Somehow, during the upgrade from libvirt 4.1 to 4.3, I was having a problem building and installing the new libvirt, getting errors such as the following:
qemu_monitor_json.c:(.text+0x2de6): undefined reference to `virJSONValueGetType' ./.libs/libvirt_driver_qemu_impl.a(libvirt_driver_qemu_impl_la-qemu_monitor_json.o):
qemu_monitor_json.c:(.text+0x58b6): more undefined references to `virJSONValueGetType' follow ./.libs/libvirt_driver_qemu_impl.a(libvirt_driver_qemu_impl_la-qemu_monitor_json.o): In function `qemuMonitorJSONGetMigrationParams':
qemu_monitor_json.c:(.text+0x666c): undefined reference to `virJSONValueObjectStealObject' ./.libs/libvirt_driver_qemu_impl.a(libvirt_driver_qemu_impl_la-qemu_monitor_json.o): In function `qemuMonitorJSONGetDumpGuestMemoryCapability':
qemu_monitor_json.c:(.text+0x7306): undefined reference to `virJSONValueGetType' ./.libs/libvirt_driver_qemu_impl.a(libvirt_driver_qemu_impl_la-qemu_monitor_json.o): In function `qemuMonitorJSONGetBlockIoThrottle':
qemu_monitor_json.c:(.text+0xa5ba): undefined reference to `virJSONValueGetType' ./.libs/libvirt_driver_qemu_impl.a(libvirt_driver_qemu_impl_la-qemu_monitor_json.o): In function `qemuMonitorJSONGetCPUDefinitions':
qemu_monitor_json.c:(.text+0xb76f): undefined reference to `virJSONValueGetType' ./.libs/libvirt_driver_qemu_impl.a(libvirt_driver_qemu_impl_la-qemu_monitor_json.o): In function `qemuMonitorJSONGetMigrationCapabilities':
qemu_monitor_json.c:(.text+0xdf4c): undefined reference to `virJSONValueGetType' ./.libs/libvirt_driver_qemu_impl.a(libvirt_driver_qemu_impl_la-qemu_monitor_json.o): In function `qemuMonitorJSONGetGICCapabilities':
qemu_monitor_json.c:(.text+0xe2cc): undefined reference to `virJSONValueGetType' collect2: error: ld returned 1 exit status
libtool: error: error: relink 'libvirt_driver_qemu.la' with the above command before installing it
Always similar linking and libtool errors. Many of them. Didn't seem to matter what USE flags I had, or if I rebuilt any of the dependencies. A bit frustrating.
The eventual fix was to remove libvirt and reinstall it. I'm still not sure what would cause this, other than the ebuild somehow using old versions of the file from the previous build, but that shouldn't be possible. I figured it out by emerging libvirt on another box that didn't already have it installed, and then it all started to work.
emerge -C app-emulation/libvirt dev-python/libvirt-python
emerge -av app-emulation/libvirt dev-python/libvirt-python
And then I was back in business.