qdev: Reject drive property override

qdev_prop_set_drive() screws up when the property already has a
non-null value: it neglects to release the old value.  Both the old
and the new backend become attached to the same device.

Example (taken from iotest 172): -fda ... -drive if=none,... -global
floppy.drive=none0.

Special case: attempting to use the same backend both times fails.
Example (also from iotest 172): -fda ... -global floppy.drive=floppy0.

Yet another example: -device with multiple drive=... (but not
device_add, which silently drops all but the last duplicate property).

Perhaps drive property override could be made to work.  Perhaps it
should.  I can't afford the time to figure this out now.  What I can
do is reject usage that leaves backends in unhealthy states.  For what
it's worth, we've long done the same for netdev properties.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200622094227.1271650-12-armbru@redhat.com>

hw/core/qdev-properties-system.c

@@ -98,6 +98,14 @@ static void set_drive_helper(Object *obj, Visitor *v, const char *name,
         return;
     }
 
+    /*
+     * TODO Should this really be an error?  If no, the old value
+     * needs to be released before we store the new one.
+     */
+    if (!check_prop_still_unset(dev, name, *ptr, str, errp)) {
+        return;
+    }
+
     if (!*str) {
         g_free(str);
         *ptr = NULL;

tests/qemu-iotests/172.out

@@ -795,48 +795,7 @@ Use -device floppy,unit=1,drive=... instead.
 QEMU_PROG: Floppy unit 1 is in use
 
 Testing: -fda TEST_DIR/t.qcow2 -drive if=none,file=TEST_DIR/t.qcow2.2 -global floppy.drive=none0
-
+QEMU_PROG: -global floppy.drive=... conflicts with drive=floppy0
-          dev: isa-fdc, id ""
-            iobase = 1008 (0x3f0)
-            irq = 6 (0x6)
-            dma = 2 (0x2)
-            driveA = ""
-            driveB = ""
-            check_media_rate = true
-            fdtypeA = "auto"
-            fdtypeB = "auto"
-            fallback = "288"
-            isa irq 6
-            bus: floppy-bus.0
-              type floppy-bus
-              dev: floppy, id ""
-                unit = 0 (0x0)
-                drive = "floppy0"
-                logical_block_size = 512 (512 B)
-                physical_block_size = 512 (512 B)
-                min_io_size = 0 (0 B)
-                opt_io_size = 0 (0 B)
-                discard_granularity = 4294967295 (4 GiB)
-                write-cache = "auto"
-                share-rw = false
-                drive-type = "144"
-floppy0 (NODE_NAME): TEST_DIR/t.qcow2 (qcow2)
-    Attached to:      /machine/unattached/device[15]
-    Removable device: not locked, tray closed
-    Cache mode:       writeback
-
-none0 (NODE_NAME): TEST_DIR/t.qcow2.2 (qcow2)
-    Attached to:      /machine/unattached/device[15]
-    Cache mode:       writeback
-
-ide1-cd0: [not inserted]
-    Attached to:      /machine/unattached/device[22]
-    Removable device: not locked, tray closed
-
-sd0: [not inserted]
-    Removable device: not locked, tray closed
-(qemu) quit
-
 
 
 === Mixing -fdX and -device ===
@@ -1475,48 +1434,7 @@ Use -device floppy,unit=1,drive=... instead.
 QEMU_PROG: -device floppy,drive=none1,unit=1: Floppy unit 1 is in use
 
 Testing: -drive if=none,file=TEST_DIR/t.qcow2 -drive if=none,file=TEST_DIR/t.qcow2.2 -global floppy.drive=none0 -device floppy,drive=none1,unit=0
-
+QEMU_PROG: -device floppy,drive=none1,unit=0: -global floppy.drive=... conflicts with drive=none1
-          dev: isa-fdc, id ""
-            iobase = 1008 (0x3f0)
-            irq = 6 (0x6)
-            dma = 2 (0x2)
-            driveA = ""
-            driveB = ""
-            check_media_rate = true
-            fdtypeA = "auto"
-            fdtypeB = "auto"
-            fallback = "288"
-            isa irq 6
-            bus: floppy-bus.0
-              type floppy-bus
-              dev: floppy, id ""
-                unit = 0 (0x0)
-                drive = "none1"
-                logical_block_size = 512 (512 B)
-                physical_block_size = 512 (512 B)
-                min_io_size = 0 (0 B)
-                opt_io_size = 0 (0 B)
-                discard_granularity = 4294967295 (4 GiB)
-                write-cache = "auto"
-                share-rw = false
-                drive-type = "144"
-none0 (NODE_NAME): TEST_DIR/t.qcow2 (qcow2)
-    Attached to:      /machine/peripheral-anon/device[0]
-    Cache mode:       writeback
-
-none1 (NODE_NAME): TEST_DIR/t.qcow2.2 (qcow2)
-    Attached to:      /machine/peripheral-anon/device[0]
-    Removable device: not locked, tray closed
-    Cache mode:       writeback
-
-ide1-cd0: [not inserted]
-    Attached to:      /machine/unattached/device[21]
-    Removable device: not locked, tray closed
-
-sd0: [not inserted]
-    Removable device: not locked, tray closed
-(qemu) quit
-
 
 
 === Attempt to use drive twice ===
@@ -1531,7 +1449,7 @@ Testing: -fda  -device floppy,drive=floppy0
 QEMU_PROG: -device floppy,drive=floppy0: Drive 'floppy0' is already in use because it has been automatically connected to another device (did you need 'if=none' in the drive options?)
 
 Testing: -fda  -global floppy.drive=floppy0
-QEMU_PROG: Drive 'floppy0' is already in use because it has been automatically connected to another device (did you need 'if=none' in the drive options?)
+QEMU_PROG: -global floppy.drive=... conflicts with drive=floppy0
 
 Testing: -device floppy,drive=floppy0
 QEMU_PROG: -device floppy,drive=floppy0: Property 'floppy.drive' can't find value 'floppy0'