@ static void coroutine_fn bdrv_qed_co_invalidate_cache(BlockDriverState *bs,
     ret = bdrv_qed_do_open(bs, NULL, bs->open_flags, &local_err);
     qemu_co_mutex_unlock(&s->table_lock);
     if (local_err) {
-        error_propagate(errp, local_err);
+        error_propagate_prepend(errp, local_err,
-        error_prepend(errp, "Could not reopen qed layer: ");
+                                "Could not reopen qed layer: ");
         return;
     } else if (ret < 0) {
         error_setg_errno(errp, -ret, "Could not reopen qed layer");

hw/9pfs/9p-local.c

@@ -1509,8 +1509,8 @@ static int local_parse_opts(QemuOpts *opts, FsDriverEntry *fse, Error **errp)
 
     fsdev_throttle_parse_opts(opts, &fse->fst, &local_err);
     if (local_err) {
-        error_propagate(errp, local_err);
+        error_propagate_prepend(errp, local_err,
-        error_prepend(errp, "invalid throttle configuration: ");
+                                "invalid throttle configuration: ");
         return -1;
     }
 

hw/intc/xics.c

@@ -320,8 +320,9 @@ static void icp_realize(DeviceState *dev, Error **errp)
 
     obj = object_property_get_link(OBJECT(dev), ICP_PROP_XICS, &err);
     if (!obj) {
-        error_propagate(errp, err);
+        error_propagate_prepend(errp, err,
-        error_prepend(errp, "required link '" ICP_PROP_XICS "' not found: ");
+                                "required link '" ICP_PROP_XICS
+                                "' not found: ");
         return;
     }
 
@@ -329,8 +330,9 @@ static void icp_realize(DeviceState *dev, Error **errp)
 
     obj = object_property_get_link(OBJECT(dev), ICP_PROP_CPU, &err);
     if (!obj) {
-        error_propagate(errp, err);
+        error_propagate_prepend(errp, err,
-        error_prepend(errp, "required link '" ICP_PROP_CPU "' not found: ");
+                                "required link '" ICP_PROP_CPU
+                                "' not found: ");
         return;
     }
 
@@ -624,8 +626,9 @@ static void ics_base_realize(DeviceState *dev, Error **errp)
 
     obj = object_property_get_link(OBJECT(dev), ICS_PROP_XICS, &err);
     if (!obj) {
-        error_propagate(errp, err);
+        error_propagate_prepend(errp, err,
-        error_prepend(errp, "required link '" ICS_PROP_XICS "' not found: ");
+                                "required link '" ICS_PROP_XICS
+                                "' not found: ");
         return;
     }
     ics->xics = XICS_FABRIC(obj);

hw/ppc/pnv_core.c

@@ -148,8 +148,8 @@ static void pnv_core_realize(DeviceState *dev, Error **errp)
 
     chip = object_property_get_link(OBJECT(dev), "chip", &local_err);
     if (!chip) {
-        error_propagate(errp, local_err);
+        error_propagate_prepend(errp, local_err,
-        error_prepend(errp, "required link 'chip' not found: ");
+                                "required link 'chip' not found: ");
         return;
     }
 

hw/ppc/spapr_pci.c

@@ -1724,16 +1724,15 @@ static void spapr_phb_realize(DeviceState *dev, Error **errp)
         if (smc->legacy_irq_allocation) {
             irq = spapr_irq_findone(spapr, &local_err);
             if (local_err) {
-                error_propagate(errp, local_err);
+                error_propagate_prepend(errp, local_err,
-                error_prepend(errp, "can't allocate LSIs: ");
+                                        "can't allocate LSIs: ");
                 return;
             }
         }
 
         spapr_irq_claim(spapr, irq, true, &local_err);
         if (local_err) {
-            error_propagate(errp, local_err);
+            error_propagate_prepend(errp, local_err, "can't allocate LSIs: ");
-            error_prepend(errp, "can't allocate LSIs: ");
             return;
         }
 

hw/timer/aspeed_timer.c

@@ -454,8 +454,7 @@ static void aspeed_timer_realize(DeviceState *dev, Error **errp)
 
