[libvirt] [PATCH] Fix build with GCC 8 new warnings

[Daniel P. Berrangé]

GCC 8 added a -Wcast-function-type warning to -Wextra by
default. This complains if you try to explicitly cast
one function signature to another function signature
with incompatible args. This is something we do many
times in libvirt especially with event callbacks. The
hack to silence the warning requires casting via a
void(*)(void) signature before casting to the desired
type. This gross, so we just disable this new warning.

GCC 8 also became more fussy about detecting switch
fallthroughs. First it doesn't like it if you have
a fallthrough attribute that is not before a case
statement. e.g.

   FOO:
   BAR:
   WIZZ:
      ATTRIBUTE_FALLTHROUGH;

Is unacceptable as there's no final case statement,
so while FOO & BAR are falling through, WIZZ is
not falling through. IOW, GCC wants us to write

  FOO:
  BAR:
    ATTRIBUTE_FALLTHROUGH;
  WIZZ:

Second, it will report risk of fallthrough even if you
have a case statement for every single enum value, but
only if the switch is nested inside another switch and
the outer case statement has no final break. This is
is arguably valid because despite the fact that we have
cast from "int" to the enum typedef, nothing guarantees
that the variable we're switching on only contains values
that have corresponding switch labels. e.g.

   int domstate = 87539319;
   switch ((virDomainState)domstate) {
      ...
   }

will not match enum value, but also not raise any kind
of compiler warning. So it is right to complain about
risk of fallthrough if no default: is present.

Signed-off-by: Daniel P. Berrangé <berrange at redhat.com>
---
 m4/virt-compile-warnings.m4    |  2 ++
 src/qemu/qemu_domain.c         | 14 +++++++-------
 src/qemu/qemu_domain_address.c | 11 +++++++++++
 3 files changed, 20 insertions(+), 7 deletions(-)

diff --git a/m4/virt-compile-warnings.m4 b/m4/virt-compile-warnings.m4
index b9c974842..4b35a6f6b 100644
--- a/m4/virt-compile-warnings.m4
+++ b/m4/virt-compile-warnings.m4
@@ -177,6 +177,8 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
     # with gl_MANYWARN_COMPLEMENT
     # So we have -W enabled, and then have to explicitly turn off...
     wantwarn="$wantwarn -Wno-sign-compare"
+    # We do "bad" function casts all the time for event callbacks
+    wantwarn="$wantwarn -Wno-cast-function-type"
 
     # GNULIB expects this to be part of -Wc++-compat, but we turn
     # that one off, so we need to manually enable this again
diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
index 178ec24ae..560436f8e 100644
--- a/src/qemu/qemu_domain.c
+++ b/src/qemu/qemu_domain.c
@@ -175,9 +175,9 @@ qemuDomainAsyncJobPhaseToString(qemuDomainAsyncJob job,
     case QEMU_ASYNC_JOB_NONE:
     case QEMU_ASYNC_JOB_LAST:
         ATTRIBUTE_FALLTHROUGH;
+    default:
+        return "none";
     }
-
-    return "none";
 }
 
 int
@@ -199,12 +199,12 @@ qemuDomainAsyncJobPhaseFromString(qemuDomainAsyncJob job,
     case QEMU_ASYNC_JOB_NONE:
     case QEMU_ASYNC_JOB_LAST:
         ATTRIBUTE_FALLTHROUGH;
+    default:
+        if (STREQ(phase, "none"))
+            return 0;
+        else
+            return -1;
     }
-
-    if (STREQ(phase, "none"))
-        return 0;
-    else
-        return -1;
 }
 
 
diff --git a/src/qemu/qemu_domain_address.c b/src/qemu/qemu_domain_address.c
index e28119e4b..42c7c0b12 100644
--- a/src/qemu/qemu_domain_address.c
+++ b/src/qemu/qemu_domain_address.c
@@ -532,6 +532,7 @@ qemuDomainDeviceCalculatePCIConnectFlags(virDomainDeviceDefPtr dev,
             case VIR_DOMAIN_CONTROLLER_MODEL_USB_QUSB2: /* xen only */
             case VIR_DOMAIN_CONTROLLER_MODEL_USB_NONE:
             case VIR_DOMAIN_CONTROLLER_MODEL_USB_LAST:
+            default:
                 return 0;
             }
 