     obj = object_property_get_link(OBJECT(dev), "scu", &err);
     if (!obj) {
-        error_propagate(errp, err);
+        error_propagate_prepend(errp, err, "required link 'scu' not found: ");
-        error_prepend(errp, "required link 'scu' not found: ");
         return;
     }
     s->scu = ASPEED_SCU(obj);

hw/usb/bus.c

@@ -340,8 +340,9 @@ static USBDevice *usb_try_create_simple(USBBus *bus, const char *name,
     }
     object_property_set_bool(OBJECT(dev), true, "realized", &err);
     if (err) {
-        error_propagate(errp, err);
+        error_propagate_prepend(errp, err,
-        error_prepend(errp, "Failed to initialize USB device '%s': ", name);
+                                "Failed to initialize USB device '%s': ",
+                                name);
         return NULL;
     }
     return dev;

hw/vfio/pci.c

@@ -1283,8 +1283,7 @@ static int vfio_msi_setup(VFIOPCIDevice *vdev, int pos, Error **errp)
         if (ret == -ENOTSUP) {
             return 0;
         }
-        error_prepend(&err, "msi_init failed: ");
+        error_propagate_prepend(errp, err, "msi_init failed: ");
-        error_propagate(errp, err);
         return ret;
     }
     vdev->msi_cap_size = 0xa + (msi_maskbit ? 0xa : 0) + (msi_64bit ? 0x4 : 0);

include/qapi/error.h

@@ -52,8 +52,12 @@
  * where Error **errp is a parameter, by convention the last one.
  *
  * Pass an existing error to the caller with the message modified:
+ *     error_propagate_prepend(errp, err);
+ *
+ * Avoid
  *     error_propagate(errp, err);
  *     error_prepend(errp, "Could not frobnicate '%s': ", name);
+ * because this fails to prepend when @errp is &error_fatal.
  *
  * Create a new error and pass it to the caller:
  *     error_setg(errp, "situation normal, all fouled up");
@@ -215,6 +219,16 @@ void error_setg_win32_internal(Error **errp,
  */
 void error_propagate(Error **dst_errp, Error *local_err);
 
+
+/*
+ * Propagate error object (if any) with some text prepended.
+ * Behaves like
+ *     error_prepend(&local_err, fmt, ...);
+ *     error_propagate(dst_errp, local_err);
+ */
+void error_propagate_prepend(Error **dst_errp, Error *local_err,
+                             const char *fmt, ...);
+
 /*
  * Prepend some text to @errp's human-readable error message.
  * The text is made by formatting @fmt, @ap like vprintf().

migration/migration.c

@@ -1546,9 +1546,9 @@ static GSList *migration_blockers;
 int migrate_add_blocker(Error *reason, Error **errp)
 {
     if (migrate_get_current()->only_migratable) {
-        error_propagate(errp, error_copy(reason));
+        error_propagate_prepend(errp, error_copy(reason),
-        error_prepend(errp, "disallowing migration blocker "
+                                "disallowing migration blocker "
-                          "(--only_migratable) for: ");
+                                "(--only_migratable) for: ");
         return -EACCES;
     }
 
@@ -1557,9 +1557,9 @@ int migrate_add_blocker(Error *reason, Error **errp)
         return 0;
     }
 
-    error_propagate(errp, error_copy(reason));
+    error_propagate_prepend(errp, error_copy(reason),
-    error_prepend(errp, "disallowing migration blocker (migration in "
+                            "disallowing migration blocker "
-                      "progress) for: ");
+                            "(migration in progress) for: ");
     return -EBUSY;
 }
 

util/error.c

@@ -292,3 +292,16 @@ void error_propagate(Error **dst_errp, Error *local_err)
         error_free(local_err);
     }
 }
+
+void error_propagate_prepend(Error **dst_errp, Error *err,
+                             const char *fmt, ...)
+{
+    va_list ap;
+
+    if (dst_errp && !*dst_errp) {
+        va_start(ap, fmt);
+        error_vprepend(&err, fmt, ap);
+        va_end(ap);
+    } /* else error is being ignored, don't bother with prepending */
+    error_propagate(dst_errp, err);
+}