@@ -553,6 +554,7 @@ qemuDomainDeviceCalculatePCIConnectFlags(virDomainDeviceDefPtr dev,
                 return pciFlags;
 
             case VIR_DOMAIN_CONTROLLER_MODEL_SCSI_LAST:
+            default:
                 return 0;
             }
 
@@ -562,6 +564,7 @@ qemuDomainDeviceCalculatePCIConnectFlags(virDomainDeviceDefPtr dev,
         case VIR_DOMAIN_CONTROLLER_TYPE_FDC:
         case VIR_DOMAIN_CONTROLLER_TYPE_CCID:
         case VIR_DOMAIN_CONTROLLER_TYPE_LAST:
+        default:
             /* should be 0 */
             return pciFlags;
         }
@@ -605,6 +608,7 @@ qemuDomainDeviceCalculatePCIConnectFlags(virDomainDeviceDefPtr dev,
         case VIR_DOMAIN_SOUND_MODEL_PCSPK:
         case VIR_DOMAIN_SOUND_MODEL_USB:
         case VIR_DOMAIN_SOUND_MODEL_LAST:
+        default:
             return 0;
         }
 
@@ -622,6 +626,7 @@ qemuDomainDeviceCalculatePCIConnectFlags(virDomainDeviceDefPtr dev,
         case VIR_DOMAIN_DISK_BUS_SATA:
         case VIR_DOMAIN_DISK_BUS_SD:
         case VIR_DOMAIN_DISK_BUS_LAST:
+        default:
             return 0;
         }
 
@@ -734,6 +739,7 @@ qemuDomainDeviceCalculatePCIConnectFlags(virDomainDeviceDefPtr dev,
         case VIR_DOMAIN_MEMBALLOON_MODEL_XEN:
         case VIR_DOMAIN_MEMBALLOON_MODEL_NONE:
         case VIR_DOMAIN_MEMBALLOON_MODEL_LAST:
+        default:
             return 0;
         }
 
@@ -743,6 +749,7 @@ qemuDomainDeviceCalculatePCIConnectFlags(virDomainDeviceDefPtr dev,
             return virtioFlags;
 
         case VIR_DOMAIN_RNG_MODEL_LAST:
+        default:
             return 0;
         }
 
@@ -755,6 +762,7 @@ qemuDomainDeviceCalculatePCIConnectFlags(virDomainDeviceDefPtr dev,
         case VIR_DOMAIN_WATCHDOG_MODEL_IB700:
         case VIR_DOMAIN_WATCHDOG_MODEL_DIAG288:
         case VIR_DOMAIN_WATCHDOG_MODEL_LAST:
+        default:
             return 0;
         }
 
@@ -775,6 +783,7 @@ qemuDomainDeviceCalculatePCIConnectFlags(virDomainDeviceDefPtr dev,
         case VIR_DOMAIN_VIDEO_TYPE_DEFAULT:
         case VIR_DOMAIN_VIDEO_TYPE_GOP:
         case VIR_DOMAIN_VIDEO_TYPE_LAST:
+        default:
             return 0;
         }
 
@@ -791,6 +800,7 @@ qemuDomainDeviceCalculatePCIConnectFlags(virDomainDeviceDefPtr dev,
         case VIR_DOMAIN_INPUT_BUS_XEN:
         case VIR_DOMAIN_INPUT_BUS_PARALLELS:
         case VIR_DOMAIN_INPUT_BUS_LAST:
+        default:
             return 0;
         }
 
@@ -806,6 +816,7 @@ qemuDomainDeviceCalculatePCIConnectFlags(virDomainDeviceDefPtr dev,
         case VIR_DOMAIN_CHR_SERIAL_TARGET_TYPE_SCLP:
         case VIR_DOMAIN_CHR_SERIAL_TARGET_TYPE_NONE:
         case VIR_DOMAIN_CHR_SERIAL_TARGET_TYPE_LAST:
+        default:
             return 0;
         }
 
-- 
2.16.1